[PATCH v1 RFC 0/6] kfuzztest: a new kernel fuzzing framework

[PATCH v1 RFC 0/6] kfuzztest: a new kernel fuzzing framework

[Ethan Graham]

From: Ethan Graham <ethangraham@google.com>

This patch series introduces KFuzzTest, a lightweight framework for
creating in-kernel fuzz targets for internal kernel functions.

The primary motivation for KFuzzTest is to simplify the fuzzing of
low-level, relatively stateless functions (e.g., data parsers, format
converters) that are difficult to exercise effectively from the syscall
boundary. It is intended for in-situ fuzzing of kernel code without
requiring that it be built as a separate userspace library or that its
dependencies be stubbed out. Using a simple macro-based API, developers
can add a new fuzz target with minimal boilerplate code.

The core design consists of three main parts:
1. A `FUZZ_TEST(name, struct_type)` macro that allows developers to
   easily define a fuzz test.
2. A binary input format that allows a userspace fuzzer to serialize
   complex, pointer-rich C structures into a single buffer.
3. Metadata for test targets, constraints, and annotations, which is
   emitted into dedicated ELF sections to allow for discovery and
   inspection by userspace tools. These are found in
   ".kfuzztest_{targets, constraints, annotations}".

To demonstrate this framework's viability, support for KFuzzTest has been
prototyped in a development fork of syzkaller, enabling coverage-guided
fuzzing. To validate its end-to-end effectiveness, we performed an
experiment by manually introducing an off-by-one buffer over-read into
pkcs7_parse_message, like so:

-ret = asn1_ber_decoder(&pkcs7_decoder, ctx, data, datalen);
+ret = asn1_ber_decoder(&pkcs7_decoder, ctx, data, datalen + 1);

A syzkaller instance fuzzing the new test_pkcs7_parse_message target
introduced in patch 6 successfully triggered the bug inside of
asn1_ber_decoder in under a 30 seconds from a cold start.

This series is an RFC to gather early feedback on the overall design and
approach. We are particularly interested in feedback on:
- The general utility of such a framework.
- The design of the binary serialization format.
- The use of ELF sections for metadata and discovery.

The patch series is structured as follows:
- Patch 1 adds and exposes a new KASAN function needed by KFuzzTest.
- Patch 2 introduces the core KFuzzTest API and data structures.
- Patch 3 adds the runtime implementation for the framework.
- Patch 4 adds documentation.
- Patch 5 provides example fuzz targets.
- Patch 6 defines fuzz targets for real kernel functions.

Ethan Graham (6):
  mm/kasan: implement kasan_poison_range
  kfuzztest: add user-facing API and data structures
  kfuzztest: implement core module and input processing
  kfuzztest: add ReST documentation
  kfuzztest: add KFuzzTest sample fuzz targets
  crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

 Documentation/dev-tools/index.rst             |   1 +
 Documentation/dev-tools/kfuzztest.rst         | 279 ++++++++++
 arch/x86/kernel/vmlinux.lds.S                 |  22 +
 crypto/asymmetric_keys/pkcs7_parser.c         |  15 +
 crypto/rsa_helper.c                           |  29 +
 include/linux/kasan.h                         |  16 +
 include/linux/kfuzztest.h                     | 508 ++++++++++++++++++
 lib/Kconfig.debug                             |   1 +
 lib/Makefile                                  |   2 +
 lib/kfuzztest/Kconfig                         |  20 +
 lib/kfuzztest/Makefile                        |   4 +
 lib/kfuzztest/main.c                          | 161 ++++++
 lib/kfuzztest/parse.c                         | 208 +++++++
 mm/kasan/shadow.c                             |  31 ++
 samples/Kconfig                               |   7 +
 samples/Makefile                              |   1 +
 samples/kfuzztest/Makefile                    |   3 +
 samples/kfuzztest/overflow_on_nested_buffer.c |  52 ++
 samples/kfuzztest/underflow_on_buffer.c       |  41 ++
 19 files changed, 1401 insertions(+)
 create mode 100644 Documentation/dev-tools/kfuzztest.rst
 create mode 100644 include/linux/kfuzztest.h
 create mode 100644 lib/kfuzztest/Kconfig
 create mode 100644 lib/kfuzztest/Makefile
 create mode 100644 lib/kfuzztest/main.c
 create mode 100644 lib/kfuzztest/parse.c
 create mode 100644 samples/kfuzztest/Makefile
 create mode 100644 samples/kfuzztest/overflow_on_nested_buffer.c
 create mode 100644 samples/kfuzztest/underflow_on_buffer.c

-- 
2.51.0.rc0.205.g4a044479a3-goog

[PATCH v1 RFC 1/6] mm/kasan: implement kasan_poison_range

[Ethan Graham]

From: Ethan Graham <ethangraham@google.com>

Introduce a new helper function, kasan_poison_range(), to encapsulate
the logic for poisoning an arbitrary memory range of a given size, and
expose it publically in <include/linux/kasan.h>.

This is a preparatory change for the upcoming KFuzzTest patches, which
requires the ability to poison the inter-region padding in its input
buffers.

No functional change to any other subsystem is intended by this commit.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 include/linux/kasan.h | 16 ++++++++++++++++
 mm/kasan/shadow.c     | 31 +++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+)

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index 890011071f2b..09baeb6c9f4d 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -102,6 +102,21 @@ static inline bool kasan_has_integrated_init(void)
 }
 
 #ifdef CONFIG_KASAN
+
+/**
+ * kasan_poison_range - poison the memory range [start, start + size)
+ *
+ * The exact behavior is subject to alignment with KASAN_GRANULE_SIZE, defined
+ * in <mm/kasan/kasan.h>.
+ *
+ * - If @start is unaligned, the initial partial granule at the beginning
+ *	of the range is only poisoned if CONFIG_KASAN_GENERIC is enabled.
+ * - The poisoning of the range only extends up to the last full granule before
+ *	the end of the range. Any remaining bytes in a final partial granule are
+ *	ignored.
+ */
+void kasan_poison_range(const void *start, size_t size);
+
 void __kasan_unpoison_range(const void *addr, size_t size);
 static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
 {
@@ -402,6 +417,7 @@ static __always_inline bool kasan_check_byte(const void *addr)
 
 #else /* CONFIG_KASAN */
 
+static inline void kasan_poison_range(const void *start, size_t size) {}
 static inline void kasan_unpoison_range(const void *address, size_t size) {}
 static inline void kasan_poison_pages(struct page *page, unsigned int order,
 				      bool init) {}
diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
index d2c70cd2afb1..a1b6bfb35f07 100644
--- a/mm/kasan/shadow.c
+++ b/mm/kasan/shadow.c
@@ -147,6 +147,37 @@ void kasan_poison(const void *addr, size_t size, u8 value, bool init)
 }
 EXPORT_SYMBOL_GPL(kasan_poison);
 
+void kasan_poison_range(const void *start, size_t size)
+{
+	void *end = (char *)start + size;
+	uintptr_t start_addr = (uintptr_t)start;
+	uintptr_t head_granule_start;
+	uintptr_t poison_body_start;
+	uintptr_t poison_body_end;
+	size_t head_prefix_size;
+	uintptr_t end_addr;
+
+	end_addr = ALIGN_DOWN((uintptr_t)end, KASAN_GRANULE_SIZE);
+	if (start_addr >= end_addr)
+		return;
+
+	head_granule_start = ALIGN_DOWN(start_addr, KASAN_GRANULE_SIZE);
+	head_prefix_size = start_addr - head_granule_start;
+
+	if (IS_ENABLED(CONFIG_KASAN_GENERIC) && head_prefix_size > 0)
+		kasan_poison_last_granule((void *)head_granule_start,
+					  head_prefix_size);
+
+	poison_body_start = ALIGN(start_addr, KASAN_GRANULE_SIZE);
+	poison_body_end = ALIGN_DOWN(end_addr, KASAN_GRANULE_SIZE);
+
+	if (poison_body_start < poison_body_end)
+		kasan_poison((void *)poison_body_start,
+			     poison_body_end - poison_body_start,
+			     KASAN_SLAB_REDZONE, false);
+}
+EXPORT_SYMBOL(kasan_poison_range);
+
 #ifdef CONFIG_KASAN_GENERIC
 void kasan_poison_last_granule(const void *addr, size_t size)
 {
-- 
2.51.0.rc0.205.g4a044479a3-goog

[PATCH v1 RFC 2/6] kfuzztest: add user-facing API and data structures

[Ethan Graham]

From: Ethan Graham <ethangraham@google.com>

Add the foundational user-facing components for the KFuzzTest framework.
This includes the main API header <linux/kfuzztest.h>, the Kconfig
option to enable the feature, and the required linker script changes
which introduce three new ELF sections in vmlinux.

Note that KFuzzTest is intended strictly for debug builds only, and
should never be enabled in a production build. The fact that it exposes
internal kernel functions and state directly to userspace may constitute
a serious security vulnerability if used for any reason other than
testing.

The header defines:
- The FUZZ_TEST() macro for creating test targets.
- The data structures required for the binary serialization format,
  which allows passing complex inputs from userspace.
- The metadata structures for test targets, constraints and annotations,
  which are placed in dedicated ELF sections (.kfuzztest_*) for discovery.

This patch only adds the public interface and build integration; no
runtime logic is included.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 arch/x86/kernel/vmlinux.lds.S |  22 ++
 include/linux/kfuzztest.h     | 508 ++++++++++++++++++++++++++++++++++
 lib/Kconfig.debug             |   1 +
 lib/kfuzztest/Kconfig         |  20 ++
 4 files changed, 551 insertions(+)
 create mode 100644 include/linux/kfuzztest.h
 create mode 100644 lib/kfuzztest/Kconfig

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 4fa0be732af1..484e3e1ffb9f 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -112,6 +112,26 @@ ASSERT(__relocate_kernel_end - __relocate_kernel_start <= KEXEC_CONTROL_CODE_MAX
 #else
 #define KEXEC_RELOCATE_KERNEL
 #endif
+
+#ifdef CONFIG_KFUZZTEST
+#define KFUZZTEST_TABLE							\
+	. = ALIGN(PAGE_SIZE);						\
+	__kfuzztest_targets_start = .;					\
+	KEEP(*(.kfuzztest_target));					\
+	__kfuzztest_targets_end = .;					\
+	. = ALIGN(PAGE_SIZE);						\
+	__kfuzztest_constraints_start = .;				\
+	KEEP(*(.kfuzztest_constraint));					\
+	__kfuzztest_constraints_end = .;				\
+	. = ALIGN(PAGE_SIZE);						\
+	__kfuzztest_annotations_start = .;				\
+	KEEP(*(.kfuzztest_annotation));					\
+	__kfuzztest_annotations_end = .;
+
+#else /* CONFIG_KFUZZTEST */
+#define KFUZZTEST_TABLE
+#endif /* CONFIG_KFUZZTEST */
+
 PHDRS {
 	text PT_LOAD FLAGS(5);          /* R_E */
 	data PT_LOAD FLAGS(6);          /* RW_ */
@@ -199,6 +219,8 @@ SECTIONS
 		CONSTRUCTORS
 		KEXEC_RELOCATE_KERNEL
 
+		KFUZZTEST_TABLE
+
 		/* rarely changed data like cpu maps */
 		READ_MOSTLY_DATA(INTERNODE_CACHE_BYTES)
 
diff --git a/include/linux/kfuzztest.h b/include/linux/kfuzztest.h
new file mode 100644
index 000000000000..11a647c1d925
--- /dev/null
+++ b/include/linux/kfuzztest.h
@@ -0,0 +1,508 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * The Kernel Fuzz Testing Framework (KFuzzTest) API for defining fuzz targets
+ * for internal kernel functions.
+ *
+ * For more information please see Documentation/dev-tools/kfuzztest.rst.
+ *
+ * Copyright 2025 Google LLC
+ */
+#ifndef KFUZZTEST_H
+#define KFUZZTEST_H
+
+#include <linux/fs.h>
+#include <linux/printk.h>
+#include <linux/types.h>
+
+#define KFUZZTEST_HEADER_MAGIC (0xBFACE)
+#define KFUZZTEST_V0 (0)
+
+/**
+ * @brief The KFuzzTest Input Serialization Format
+ *
+ * KFuzzTest receives its input from userspace as a single binary blob. This
+ * format allows for the serialization of complex, pointer-rich C structures
+ * into a flat buffer that can be safely passed into the kernel. This format
+ * requires only a single copy from userspace into a kenrel buffer, and no
+ * further kernel allocations. Pointers are patched internally using a "region"
+ * system where each region corresponds to some pointed-to data.
+ *
+ * Regions should be padded to respect alignment constraints of their underlying
+ * types, and should be followed by at least 8 bytes of padding. These padded
+ * regions are poisoned by KFuzzTest to ensure that KASAN catches OOB accesses.
+ *
+ * The format consists of a prefix and three main components:
+ * 1. An 8-byte header: Contains KFUZZTEST_MAGIC in the first 4 bytes, and the
+ *	version number in the subsequent 4 bytes. This ensures backwards
+ *	compatibility in the event of future format changes.
+ * 2. A reloc_region_array: Defines the memory layout of the target structure
+ *	by partitioning the payload into logical regions. Each logical region
+ *	should contain the byte representation of the type that it represents,
+ *	including any necessary padding. The region descriptors should be
+ *	ordered by offset ascending.
+ * 3. A reloc_table: Provides "linking" instructions that tell the kernel how
+ *	to patch pointer fields to point to the correct regions. By design,
+ *	the first region (index 0) is passed as input into a FUZZ_TEST.
+ * 4. A Payload: The raw binary data for the structure and its associated
+ *	buffers. This should be aligned to the maximum alignment of all
+ *	regions to satisfy alignment requirements of the input types, but this
+ *	isn't checked by the parser.
+ *
+ * For a detailed specification of the binary layout see the full documentation
+ * at: Documentation/dev-tools/kfuzztest.rst
+ */
+
+/**
+ * struct reloc_region - single contiguous memory region in the payload
+ *
+ * @offset: The byte offset of this region from the start of the payload, which
+ *	should be aligned to the alignment requirements of the region's
+ *	underlying type.
+ * @size: The size of this region in bytes.
+ */
+struct reloc_region {
+	uint32_t offset;
+	uint32_t size;
+};
+
+/**
+ * struct reloc_region_array - array of regions in an input
+ * @num_regions: The total number of regions defined.
+ * @regions: A flexible array of `num_regions` region descriptors.
+ */
+struct reloc_region_array {
+	uint32_t num_regions;
+	struct reloc_region regions[];
+};
+
+/**
+ * struct reloc_entry - a single pointer to be patched in an input
+ *
+ * @region_id: The index of the region in the `reloc_region_array` that
+ *	contains the pointer.
+ * @region_offset: The start offset of the pointer inside of the region.
+ * @value: contains the index of the pointee region, or KFUZZTEST_REGIONID_NULL
+ *	if the pointer is NULL.
+ */
+struct reloc_entry {
+	uint32_t region_id;
+	uint32_t region_offset;
+	uint32_t value;
+};
+
+/**
+ * struct reloc_entry - array of relocations required by an input
+ *
+ * @num_entries: the number of pointer relocations.
+ * @padding_size: the number of padded bytes between the last relocation in
+ *	entries, and the start of the payload data. This should be at least
+ *	8 bytes, as it is used for poisoning.
+ * @entries: array of relocations.
+ */
+struct reloc_table {
+	uint32_t num_entries;
+	uint32_t padding_size;
+	struct reloc_entry entries[];
+};
+
+/**
+ * kfuzztest_parse_and_relocate - validate and relocate a KFuzzTest input
+ *
+ * @input: A buffer containing the serialized input for a fuzz target.
+ * @input_size: the size in bytes of the @input buffer.
+ * @arg_ret: return pointer for the test case's input structure.
+ */
+int kfuzztest_parse_and_relocate(void *input, size_t input_size, void **arg_ret);
+
+/*
+ * Dump some information on the parsed headers and payload. Can be useful for
+ * debugging inputs when writing an encoder for the KFuzzTest input format.
+ */
+__attribute__((unused)) static inline void kfuzztest_debug_header(struct reloc_region_array *regions,
+								  struct reloc_table *rt, void *payload_start,
+								  void *payload_end)
+{
+	uint32_t i;
+
+	pr_info("regions: { num_regions = %u } @ %px", regions->num_regions, regions);
+	for (i = 0; i < regions->num_regions; i++) {
+		pr_info("  region_%u: { start: 0x%x, size: 0x%x }", i, regions->regions[i].offset,
+			regions->regions[i].size);
+	}
+
+	pr_info("reloc_table: { num_entries = %u, padding = %u } @ offset 0x%lx", rt->num_entries, rt->padding_size,
+		(char *)rt - (char *)regions);
+	for (i = 0; i < rt->num_entries; i++) {
+		pr_info("  reloc_%u: { src: %u, offset: 0x%x, dst: %u }", i, rt->entries[i].region_id,
+			rt->entries[i].region_offset, rt->entries[i].value);
+	}
+
+	pr_info("payload: [0x%lx, 0x%lx)", (char *)payload_start - (char *)regions,
+		(char *)payload_end - (char *)regions);
+}
+
+struct kfuzztest_target {
+	const char *name;
+	const char *arg_type_name;
+	ssize_t (*write_input_cb)(struct file *filp, const char __user *buf, size_t len, loff_t *off);
+} __aligned(32);
+
+/**
+ * FUZZ_TEST - defines a KFuzzTest target
+ *
+ * @test_name: The unique identifier for the fuzz test, which is used to name
+ *	the debugfs entry, e.g., /sys/kernel/debug/kfuzztest/@test_name.
+ * @test_arg_type: The struct type that defines the inputs for the test. This
+ *	must be the full struct type (e.g., "struct my_inputs"), not a typedef.
+ *
+ * Context:
+ * This macro is the primary entry point for the KFuzzTest framework. It
+ * generates all the necessary boilerplate for a fuzz test, including:
+ *   - A static `struct kfuzztest_target` instance that is placed in a
+ *	dedicated ELF section for discovery by userspace tools.
+ *   - A `debugfs` write callback that handles receiving serialized data from
+ *	a fuzzer, parsing it, and "hydrating" it into a valid C struct.
+ *   - A function stub where the developer places the test logic.
+ *
+ * User-Provided Logic:
+ * The developer must provide the body of the fuzz test logic within the curly
+ * braces following the macro invocation. Within this scope, the framework
+ * provides the following variables:
+ *
+ * - `arg`: A pointer of type `@test_arg_type *` to the fully hydrated input
+ * structure. All pointer fields within this struct have been relocated
+ * and are valid kernel pointers. This is the primary variable to use
+ * for accessing fuzzing inputs.
+ *
+ * - `regions`: A pointer of type `struct reloc_region_array *`. This is an
+ * advanced feature that allows access to the raw region metadata, which
+ * can be useful for checking the actual allocated size of a buffer via
+ * `KFUZZTEST_REGION_SIZE(n)`.
+ *
+ * Example Usage:
+ *
+ * // 1. The kernel function we want to fuzz.
+ * int process_data(const char *data, size_t len);
+ *
+ * // 2. Define a struct to hold all inputs for the function.
+ * struct process_data_inputs {
+ *	const char *data;
+ *	size_t len;
+ * };
+ *
+ * // 3. Define the fuzz test using the FUZZ_TEST macro.
+ * FUZZ_TEST(process_data_fuzzer, struct process_data_inputs)
+ * {
+ *	int ret;
+ *	// Use KFUZZTEST_EXPECT_* to enforce preconditions.
+ *	// The test will exit early if data is NULL.
+ *	KFUZZTEST_EXPECT_NOT_NULL(process_data_inputs, data);
+ *
+ *	// Use KFUZZTEST_ANNOTATE_* to provide hints to the fuzzer.
+ *	// This links the 'len' field to the 'data' buffer.
+ *	KFUZZTEST_ANNOTATE_LEN(process_data_inputs, len, data);
+ *
+ *	// Call the function under test using the 'arg' variable. OOB memory
+ *	// accesses will be caught by KASAN, but the user can also choose to
+ *	// validate the return value and log any failures.
+ *	ret = process_data(arg->data, arg->len);
+ * }
+ */
+#define FUZZ_TEST(test_name, test_arg_type)                                                                  \
+	static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
+						      loff_t *off);                                          \
+	static void kfuzztest_logic_##test_name(test_arg_type *arg);                                         \
+	const struct kfuzztest_target __fuzz_test__##test_name __section(".kfuzztest_target") __used = {     \
+		.name = #test_name,                                                                          \
+		.arg_type_name = #test_arg_type,                                                             \
+		.write_input_cb = kfuzztest_write_cb_##test_name,                                            \
+	};                                                                                                   \
+	static ssize_t kfuzztest_write_cb_##test_name(struct file *filp, const char __user *buf, size_t len, \
+						      loff_t *off)                                           \
+	{                                                                                                    \
+		test_arg_type *arg;                                                                          \
+		void *buffer;                                                                                \
+		int ret;                                                                                     \
+                                                                                                             \
+		buffer = kmalloc(len, GFP_KERNEL);                                                           \
+		if (!buffer)                                                                                 \
+			return -ENOMEM;                                                                      \
+		ret = simple_write_to_buffer(buffer, len, off, buf, len);                                    \
+		if (ret < 0)                                                                                 \
+			goto out;                                                                            \
+		ret = kfuzztest_parse_and_relocate(buffer, len, (void **)&arg);                              \
+		if (ret < 0)                                                                                 \
+			goto out;                                                                            \
+		kfuzztest_logic_##test_name(arg);                                                            \
+		ret = len;                                                                                   \
+out:                                                                                                         \
+		kfree(buffer);                                                                               \
+		return ret;                                                                                  \
+	}                                                                                                    \
+	static void kfuzztest_logic_##test_name(test_arg_type *arg)
+
+enum kfuzztest_constraint_type {
+	EXPECT_EQ,
+	EXPECT_NE,
+	EXPECT_LT,
+	EXPECT_LE,
+	EXPECT_GT,
+	EXPECT_GE,
+	EXPECT_IN_RANGE,
+};
+
+/**
+ * struct kfuzztest_constraint - a metadata record for a domain constraint
+ *
+ * Domain constraints are rules about the input data that must be satisfied for
+ * a fuzz test to proceed. While they are enforced in the kernel with a runtime
+ * check, they are primarily intended as a discoverable contract for userspace
+ * fuzzers.
+ *
+ * Instances of this struct are generated by the KFUZZTEST_EXPECT_* macros
+ * and placed into the read-only ".kfuzztest_constraint" ELF section of the
+ * vmlinux binary. A fuzzer can parse this section to learn about the
+ * constraints and generate valid inputs more intelligently.
+ *
+ * For an example of how these constraints are used within a fuzz test, see the
+ * documentation for the FUZZ_TEST() macro.
+ *
+ * @input_type: The name of the input struct type, without the leading
+ *	"struct ".
+ * @field_name: The name of the field within the struct that this constraint
+ *	applies to.
+ * @value1: The primary value used in the comparison (e.g., the upper
+ *	bound for EXPECT_LE).
+ * @value2: The secondary value, used only for multi-value comparisons
+ *	(e.g., the upper bound for EXPECT_IN_RANGE).
+ * @type: The type of the constraint.
+ */
+struct kfuzztest_constraint {
+	const char *input_type;
+	const char *field_name;
+	uintptr_t value1;
+	uintptr_t value2;
+	enum kfuzztest_constraint_type type;
+} __aligned(64);
+
+#define __KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val1, val2, tpe)                                         \
+	static struct kfuzztest_constraint __constraint_##arg_type##_##field __section(".kfuzztest_constraint") \
+		__used = {                                                                                      \
+			.input_type = "struct " #arg_type,                                                      \
+			.field_name = #field,                                                                   \
+			.value1 = (uintptr_t)val1,                                                              \
+			.value2 = (uintptr_t)val2,                                                              \
+			.type = tpe,                                                                            \
+		}
+
+/**
+ * KFUZZTEST_EXPECT_EQ - constrain a field to be equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable
+ * @val: a value of the same type as @arg_type.@field
+ */
+#define KFUZZTEST_EXPECT_EQ(arg_type, field, val)                                    \
+	do {                                                                         \
+		if (arg->field != val)                                               \
+			return;                                                      \
+		__KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_EQ); \
+	} while (0)
+
+/**
+ * KFUZZTEST_EXPECT_NE - constrain a field to be not equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_NE(arg_type, field, val)                                    \
+	do {                                                                         \
+		if (arg->field == val)                                               \
+			return;                                                      \
+		__KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_NE); \
+	} while (0)
+
+/**
+ * KFUZZTEST_EXPECT_LT - constrain a field to be less than a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_LT(arg_type, field, val)                                    \
+	do {                                                                         \
+		if (arg->field >= val)                                               \
+			return;                                                      \
+		__KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_LT); \
+	} while (0)
+
+/**
+ * KFUZZTEST_EXPECT_LE - constrain a field to be less than or equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_LE(arg_type, field, val)                                    \
+	do {                                                                         \
+		if (arg->field > val)                                                \
+			return;                                                      \
+		__KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_LE); \
+	} while (0)
+
+/**
+ * KFUZZTEST_EXPECT_GT - constrain a field to be greater than a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_GT(arg_type, field, val)                                   \
+	do {                                                                        \
+		if (arg->field <= val)                                              \
+			return;                                                     \
+		__KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_GT) \
+	} while (0)
+
+/**
+ * KFUZZTEST_EXPECT_GE - constrain a field to be greater than or equal to a value
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_GE(arg_type, field, val)                                   \
+	do {                                                                        \
+		if (arg->field < val)                                               \
+			return;                                                     \
+		__KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, val, 0x0, EXPECT_GE)` \
+	} while (0)
+
+/**
+ * KFUZZTEST_EXPECT_GE - constrain a pointer field to be non-NULL
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @val: a value of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_NOT_NULL(arg_type, field) KFUZZTEST_EXPECT_NE(arg_type, field, NULL)
+
+/**
+ * KFUZZTEST_EXPECT_IN_RANGE - constrain a field to be within a range
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: some field that is comparable.
+ * @lower_bound: a lower bound of the same type as @arg_type.@field.
+ * @upper_bound: an upper bound of the same type as @arg_type.@field.
+ */
+#define KFUZZTEST_EXPECT_IN_RANGE(arg_type, field, lower_bound, upper_bound)                              \
+	do {                                                                                              \
+		if (arg->field < lower_bound || arg->field > upper_bound)                                 \
+			return;                                                                           \
+		__KFUZZTEST_DEFINE_CONSTRAINT(arg_type, field, lower_bound, upper_bound, EXPECT_IN_RANGE) \
+	} while (0)
+
+/**
+ * Annotations express attributes about structure fields that can't be easily
+ * or safely verified at runtime. They are intended as hints to the fuzzing
+ * engine to help it generate more semantically correct and effective inputs.
+ * Unlike constraints, annotations do not add any runtime checks and do not
+ * cause a test to exit early.
+ *
+ * For example, a `char *` field could be a raw byte buffer or a C-style
+ * null-terminated string. A fuzzer that is aware of this distinction can avoid
+ * creating inputs that would cause trivial, uninteresting crashes from reading
+ * past the end of a non-null-terminated buffer.
+ */
+enum kfuzztest_annotation_attribute : uint8_t {
+	ATTRIBUTE_LEN,
+	ATTRIBUTE_STRING,
+	ATTRIBUTE_ARRAY,
+};
+
+/**
+ * struct kfuzztest_annotation - a metadata record for a fuzzer hint
+ *
+ * This struct captures a single hint about a field in the input structure.
+ * Instances are generated by the KFUZZTEST_ANNOTATE_* macros and are placed
+ * into the read-only ".kfuzztest_annotation" ELF section of the vmlinux binary.
+ *
+ * A userspace fuzzer can parse this section to understand the semantic
+ * relationships between fields (e.g., which field is a length for which
+ * buffer) and the expected format of the data (e.g., a null-terminated
+ * string). This allows the fuzzer to be much more intelligent during input
+ * generation and mutation.
+ *
+ * For an example of how annotations are used within a fuzz test, see the
+ * documentation for the FUZZ_TEST() macro.
+ *
+ * @input_type: The name of the input struct type.
+ * @field_name: The name of the field being annotated (e.g., the data
+ *	buffer field).
+ * @linked_field_name: For annotations that link two fields (like
+ *	ATTRIBUTE_LEN), this is the name of the related field (e.g., the
+ *	length field). For others, this may be unused.
+ * @attrib: The type of the annotation hint.
+ */
+struct kfuzztest_annotation {
+	const char *input_type;
+	const char *field_name;
+	const char *linked_field_name;
+	enum kfuzztest_annotation_attribute attrib;
+} __aligned(32);
+
+#define __KFUZZTEST_ANNOTATE(arg_type, field, linked_field, attribute)                                          \
+	static struct kfuzztest_annotation __annotation_##arg_type##_##field __section(".kfuzztest_annotation") \
+		__used = {                                                                                      \
+			.input_type = "struct " #arg_type,                                                      \
+			.field_name = #field,                                                                   \
+			.linked_field_name = #linked_field,                                                     \
+			.attrib = attribute,                                                                    \
+		}
+
+/**
+ * KFUZZTEST_ANNOTATE_STRING - annotate a char* field as a C string
+ *
+ * We define a C string as a sequence of non-zero characters followed by exactly
+ * one null terminator.
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: the name of the field to annotate.
+ */
+#define KFUZZTEST_ANNOTATE_STRING(arg_type, field) __KFUZZTEST_ANNOTATE(arg_type, field, NULL, ATTRIBUTE_STRING)
+
+/**
+ * KFUZZTEST_ANNOTATE_ARRAY - annotate a pointer as an array
+ *
+ * We define an array as a contiguous memory region containing zero or more
+ * elements of the same type.
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: the name of the field to annotate.
+ */
+#define KFUZZTEST_ANNOTATE_ARRAY(arg_type, field) __KFUZZTEST_ANNOTATE(arg_type, field, NULL, ATTRIBUTE_ARRAY)
+
+/**
+ * KFUZZTEST_ANNOTATE_LEN - annotate a field as the length of another
+ *
+ * This expresses the relationship `arg_type.field == len(linked_field)`, where
+ * `linked_field` is an array.
+ *
+ * @arg_type: name of the input structure, without the leading "struct ".
+ * @field: the name of the field to annotate.
+ * @linked_field: the name of an array field with length @field.
+ */
+#define KFUZZTEST_ANNOTATE_LEN(arg_type, field, linked_field) \
+	__KFUZZTEST_ANNOTATE(arg_type, field, linked_field, ATTRIBUTE_LEN)
+
+#define KFUZZTEST_REGIONID_NULL U32_MAX
+
+/**
+ * The end of the input should be padded by at least this number of bytes as
+ * it is poisoned to detect out of bounds accesses at the end of the last
+ * region.
+ */
+#define KFUZZTEST_POISON_SIZE 0x8
+
+#endif /* KFUZZTEST_H */
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index ebe33181b6e6..3542e94204c8 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1947,6 +1947,7 @@ endmenu
 menu "Kernel Testing and Coverage"
 
 source "lib/kunit/Kconfig"
+source "lib/kfuzztest/Kconfig"
 
 config NOTIFIER_ERROR_INJECTION
 	tristate "Notifier error injection"
diff --git a/lib/kfuzztest/Kconfig b/lib/kfuzztest/Kconfig
new file mode 100644
index 000000000000..f9fb5abf8d27
--- /dev/null
+++ b/lib/kfuzztest/Kconfig
@@ -0,0 +1,20 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config KFUZZTEST
+	bool "KFuzzTest - enable support for internal fuzz targets"
+	depends on DEBUG_FS && DEBUG_KERNEL
+	help
+	  Enables support for the kernel fuzz testing framework (KFuzzTest), an
+	  interface for exposing internal kernel functions to a userspace fuzzing
+	  engine. KFuzzTest targets are exposed via a debugfs interface that
+	  accepts serialized userspace inputs, and is designed to make it easier
+	  to fuzz deeply nested kernel code that is hard to reach from the system
+	  call boundary. Using a simple macro-based API, developers can add a new
+	  fuzz target with minimal boilerplate code.
+
+	  It is strongly recommended to also enable CONFIG_KASAN for byte-accurate
+	  out-of-bounds detection, as KFuzzTest was designed with this in mind. It
+	  is also recommended to enable CONFIG_KCOV for coverage guided fuzzing.
+
+	  WARNING: This exposes internal kernel functions directly to userspace
+	  and must NEVER be enabled in production builds.
-- 
2.51.0.rc0.205.g4a044479a3-goog

[PATCH v1 RFC 3/6] kfuzztest: implement core module and input processing

[Ethan Graham]

From: Ethan Graham <ethangraham@google.com>

Add the core runtime implementation for KFuzzTest. This includes the
module initialization, and the logic for receiving and processing
user-provided inputs through debugfs.

On module load, the framework discovers all test targets by iterating
over the .kfuzztest_target section, creating a corresponding debugfs
directory with a write-only 'input' file for each of them.

Writing to an 'input' file triggers the main fuzzing sequence:
1. The serialized input is copied from userspace into a kernel buffer.
2. The buffer is parsed to validate the region array and relocation
   table.
3. Pointers are patched based on the relocation entries, and in KASAN
   builds the inter-region padding is poisoned.
4. The resulting struct is passed to the user-defined test logic.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 lib/Makefile           |   2 +
 lib/kfuzztest/Makefile |   4 +
 lib/kfuzztest/main.c   | 161 +++++++++++++++++++++++++++++++
 lib/kfuzztest/parse.c  | 208 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 375 insertions(+)
 create mode 100644 lib/kfuzztest/Makefile
 create mode 100644 lib/kfuzztest/main.c
 create mode 100644 lib/kfuzztest/parse.c

diff --git a/lib/Makefile b/lib/Makefile
index c38582f187dd..511c44ef4b19 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -354,6 +354,8 @@ obj-$(CONFIG_GENERIC_LIB_CMPDI2) += cmpdi2.o
 obj-$(CONFIG_GENERIC_LIB_UCMPDI2) += ucmpdi2.o
 obj-$(CONFIG_OBJAGG) += objagg.o
 
+obj-$(CONFIG_KFUZZTEST) += kfuzztest/
+
 # pldmfw library
 obj-$(CONFIG_PLDMFW) += pldmfw/
 
diff --git a/lib/kfuzztest/Makefile b/lib/kfuzztest/Makefile
new file mode 100644
index 000000000000..142d16007eea
--- /dev/null
+++ b/lib/kfuzztest/Makefile
@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: GPL-2.0
+
+obj-$(CONFIG_KFUZZTEST) += kfuzztest.o
+kfuzztest-objs := main.o parse.o
diff --git a/lib/kfuzztest/main.c b/lib/kfuzztest/main.c
new file mode 100644
index 000000000000..fccda1319fb0
--- /dev/null
+++ b/lib/kfuzztest/main.c
@@ -0,0 +1,161 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KFuzzTest core module initialization and debugfs interface.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/debugfs.h>
+#include <linux/fs.h>
+#include <linux/kfuzztest.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Ethan Graham <ethangraham@google.com>");
+MODULE_DESCRIPTION("Kernel Fuzz Testing Framework (KFuzzTest)");
+
+extern const struct kfuzztest_target __kfuzztest_targets_start[];
+extern const struct kfuzztest_target __kfuzztest_targets_end[];
+
+/**
+ * struct kfuzztest_dentry - A container for a debugfs dentry and its fops.
+ * @dentry: Pointer to the created debugfs dentry.
+ * @fops: The file_operations struct associated with this dentry.
+ *
+ * This simplifies state management by keeping a file's dentry and its
+ * operations bundled together.
+ */
+struct kfuzztest_dentry {
+	struct dentry *dentry;
+	struct file_operations fops;
+};
+
+/**
+ * struct kfuzztest_debugfs_state - Per-test-case debugfs state.
+ * @test_dir: The top-level debugfs directory for a single test case, e.g.,
+ * /sys/kernel/debug/kfuzztest/<test-name>/.
+ * @input_dentry: The state for the "input" file, which is write-only.
+ *
+ * Wraps all debugfs components created for a single test case.
+ */
+struct kfuzztest_debugfs_state {
+	struct dentry *target_dir;
+	struct kfuzztest_dentry input_dentry;
+};
+
+/**
+ * struct kfuzztest_simple_fuzzer_state - Global state for the KFTF module.
+ * @kfuzztest_dir: The root debugfs directory, /sys/kernel/debug/kfuzztest/.
+ * @debugfs_state: A statically sized array holding the state for each
+ *	registered test case.
+ */
+struct kfuzztest_state {
+	struct file_operations fops;
+	struct dentry *kfuzztest_dir;
+	struct kfuzztest_debugfs_state *debugfs_state;
+};
+
+/* Global static variable to hold all state for the module. */
+static struct kfuzztest_state state;
+
+const umode_t KFUZZTEST_INPUT_PERMS = 0222;
+
+/**
+ * kfuzztest_init - Initializes the debug filesystem for KFuzzTest.
+ *
+ * Each registered test in the ".kfuzztest" section gets its own subdirectory
+ * under "/sys/kernel/debug/kfuzztest/<test-name>" with one files:
+ *	- input: write-only file to send input to the fuzz driver
+ *
+ * Returns:
+ *	0 on success.
+ *	-ENODEV or other error codes if debugfs creation fails.
+ */
+static int __init kfuzztest_init(void)
+{
+	const struct kfuzztest_target *targ;
+	int ret = 0;
+	int i = 0;
+	size_t num_test_cases;
+
+	num_test_cases = __kfuzztest_targets_end - __kfuzztest_targets_start;
+
+	state.debugfs_state =
+		kzalloc(num_test_cases * sizeof(struct kfuzztest_debugfs_state),
+			GFP_KERNEL);
+	if (!state.debugfs_state)
+		return -ENOMEM;
+
+	/* Create the main "kfuzztest" directory in /sys/kernel/debug. */
+	state.kfuzztest_dir = debugfs_create_dir("kfuzztest", NULL);
+	if (!state.kfuzztest_dir) {
+		pr_warn("KFuzzTest: could not create debugfs");
+		return -ENODEV;
+	}
+
+	if (IS_ERR(state.kfuzztest_dir)) {
+		state.kfuzztest_dir = NULL;
+		return PTR_ERR(state.kfuzztest_dir);
+	}
+
+	for (targ = __kfuzztest_targets_start; targ < __kfuzztest_targets_end;
+	     targ++, i++) {
+		/* Create debugfs directory for the target. */
+		state.debugfs_state[i].target_dir =
+			debugfs_create_dir(targ->name, state.kfuzztest_dir);
+
+		if (!state.debugfs_state[i].target_dir) {
+			ret = -ENOMEM;
+			goto cleanup_failure;
+		} else if (IS_ERR(state.debugfs_state[i].target_dir)) {
+			ret = PTR_ERR(state.debugfs_state[i].target_dir);
+			goto cleanup_failure;
+		}
+
+		/* Create an input file under the target's directory. */
+		state.debugfs_state[i].input_dentry.fops =
+			(struct file_operations){
+				.owner = THIS_MODULE,
+				.write = targ->write_input_cb,
+			};
+		state.debugfs_state[i].input_dentry.dentry =
+			debugfs_create_file(
+				"input", KFUZZTEST_INPUT_PERMS,
+				state.debugfs_state[i].target_dir, NULL,
+				&state.debugfs_state[i].input_dentry.fops);
+		if (!state.debugfs_state[i].input_dentry.dentry) {
+			ret = -ENOMEM;
+			goto cleanup_failure;
+		} else if (IS_ERR(state.debugfs_state[i].input_dentry.dentry)) {
+			ret = PTR_ERR(
+				state.debugfs_state[i].input_dentry.dentry);
+			goto cleanup_failure;
+		}
+
+		pr_info("KFuzzTest: registered target %s", targ->name);
+	}
+
+	return 0;
+
+cleanup_failure:
+	debugfs_remove_recursive(state.kfuzztest_dir);
+	return ret;
+}
+
+static void __exit kfuzztest_exit(void)
+{
+	pr_info("KFuzzTest: exiting");
+	if (!state.kfuzztest_dir)
+		return;
+
+	debugfs_remove_recursive(state.kfuzztest_dir);
+	state.kfuzztest_dir = NULL;
+
+	if (state.debugfs_state) {
+		kfree(state.debugfs_state);
+		state.debugfs_state = NULL;
+	}
+}
+
+module_init(kfuzztest_init);
+module_exit(kfuzztest_exit);
diff --git a/lib/kfuzztest/parse.c b/lib/kfuzztest/parse.c
new file mode 100644
index 000000000000..6010171190ad
--- /dev/null
+++ b/lib/kfuzztest/parse.c
@@ -0,0 +1,208 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * KFuzzTest input parsing and validation.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+#include <linux/kasan.h>
+
+/*
+ * Enforce a fixed struct size to ensure a consistent stride when iterating over
+ * the array of these structs in the dedicated ELF section.
+ */
+static_assert(sizeof(struct kfuzztest_target) == 32, "struct kfuzztest_target should have size 32");
+static_assert(sizeof(struct kfuzztest_constraint) == 64, "struct kfuzztest_constraint should have size 64");
+static_assert(sizeof(struct kfuzztest_annotation) == 32, "struct kfuzztest_annotation should have size 32");
+
+static int kfuzztest_relocate_v0(struct reloc_region_array *regions, struct reloc_table *rt, void *payload_start,
+				 void *payload_end)
+{
+	struct reloc_region reg, src, dst;
+	void *poison_start, *poison_end;
+	uintptr_t *ptr_location;
+	struct reloc_entry re;
+	size_t i;
+
+	/* Patch pointers. */
+	for (i = 0; i < rt->num_entries; i++) {
+		re = rt->entries[i];
+		src = regions->regions[re.region_id];
+		ptr_location = (uintptr_t *)((char *)payload_start + src.offset + re.region_offset);
+		if (re.value == KFUZZTEST_REGIONID_NULL)
+			*ptr_location = (uintptr_t)NULL;
+		else if (re.value < regions->num_regions) {
+			dst = regions->regions[re.value];
+			*ptr_location = (uintptr_t)((char *)payload_start + dst.offset);
+		} else
+			return -EINVAL;
+	}
+
+	/* Poison the padding between regions. */
+	for (i = 0; i < regions->num_regions; i++) {
+		reg = regions->regions[i];
+
+		/* Points to the beginning of the inter-region padding */
+		poison_start = payload_start + reg.offset + reg.size;
+		if (i < regions->num_regions - 1)
+			poison_end = payload_start + regions->regions[i + 1].offset;
+		else
+			poison_end = payload_end;
+
+		if ((char *)poison_end > (char *)payload_end)
+			return -EINVAL;
+
+		kasan_poison_range(poison_start, poison_end - poison_start);
+	}
+
+	/* Poison the padded area preceding the payload. */
+	kasan_poison_range((char *)payload_start - rt->padding_size, rt->padding_size);
+	return 0;
+}
+
+static bool kfuzztest_input_is_valid(struct reloc_region_array *regions, struct reloc_table *rt, void *payload_start,
+				     void *payload_end)
+{
+	size_t payload_size = (char *)payload_end - (char *)payload_start;
+	struct reloc_region reg, next_reg;
+	size_t usable_payload_size;
+	uint32_t region_end_offset;
+	struct reloc_entry reloc;
+	uint32_t i;
+
+	if ((char *)payload_start > (char *)payload_end)
+		return false;
+	if (payload_size < KFUZZTEST_POISON_SIZE)
+		return false;
+	usable_payload_size = payload_size - KFUZZTEST_POISON_SIZE;
+
+	for (i = 0; i < regions->num_regions; i++) {
+		reg = regions->regions[i];
+		if (check_add_overflow(reg.offset, reg.size, &region_end_offset))
+			return false;
+		if ((size_t)region_end_offset > usable_payload_size)
+			return false;
+
+		if (i < regions->num_regions - 1) {
+			next_reg = regions->regions[i + 1];
+			if (reg.offset > next_reg.offset)
+				return false;
+			/*
+			 * Enforce the minimum poisonable gap between
+			 * consecutive regions.
+			 */
+			if (reg.offset + reg.size + KFUZZTEST_POISON_SIZE > next_reg.offset)
+				return false;
+		}
+	}
+
+	if (rt->padding_size < KFUZZTEST_POISON_SIZE) {
+		pr_info("validation failed because rt->padding_size = %u", rt->padding_size);
+		return false;
+	}
+
+	for (i = 0; i < rt->num_entries; i++) {
+		reloc = rt->entries[i];
+		if (reloc.region_id >= regions->num_regions)
+			return false;
+		if (reloc.value != KFUZZTEST_REGIONID_NULL && reloc.value >= regions->num_regions)
+			return false;
+
+		reg = regions->regions[reloc.region_id];
+		if (reloc.region_offset % (sizeof(uintptr_t)) || reloc.region_offset + sizeof(uintptr_t) > reg.size)
+			return false;
+	}
+
+	return true;
+}
+
+static int kfuzztest_parse_input_v0(void *input, size_t input_size, struct reloc_region_array **ret_regions,
+				    struct reloc_table **ret_reloc_table, void **ret_payload_start,
+				    void **ret_payload_end)
+{
+	size_t reloc_entries_size, reloc_regions_size;
+	size_t reloc_table_size, regions_size;
+	struct reloc_region_array *regions;
+	void *payload_end, *payload_start;
+	struct reloc_table *rt;
+	size_t curr_offset = 0;
+
+	if (input_size < sizeof(struct reloc_region_array) + sizeof(struct reloc_table))
+		return -EINVAL;
+
+	regions = input;
+	if (check_mul_overflow(regions->num_regions, sizeof(struct reloc_region), &reloc_regions_size))
+		return -EINVAL;
+	if (check_add_overflow(sizeof(*regions), reloc_regions_size, &regions_size))
+		return -EINVAL;
+
+	curr_offset = regions_size;
+	if (curr_offset > input_size)
+		return -EINVAL;
+	if (input_size - curr_offset < sizeof(struct reloc_table))
+		return -EINVAL;
+
+	rt = (struct reloc_table *)((char *)input + curr_offset);
+
+	if (check_mul_overflow((size_t)rt->num_entries, sizeof(struct reloc_entry), &reloc_entries_size))
+		return -EINVAL;
+	if (check_add_overflow(sizeof(*rt), reloc_entries_size, &reloc_table_size))
+		return -EINVAL;
+	if (check_add_overflow(reloc_table_size, rt->padding_size, &reloc_table_size))
+		return -EINVAL;
+
+	if (check_add_overflow(curr_offset, reloc_table_size, &curr_offset))
+		return -EINVAL;
+	if (curr_offset > input_size)
+		return -EINVAL;
+
+	payload_start = (char *)input + curr_offset;
+	payload_end = (char *)input + input_size;
+
+	if (!kfuzztest_input_is_valid(regions, rt, payload_start, payload_end))
+		return -EINVAL;
+
+	*ret_regions = regions;
+	*ret_reloc_table = rt;
+	*ret_payload_start = payload_start;
+	*ret_payload_end = payload_end;
+	return 0;
+}
+
+static int kfuzztest_parse_and_relocate_v0(void *input, size_t input_size, void **arg_ret)
+{
+	struct reloc_region_array *regions;
+	void *payload_start, *payload_end;
+	struct reloc_table *reloc_table;
+	int ret;
+
+	ret = kfuzztest_parse_input_v0(input, input_size, &regions, &reloc_table, &payload_start, &payload_end);
+	if (ret < 0)
+		return ret;
+
+	ret = kfuzztest_relocate_v0(regions, reloc_table, payload_start, payload_end);
+	if (ret < 0)
+		return ret;
+	*arg_ret = payload_start;
+	return 0;
+}
+
+int kfuzztest_parse_and_relocate(void *input, size_t input_size, void **arg_ret)
+{
+	u32 version, magic;
+
+	if (input_size < sizeof(u32) + sizeof(u32))
+		return -EINVAL;
+
+	magic = *(u32 *)input;
+	if (magic != KFUZZTEST_HEADER_MAGIC)
+		return -EINVAL;
+
+	version = *(u32 *)((char *)input + sizeof(u32));
+	switch (version) {
+	case KFUZZTEST_V0:
+		return kfuzztest_parse_and_relocate_v0(input + sizeof(u64), input_size - sizeof(u64), arg_ret);
+	}
+
+	return -EINVAL;
+}
-- 
2.51.0.rc0.205.g4a044479a3-goog

[PATCH v1 RFC 4/6] kfuzztest: add ReST documentation

[Ethan Graham]

From: Ethan Graham <ethangraham@google.com>

Add Documentation/dev-tools/kfuzztest.rst and reference it in the
dev-tools index.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 Documentation/dev-tools/index.rst     |   1 +
 Documentation/dev-tools/kfuzztest.rst | 279 ++++++++++++++++++++++++++
 2 files changed, 280 insertions(+)
 create mode 100644 Documentation/dev-tools/kfuzztest.rst

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 65c54b27a60b..00ccc4da003b 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -32,6 +32,7 @@ Documentation/process/debugging/index.rst
    kfence
    kselftest
    kunit/index
+   kfuzztest
    ktap
    checkuapi
    gpio-sloppy-logic-analyzer
diff --git a/Documentation/dev-tools/kfuzztest.rst b/Documentation/dev-tools/kfuzztest.rst
new file mode 100644
index 000000000000..7fdc4914b966
--- /dev/null
+++ b/Documentation/dev-tools/kfuzztest.rst
@@ -0,0 +1,279 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright 2025 Google LLC
+
+=========================================
+Kernel Fuzz Testing Framework (KFuzzTest)
+=========================================
+
+Overview
+========
+
+The Kernel Fuzz Testing Framework (KFuzzTest) is a framework designed to expose
+internal kernel functions to a userspace fuzzing engine.
+
+It is intended for testing stateless or low-state functions that are difficult
+to reach from the system call interface, such as routines involved in file
+format parsing or complex data transformations. This provides a method for
+in-situ fuzzing of kernel code without requiring that it be built as a separate
+userspace library or that its dependencies be stubbed out.
+
+The framework consists of four main components:
+
+1.  An API, based on the ``FUZZ_TEST`` macro, for defining test targets
+    directly in the kernel tree.
+2.  A binary serialization format for passing complex, pointer-rich data
+    structures from userspace to the kernel.
+3.  A ``debugfs`` interface through which a userspace fuzzer submits
+    serialized test inputs.
+4.  Metadata embedded in dedicated ELF sections of the ``vmlinux`` binary to
+    allow for the discovery of available fuzz targets by external tooling.
+
+.. warning::
+   KFuzzTest is a debugging and testing tool. It exposes internal kernel
+   functions to userspace with minimal sanitization and is designed for
+   use in controlled test environments only. It must **NEVER** be enabled
+   in production kernels.
+
+Supported Architectures
+=======================
+
+KFuzzTest is currently only supported for x86_64.
+
+Usage
+=====
+
+To enable KFuzzTest, configure the kernel with::
+
+	CONFIG_KFUZZTEST=y
+
+which depends on ``CONFIG_DEBUGFS`` for receiving userspace inputs, and
+``CONFIG_DEBUG_KERNEL`` as an additional guardrail for preventing KFuzzTest
+from finding its way into a production build accidentally.
+
+The KFuzzTest sample fuzz targets can be built in with
+``CONFIG_SAMPLE_KFUZZTEST``.
+
+KFuzzTest currently only supports code that is built into the kernel, as the
+core module's startup process discovers fuzz targets, constraints, and
+annotations from a dedicated ELF section during startup.
+
+Declaring a KFuzzTest target
+----------------------------
+
+A fuzz target is defined directly in a .c file, typically alongside the function
+being tested. This process involves three main parts: defining an input
+structure, writing the test body using the ``FUZZ_TEST`` macro, and optionally
+adding metadata for the fuzzer.
+
+The following example illustrates how to create a fuzz target for a function
+``int process_data(const char *data, size_t len)``.
+
+.. code-block:: c
+
+	/*
+	 * 1. Define a struct to model the inputs for the function under test.
+	 *    Each field corresponds to an argument needed by the function.
+	 */
+	struct process_data_inputs {
+		const char *data;
+		size_t len;
+	};
+
+	/*
+	 * 2. Define the fuzz target using the FUZZ_TEST macro.
+	 *    The first parameter is a unique name for the target.
+	 *    The second parameter is the input struct defined above.
+	 */
+	FUZZ_TEST(test_process_data, struct process_data_inputs)
+	{
+		/*
+		 * Within this body, the 'arg' variable is a pointer to a
+		 * fully initialized 'struct process_data_inputs'.
+		 */
+
+		/*
+		 * 3. (Optional) Add constraints to define preconditions.
+		 *    This check ensures 'arg->data' is not NULL. If the condition
+		 *    is not met, the test exits early. This also creates metadata
+		 *    to inform the fuzzer.
+		 */
+		KFUZZTEST_EXPECT_NOT_NULL(process_data_inputs, data);
+
+		/*
+		 * 4. (Optional) Add annotations to provide semantic hints.
+		 *    This annotation informs the fuzzer that the 'len' field
+		 *    is the length of the buffer pointed to by 'data'.
+		 *    Annotations do not add any runtime checks.
+		 */
+		KFUZZTEST_ANNOTATE_LEN(process_data_inputs, len, data);
+
+		/*
+		 * 5. Call the kernel function with the provided inputs.
+		 *    Memory errors like out-of-bounds accesses on 'arg->data' will
+		 *    be detected by KASAN or other memory error detection tools.
+		 */
+		process_data(arg->data, arg->len);
+	}
+
+KFuzzTest provides two families of macros to improve the quality of fuzzing:
+
+- ``KFUZZTEST_EXPECT_*``: These macros define constraints, which are
+  preconditions that must be true for the test to proceed. They are enforced
+  with a runtime check in the kernel. If a check fails, the current test run is
+  aborted. This metadata helps the userspace fuzzer avoid generating invalid
+  inputs.
+
+- ``KFUZZTEST_ANNOTATE_*``: These macros define annotations, which are purely
+  semantic hints for the fuzzer. They do not add any runtime checks and exist
+  only to help the fuzzer generate more intelligent and structurally correct
+  inputs. For example, KFUZZTEST_ANNOTATE_LEN links a size field to a pointer
+  field, which is a common pattern in C APIs.
+
+Metadata
+--------
+
+Macros ``FUZZ_TEST``, `KFUZZTEST_EXPECT_*`` and ``KFUZZTEST_ANNOTATE_*`` embed
+metadata into several sections within the main ``.data`` section of the final
+``vmlinux`` binary; ``.kfuzztest_target``, ``.kfuzztest_constraint`` and
+``.kfuzztest_annotation`` respectively.
+
+This serves two purposes:
+
+1. The core module uses the ``.kfuzztest_target`` section at boot to discover
+   every ``FUZZ_TEST`` instance and create its ``debugfs`` directory and
+   ``input`` file.
+2. Userspace fuzzers can read this metadata from the ``vmlinux`` binary to
+   discover targets and learn about their rules and structure in order to
+   generate correct and effective inputs.
+
+The metadata in the ``.kfuzztest_*`` sections consists of arrays of fixed-size C
+structs (e.g., ``struct kfuzztest_target``). Fields within these structs that
+are pointers, such as ``name`` or ``arg_type_name``, contain addresses that
+point to other locations in the ``vmlinux`` binary. A userspace tool that
+parsing the ELF file must resolve these pointers to read the data that they
+reference. For example, to get a target's name, a tool must:
+
+1. Read the ``struct kfuzztest_target`` from the ``.kfuzztest_target`` section.
+2. Read the address in the ``.name`` field.
+3. Use that address to locate and read null-terminated string from its position
+   elsewhere in the binary (e.g., ``.rodata``).
+
+Tooling Dependencies
+--------------------
+
+For userspace tools to parse the ``vmlinux`` binary and make use of emitted
+KFuzzTest metadata, the kernel must be compiled with DWARF debug information.
+This is required for tools to understand the layout of C structs, resolve type
+information, and correctly interpret constraints and annotations.
+
+When using KFuzzTest with automated fuzzing tools, either
+``CONFIG_DEBUG_INFO_DWARF4`` or ``CONFIG_DEBUG_INFO_DWARF5`` should be enabled.
+
+Input Format
+============
+
+KFuzzTest targets receive their inputs from userspace via a write to a dedicated
+debugfs ``/sys/kernel/debug/kfuzztest/<test-name>/input``.
+
+The data written to this file must be a single binary blob that follows a
+specific serialization format. This format is designed to allow complex,
+pointer-rich C structures to be represented in a flat buffer, requiring only a
+single kernel allocation and copy from userspace.
+
+An input is first prefixed by an 8-byte header containing a magic value in the
+first four bytes, defined as ``KFUZZTEST_HEADER_MAGIC`` in
+`<include/linux/kfuzztest.h>``, and a version number in the subsequent four
+bytes.
+
+Version 0
+---------
+
+In version 0 (i.e., when the version number in the 8-byte header is equal to 0),
+the input format consists of three main parts laid out sequentially: a region
+array, a relocation table, and the payload.::
+
+    +----------------+---------------------+-----------+----------------+
+    |  region array  |  relocation table   |  padding  |    payload     |
+    +----------------+---------------------+-----------+----------------+
+
+Region Array
+^^^^^^^^^^^^
+
+This component is a header that describes how the raw data in the Payload is
+partitioned into logical memory regions. It consists of a count of regions
+followed by an array of ``struct reloc_region``, where each entry defines a
+single region with its size and offset from the start of the payload.
+
+.. code-block:: c
+
+	struct reloc_region {
+		uint32_t offset;
+		uint32_t size;
+	};
+
+	struct reloc_region_array {
+		uint32_t num_regions;
+		struct reloc_region regions[];
+	};
+
+By convention, region 0 represents the top-level input struct that is passed
+as the arg variable to the FUZZ_TEST body. Subsequent regions typically
+represent data buffers pointed to by fields within that struct. Region array
+entries must be ordered by offset ascending, and must not overlap with one
+another.
+
+To satisfy C language alignment requirements and prevent potential hardware
+faults, the memory address of each region's data must be correctly aligned for
+the type it represents. The framework allocates a base buffer that is suitably
+aligned for any C type. Therefore, the userspace tool that generates the input
+is responsible for calculating each region's offset within the payload to ensure
+this alignment is maintained.
+
+Relocation Table
+^^^^^^^^^^^^^^^^
+
+The relocation table provides the instructions for the kernel to "hydrate" the
+payload by patching pointer fields. It contains an array of
+``struct reloc_entry`` items. Each entry acts as a linking instruction,
+specifying:
+
+- The location of a pointer that needs to be patched (identified by a region
+  ID and an offset within that region).
+
+- The target region that the pointer should point to (identified by the
+  target's region ID) or ``KFUZZTEST_REGIONID_NULL`` if the pointer is ``NULL``.
+
+This table also specifies the amount of padding between its end and the start
+of the payload, which should be at least 8 bytes.
+
+.. code-block:: c
+
+	struct reloc_entry {
+		uint32_t region_id;
+		uint32_t region_offset;
+		uint32_t value;
+	};
+
+	struct reloc_table {
+		uint32_t num_entries;
+		uint32_t padding_size;
+		struct reloc_entry entries[];
+    };
+
+Payload
+^^^^^^^
+
+The payload contains the raw binary data for all regions, concatenated together
+according to their specified offsets.
+
+- Alignment: The start of the payload must be aligned to the most restrictive
+  alignment requirement of all its constituent regions. The framework ensures
+  that each region within the payload is then placed at an offset that respects
+  its own type's alignment.
+
+- Padding and Poisoning: The space between the end of one region's data and the
+  beginning of the next must be sufficient for padding. In KASAN builds,
+  KFuzzTest poisons this unused padding, allowing for precise detection of
+  out-of-bounds memory accesses between adjacent buffers. This padding should
+  be at least ``KFUZZTEST_POISON_SIZE`` bytes as defined in
+  `include/linux/kfuzztest.h``.
-- 
2.51.0.rc0.205.g4a044479a3-goog

[PATCH v1 RFC 5/6] kfuzztest: add KFuzzTest sample fuzz targets

[Ethan Graham]

From: Ethan Graham <ethangraham@google.com>

Add two simple fuzz target samples to demonstrate the KFuzzTest API and
provide basic self-tests for the framework.

These examples showcase how a developer can define a fuzz target using
the FUZZ_TEST(), constraint, and annotation macros, and serve as runtime
sanity checks for the core logic. For example, they test that out-of-bounds
memory accesses into poisoned padding regions are correctly detected in a
KASAN build.

These have been tested by writing syzkaller-generated inputs into their
debugfs 'input' files and verifying that the correct KASAN reports were
triggered.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 samples/Kconfig                               |  7 +++
 samples/Makefile                              |  1 +
 samples/kfuzztest/Makefile                    |  3 ++
 samples/kfuzztest/overflow_on_nested_buffer.c | 52 +++++++++++++++++++
 samples/kfuzztest/underflow_on_buffer.c       | 41 +++++++++++++++
 5 files changed, 104 insertions(+)
 create mode 100644 samples/kfuzztest/Makefile
 create mode 100644 samples/kfuzztest/overflow_on_nested_buffer.c
 create mode 100644 samples/kfuzztest/underflow_on_buffer.c

diff --git a/samples/Kconfig b/samples/Kconfig
index ffef99950206..4be51a21d010 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -321,6 +321,13 @@ config SAMPLE_HUNG_TASK
 	  if 2 or more processes read the same file concurrently, it will
 	  be detected by the hung_task watchdog.
 
+config SAMPLE_KFUZZTEST
+	bool "Build KFuzzTest sample targets"
+	depends on KFUZZTEST
+	help
+	  Build KFuzzTest sample targets that serve as selftests for input
+	  deserialization and inter-region redzone poisoning logic.
+
 source "samples/rust/Kconfig"
 
 source "samples/damon/Kconfig"
diff --git a/samples/Makefile b/samples/Makefile
index 07641e177bd8..3a0e7f744f44 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -44,4 +44,5 @@ obj-$(CONFIG_SAMPLE_DAMON_WSSE)		+= damon/
 obj-$(CONFIG_SAMPLE_DAMON_PRCL)		+= damon/
 obj-$(CONFIG_SAMPLE_DAMON_MTIER)	+= damon/
 obj-$(CONFIG_SAMPLE_HUNG_TASK)		+= hung_task/
+obj-$(CONFIG_SAMPLE_KFUZZTEST)		+= kfuzztest/
 obj-$(CONFIG_SAMPLE_TSM_MR)		+= tsm-mr/
diff --git a/samples/kfuzztest/Makefile b/samples/kfuzztest/Makefile
new file mode 100644
index 000000000000..4f8709876c9e
--- /dev/null
+++ b/samples/kfuzztest/Makefile
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-$(CONFIG_SAMPLE_KFUZZTEST) += overflow_on_nested_buffer.o underflow_on_buffer.o
diff --git a/samples/kfuzztest/overflow_on_nested_buffer.c b/samples/kfuzztest/overflow_on_nested_buffer.c
new file mode 100644
index 000000000000..8b4bab1d6d4a
--- /dev/null
+++ b/samples/kfuzztest/overflow_on_nested_buffer.c
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This file contains a KFuzzTest example target that ensures that a buffer
+ * overflow on a nested region triggers a KASAN OOB access report.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+
+static void overflow_on_nested_buffer(const char *a, size_t a_len, const char *b, size_t b_len)
+{
+	size_t i;
+	pr_info("a = [%px, %px)", a, a + a_len);
+	pr_info("b = [%px, %px)", b, b + b_len);
+
+	/* Ensure that all bytes in arg->b are accessible. */
+	for (i = 0; i < b_len; i++)
+		READ_ONCE(b[i]);
+	/*
+	 * Check that all bytes in arg->a are accessible, and provoke an OOB on
+	 * the first byte to the right of the buffer which will trigger a KASAN
+	 * report.
+	 */
+	for (i = 0; i <= a_len; i++)
+		READ_ONCE(a[i]);
+}
+
+struct nested_buffers {
+	const char *a;
+	size_t a_len;
+	const char *b;
+	size_t b_len;
+};
+
+/**
+ * The KFuzzTest input format specifies that struct nested buffers should
+ * be expanded as:
+ *
+ * | a | b | pad[8] | *a | pad[8] | *b |
+ *
+ * where the padded regions are poisoned. We expect to trigger a KASAN report by
+ * overflowing one byte into the `a` buffer.
+ */
+FUZZ_TEST(test_overflow_on_nested_buffer, struct nested_buffers)
+{
+	KFUZZTEST_EXPECT_NOT_NULL(nested_buffers, a);
+	KFUZZTEST_EXPECT_NOT_NULL(nested_buffers, b);
+	KFUZZTEST_ANNOTATE_LEN(nested_buffers, a_len, a);
+	KFUZZTEST_ANNOTATE_LEN(nested_buffers, b_len, b);
+
+	overflow_on_nested_buffer(arg->a, arg->a_len, arg->b, arg->b_len);
+}
diff --git a/samples/kfuzztest/underflow_on_buffer.c b/samples/kfuzztest/underflow_on_buffer.c
new file mode 100644
index 000000000000..fbe214274037
--- /dev/null
+++ b/samples/kfuzztest/underflow_on_buffer.c
@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * This file contains a KFuzzTest example target that ensures that a buffer
+ * underflow on a region triggers a KASAN OOB access report.
+ *
+ * Copyright 2025 Google LLC
+ */
+#include <linux/kfuzztest.h>
+
+static void underflow_on_buffer(char *buf, size_t buflen)
+{
+	size_t i;
+
+	pr_info("buf = [%px, %px)", buf, buf + buflen);
+
+	/* First ensure that all bytes in arg->b are accessible. */
+	for (i = 0; i < buflen; i++)
+		READ_ONCE(buf[i]);
+	/*
+	 * Provoke a buffer overflow on the first byte preceding b, triggering
+	 * a KASAN report.
+	 */
+	READ_ONCE(*((char *)buf - 1));
+}
+
+struct some_buffer {
+	char *buf;
+	size_t buflen;
+};
+
+/**
+ * Tests that the region between struct some_buffer and the expanded *buf field
+ * is correctly poisoned by accessing the first byte before *buf.
+ */
+FUZZ_TEST(test_underflow_on_buffer, struct some_buffer)
+{
+	KFUZZTEST_EXPECT_NOT_NULL(some_buffer, buf);
+	KFUZZTEST_ANNOTATE_LEN(some_buffer, buflen, buf);
+
+	underflow_on_buffer(arg->buf, arg->buflen);
+}
-- 
2.51.0.rc0.205.g4a044479a3-goog

[PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Ethan Graham]

From: Ethan Graham <ethangraham@google.com>

Add KFuzzTest targets for pkcs7_parse_message, rsa_parse_pub_key, and
rsa_parse_priv_key to serve as real-world examples of how the framework is used.

These functions are ideal candidates for KFuzzTest as they perform complex
parsing of user-controlled data but are not directly exposed at the syscall
boundary. This makes them difficult to exercise with traditional fuzzing tools
and showcases the primary strength of the KFuzzTest framework: providing an
interface to fuzz internal, non-exported kernel functions.

The targets are defined directly within the source files of the functions they
test, demonstrating how to colocate fuzz tests with the code under test.

Signed-off-by: Ethan Graham <ethangraham@google.com>
---
 crypto/asymmetric_keys/pkcs7_parser.c | 15 ++++++++++++++
 crypto/rsa_helper.c                   | 29 +++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/crypto/asymmetric_keys/pkcs7_parser.c b/crypto/asymmetric_keys/pkcs7_parser.c
index 423d13c47545..e8477f8b0eaf 100644
--- a/crypto/asymmetric_keys/pkcs7_parser.c
+++ b/crypto/asymmetric_keys/pkcs7_parser.c
@@ -13,6 +13,7 @@
 #include <linux/err.h>
 #include <linux/oid_registry.h>
 #include <crypto/public_key.h>
+#include <linux/kfuzztest.h>
 #include "pkcs7_parser.h"
 #include "pkcs7.asn1.h"
 
@@ -169,6 +170,20 @@ struct pkcs7_message *pkcs7_parse_message(const void *data, size_t datalen)
 }
 EXPORT_SYMBOL_GPL(pkcs7_parse_message);
 
+struct pkcs7_parse_message_arg {
+	const void *data;
+	size_t datalen;
+};
+
+FUZZ_TEST(test_pkcs7_parse_message, struct pkcs7_parse_message_arg)
+{
+	KFUZZTEST_EXPECT_NOT_NULL(pkcs7_parse_message_arg, data);
+	KFUZZTEST_ANNOTATE_LEN(pkcs7_parse_message_arg, datalen, data);
+	KFUZZTEST_EXPECT_LE(pkcs7_parse_message_arg, datalen, 16 * PAGE_SIZE);
+
+	pkcs7_parse_message(arg->data, arg->datalen);
+}
+
 /**
  * pkcs7_get_content_data - Get access to the PKCS#7 content
  * @pkcs7: The preparsed PKCS#7 message to access
diff --git a/crypto/rsa_helper.c b/crypto/rsa_helper.c
index 94266f29049c..79b7ddc7c48d 100644
--- a/crypto/rsa_helper.c
+++ b/crypto/rsa_helper.c
@@ -9,6 +9,7 @@
 #include <linux/export.h>
 #include <linux/err.h>
 #include <linux/fips.h>
+#include <linux/kfuzztest.h>
 #include <crypto/internal/rsa.h>
 #include "rsapubkey.asn1.h"
 #include "rsaprivkey.asn1.h"
@@ -166,6 +167,20 @@ int rsa_parse_pub_key(struct rsa_key *rsa_key, const void *key,
 }
 EXPORT_SYMBOL_GPL(rsa_parse_pub_key);
 
+struct rsa_parse_pub_key_arg {
+	const void *key;
+	size_t key_len;
+};
+
+FUZZ_TEST(test_rsa_parse_pub_key, struct rsa_parse_pub_key_arg)
+{
+	KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_pub_key_arg, key);
+	KFUZZTEST_EXPECT_LE(rsa_parse_pub_key_arg, key_len, 16 * PAGE_SIZE);
+
+	struct rsa_key out;
+	rsa_parse_pub_key(&out, arg->key, arg->key_len);
+}
+
 /**
  * rsa_parse_priv_key() - decodes the BER encoded buffer and stores in the
  *                        provided struct rsa_key, pointers to the raw key
@@ -184,3 +199,17 @@ int rsa_parse_priv_key(struct rsa_key *rsa_key, const void *key,
 	return asn1_ber_decoder(&rsaprivkey_decoder, rsa_key, key, key_len);
 }
 EXPORT_SYMBOL_GPL(rsa_parse_priv_key);
+
+struct rsa_parse_priv_key_arg {
+	const void *key;
+	size_t key_len;
+};
+
+FUZZ_TEST(test_rsa_parse_priv_key, struct rsa_parse_priv_key_arg)
+{
+	KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_priv_key_arg, key);
+	KFUZZTEST_EXPECT_LE(rsa_parse_priv_key_arg, key_len, 16 * PAGE_SIZE);
+
+	struct rsa_key out;
+	rsa_parse_priv_key(&out, arg->key, arg->key_len);
+}
-- 
2.51.0.rc0.205.g4a044479a3-goog

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Marco Elver]

[+Cc crypto maintainers]

On Wed, 13 Aug 2025 at 15:38, Ethan Graham <ethan.w.s.graham@gmail.com> wrote:
>
> From: Ethan Graham <ethangraham@google.com>

Should also Cc crypto maintainers, as they'll be the ones giving
feedback on how interesting this is to them. Use
./scripts/get_maintainer.pl for that in the next round, and either add
the Cc list below your Signed-off-by so that git send-email picks it
up only for this patch, or just for the whole series (normally
preferred, so maintainers get context of the full series).

> Add KFuzzTest targets for pkcs7_parse_message, rsa_parse_pub_key, and
> rsa_parse_priv_key to serve as real-world examples of how the framework is used.
>
> These functions are ideal candidates for KFuzzTest as they perform complex
> parsing of user-controlled data but are not directly exposed at the syscall
> boundary. This makes them difficult to exercise with traditional fuzzing tools
> and showcases the primary strength of the KFuzzTest framework: providing an
> interface to fuzz internal, non-exported kernel functions.
>
> The targets are defined directly within the source files of the functions they
> test, demonstrating how to colocate fuzz tests with the code under test.
>
> Signed-off-by: Ethan Graham <ethangraham@google.com>
> ---
>  crypto/asymmetric_keys/pkcs7_parser.c | 15 ++++++++++++++
>  crypto/rsa_helper.c                   | 29 +++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+)
>
> diff --git a/crypto/asymmetric_keys/pkcs7_parser.c b/crypto/asymmetric_keys/pkcs7_parser.c
> index 423d13c47545..e8477f8b0eaf 100644
> --- a/crypto/asymmetric_keys/pkcs7_parser.c
> +++ b/crypto/asymmetric_keys/pkcs7_parser.c
> @@ -13,6 +13,7 @@
>  #include <linux/err.h>
>  #include <linux/oid_registry.h>
>  #include <crypto/public_key.h>
> +#include <linux/kfuzztest.h>
>  #include "pkcs7_parser.h"
>  #include "pkcs7.asn1.h"
>
> @@ -169,6 +170,20 @@ struct pkcs7_message *pkcs7_parse_message(const void *data, size_t datalen)
>  }
>  EXPORT_SYMBOL_GPL(pkcs7_parse_message);
>
> +struct pkcs7_parse_message_arg {
> +       const void *data;
> +       size_t datalen;
> +};
> +
> +FUZZ_TEST(test_pkcs7_parse_message, struct pkcs7_parse_message_arg)
> +{
> +       KFUZZTEST_EXPECT_NOT_NULL(pkcs7_parse_message_arg, data);
> +       KFUZZTEST_ANNOTATE_LEN(pkcs7_parse_message_arg, datalen, data);
> +       KFUZZTEST_EXPECT_LE(pkcs7_parse_message_arg, datalen, 16 * PAGE_SIZE);
> +
> +       pkcs7_parse_message(arg->data, arg->datalen);
> +}
> +
>  /**
>   * pkcs7_get_content_data - Get access to the PKCS#7 content
>   * @pkcs7: The preparsed PKCS#7 message to access
> diff --git a/crypto/rsa_helper.c b/crypto/rsa_helper.c
> index 94266f29049c..79b7ddc7c48d 100644
> --- a/crypto/rsa_helper.c
> +++ b/crypto/rsa_helper.c
> @@ -9,6 +9,7 @@
>  #include <linux/export.h>
>  #include <linux/err.h>
>  #include <linux/fips.h>
> +#include <linux/kfuzztest.h>
>  #include <crypto/internal/rsa.h>
>  #include "rsapubkey.asn1.h"
>  #include "rsaprivkey.asn1.h"
> @@ -166,6 +167,20 @@ int rsa_parse_pub_key(struct rsa_key *rsa_key, const void *key,
>  }
>  EXPORT_SYMBOL_GPL(rsa_parse_pub_key);
>
> +struct rsa_parse_pub_key_arg {
> +       const void *key;
> +       size_t key_len;
> +};
> +
> +FUZZ_TEST(test_rsa_parse_pub_key, struct rsa_parse_pub_key_arg)
> +{
> +       KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_pub_key_arg, key);
> +       KFUZZTEST_EXPECT_LE(rsa_parse_pub_key_arg, key_len, 16 * PAGE_SIZE);
> +
> +       struct rsa_key out;
> +       rsa_parse_pub_key(&out, arg->key, arg->key_len);
> +}
> +
>  /**
>   * rsa_parse_priv_key() - decodes the BER encoded buffer and stores in the
>   *                        provided struct rsa_key, pointers to the raw key
> @@ -184,3 +199,17 @@ int rsa_parse_priv_key(struct rsa_key *rsa_key, const void *key,
>         return asn1_ber_decoder(&rsaprivkey_decoder, rsa_key, key, key_len);
>  }
>  EXPORT_SYMBOL_GPL(rsa_parse_priv_key);
> +
> +struct rsa_parse_priv_key_arg {
> +       const void *key;
> +       size_t key_len;
> +};
> +
> +FUZZ_TEST(test_rsa_parse_priv_key, struct rsa_parse_priv_key_arg)
> +{
> +       KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_priv_key_arg, key);
> +       KFUZZTEST_EXPECT_LE(rsa_parse_priv_key_arg, key_len, 16 * PAGE_SIZE);
> +
> +       struct rsa_key out;
> +       rsa_parse_priv_key(&out, arg->key, arg->key_len);
> +}
> --
> 2.51.0.rc0.205.g4a044479a3-goog
>

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Ignat Korchagin]

On Wed, Aug 13, 2025 at 7:14 PM Marco Elver <elver@google.com> wrote:
>
> [+Cc crypto maintainers]
>
> On Wed, 13 Aug 2025 at 15:38, Ethan Graham <ethan.w.s.graham@gmail.com> wrote:
> >
> > From: Ethan Graham <ethangraham@google.com>
>
> Should also Cc crypto maintainers, as they'll be the ones giving

Thanks Marco!

> feedback on how interesting this is to them. Use
> ./scripts/get_maintainer.pl for that in the next round, and either add
> the Cc list below your Signed-off-by so that git send-email picks it
> up only for this patch, or just for the whole series (normally
> preferred, so maintainers get context of the full series).
>
> > Add KFuzzTest targets for pkcs7_parse_message, rsa_parse_pub_key, and
> > rsa_parse_priv_key to serve as real-world examples of how the framework is used.
> >
> > These functions are ideal candidates for KFuzzTest as they perform complex
> > parsing of user-controlled data but are not directly exposed at the syscall
> > boundary. This makes them difficult to exercise with traditional fuzzing tools
> > and showcases the primary strength of the KFuzzTest framework: providing an
> > interface to fuzz internal, non-exported kernel functions.
> >
> > The targets are defined directly within the source files of the functions they
> > test, demonstrating how to colocate fuzz tests with the code under test.
> >
> > Signed-off-by: Ethan Graham <ethangraham@google.com>
> > ---
> >  crypto/asymmetric_keys/pkcs7_parser.c | 15 ++++++++++++++
> >  crypto/rsa_helper.c                   | 29 +++++++++++++++++++++++++++
> >  2 files changed, 44 insertions(+)
> >
> > diff --git a/crypto/asymmetric_keys/pkcs7_parser.c b/crypto/asymmetric_keys/pkcs7_parser.c
> > index 423d13c47545..e8477f8b0eaf 100644
> > --- a/crypto/asymmetric_keys/pkcs7_parser.c
> > +++ b/crypto/asymmetric_keys/pkcs7_parser.c
> > @@ -13,6 +13,7 @@
> >  #include <linux/err.h>
> >  #include <linux/oid_registry.h>
> >  #include <crypto/public_key.h>
> > +#include <linux/kfuzztest.h>
> >  #include "pkcs7_parser.h"
> >  #include "pkcs7.asn1.h"
> >
> > @@ -169,6 +170,20 @@ struct pkcs7_message *pkcs7_parse_message(const void *data, size_t datalen)
> >  }
> >  EXPORT_SYMBOL_GPL(pkcs7_parse_message);
> >
> > +struct pkcs7_parse_message_arg {
> > +       const void *data;
> > +       size_t datalen;
> > +};
> > +
> > +FUZZ_TEST(test_pkcs7_parse_message, struct pkcs7_parse_message_arg)

Not sure if it has been mentioned elsewhere, but one thing I already
don't like about it is that these definitions "pollute" the actual
source files. Might not be such a big deal here, but kernel source
files for core subsystems tend to become quite large and complex
already, so not a great idea to make them even larger and harder to
follow with fuzz definitions.

As far as I'm aware, for the same reason KUnit [1] is not that popular
(or at least less popular than other approaches, like selftests [2]).
Is it possible to make it that these definitions live in separate
files or even closer to selftests?

Ignat

> > +{
> > +       KFUZZTEST_EXPECT_NOT_NULL(pkcs7_parse_message_arg, data);
> > +       KFUZZTEST_ANNOTATE_LEN(pkcs7_parse_message_arg, datalen, data);
> > +       KFUZZTEST_EXPECT_LE(pkcs7_parse_message_arg, datalen, 16 * PAGE_SIZE);
> > +
> > +       pkcs7_parse_message(arg->data, arg->datalen);
> > +}
> > +
> >  /**
> >   * pkcs7_get_content_data - Get access to the PKCS#7 content
> >   * @pkcs7: The preparsed PKCS#7 message to access
> > diff --git a/crypto/rsa_helper.c b/crypto/rsa_helper.c
> > index 94266f29049c..79b7ddc7c48d 100644
> > --- a/crypto/rsa_helper.c
> > +++ b/crypto/rsa_helper.c
> > @@ -9,6 +9,7 @@
> >  #include <linux/export.h>
> >  #include <linux/err.h>
> >  #include <linux/fips.h>
> > +#include <linux/kfuzztest.h>
> >  #include <crypto/internal/rsa.h>
> >  #include "rsapubkey.asn1.h"
> >  #include "rsaprivkey.asn1.h"
> > @@ -166,6 +167,20 @@ int rsa_parse_pub_key(struct rsa_key *rsa_key, const void *key,
> >  }
> >  EXPORT_SYMBOL_GPL(rsa_parse_pub_key);
> >
> > +struct rsa_parse_pub_key_arg {
> > +       const void *key;
> > +       size_t key_len;
> > +};
> > +
> > +FUZZ_TEST(test_rsa_parse_pub_key, struct rsa_parse_pub_key_arg)
> > +{
> > +       KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_pub_key_arg, key);
> > +       KFUZZTEST_EXPECT_LE(rsa_parse_pub_key_arg, key_len, 16 * PAGE_SIZE);
> > +
> > +       struct rsa_key out;
> > +       rsa_parse_pub_key(&out, arg->key, arg->key_len);
> > +}
> > +
> >  /**
> >   * rsa_parse_priv_key() - decodes the BER encoded buffer and stores in the
> >   *                        provided struct rsa_key, pointers to the raw key
> > @@ -184,3 +199,17 @@ int rsa_parse_priv_key(struct rsa_key *rsa_key, const void *key,
> >         return asn1_ber_decoder(&rsaprivkey_decoder, rsa_key, key, key_len);
> >  }
> >  EXPORT_SYMBOL_GPL(rsa_parse_priv_key);
> > +
> > +struct rsa_parse_priv_key_arg {
> > +       const void *key;
> > +       size_t key_len;
> > +};
> > +
> > +FUZZ_TEST(test_rsa_parse_priv_key, struct rsa_parse_priv_key_arg)
> > +{
> > +       KFUZZTEST_EXPECT_NOT_NULL(rsa_parse_priv_key_arg, key);
> > +       KFUZZTEST_EXPECT_LE(rsa_parse_priv_key_arg, key_len, 16 * PAGE_SIZE);
> > +
> > +       struct rsa_key out;
> > +       rsa_parse_priv_key(&out, arg->key, arg->key_len);
> > +}
> > --
> > 2.51.0.rc0.205.g4a044479a3-goog
> >

[1]: https://docs.kernel.org/dev-tools/kunit/index.html
[2]: https://docs.kernel.org/dev-tools/kselftest.html

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Eric Biggers]

On Thu, Aug 14, 2025 at 04:28:13PM +0100, Ignat Korchagin wrote:
> Not sure if it has been mentioned elsewhere, but one thing I already
> don't like about it is that these definitions "pollute" the actual
> source files. Might not be such a big deal here, but kernel source
> files for core subsystems tend to become quite large and complex
> already, so not a great idea to make them even larger and harder to
> follow with fuzz definitions.
> 
> As far as I'm aware, for the same reason KUnit [1] is not that popular
> (or at least less popular than other approaches, like selftests [2]).
> Is it possible to make it that these definitions live in separate
> files or even closer to selftests?

That's not the impression I get.  KUnit suites are normally defined in
separate files, and KUnit seems to be increasing in popularity.
KFuzzTest can use separate files too, it looks like?

Would it make any sense for fuzz tests to be a special type of KUnit
test, instead of a separate framework?

- Eric

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Ignat Korchagin]

On Fri, Aug 15, 2025 at 2:18 AM Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Thu, Aug 14, 2025 at 04:28:13PM +0100, Ignat Korchagin wrote:
> > Not sure if it has been mentioned elsewhere, but one thing I already
> > don't like about it is that these definitions "pollute" the actual
> > source files. Might not be such a big deal here, but kernel source
> > files for core subsystems tend to become quite large and complex
> > already, so not a great idea to make them even larger and harder to
> > follow with fuzz definitions.
> >
> > As far as I'm aware, for the same reason KUnit [1] is not that popular
> > (or at least less popular than other approaches, like selftests [2]).
> > Is it possible to make it that these definitions live in separate
> > files or even closer to selftests?
>
> That's not the impression I get.  KUnit suites are normally defined in
> separate files, and KUnit seems to be increasing in popularity.

Great! Either I was wrong from the start or it changed and I haven't
looked there recently.

> KFuzzTest can use separate files too, it looks like?
>
> Would it make any sense for fuzz tests to be a special type of KUnit
> test, instead of a separate framework?

I think so, if possible. There is always some hurdles adopting new
framework, but if it would be a new feature of an existing one (either
KUnit or selftests - whatever fits better semantically), the existing
users of that framework are more likely to pick it up.

> - Eric

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Marco Elver]

On Fri, 15 Aug 2025 at 15:00, Ignat Korchagin <ignat@cloudflare.com> wrote:
>
> On Fri, Aug 15, 2025 at 2:18 AM Eric Biggers <ebiggers@kernel.org> wrote:
> >
> > On Thu, Aug 14, 2025 at 04:28:13PM +0100, Ignat Korchagin wrote:
> > > Not sure if it has been mentioned elsewhere, but one thing I already
> > > don't like about it is that these definitions "pollute" the actual
> > > source files. Might not be such a big deal here, but kernel source
> > > files for core subsystems tend to become quite large and complex
> > > already, so not a great idea to make them even larger and harder to
> > > follow with fuzz definitions.
> > >
> > > As far as I'm aware, for the same reason KUnit [1] is not that popular
> > > (or at least less popular than other approaches, like selftests [2]).
> > > Is it possible to make it that these definitions live in separate
> > > files or even closer to selftests?
> >
> > That's not the impression I get.  KUnit suites are normally defined in
> > separate files, and KUnit seems to be increasing in popularity.
>
> Great! Either I was wrong from the start or it changed and I haven't
> looked there recently.
>
> > KFuzzTest can use separate files too, it looks like?
> >
> > Would it make any sense for fuzz tests to be a special type of KUnit
> > test, instead of a separate framework?
>
> I think so, if possible. There is always some hurdles adopting new
> framework, but if it would be a new feature of an existing one (either
> KUnit or selftests - whatever fits better semantically), the existing
> users of that framework are more likely to pick it up.

The dependency would be in name only (i.e. "branding"). Right now it's
possible to use KFuzzTest without the KUnit dependency. So there is
technical merit to decouple.

Would sufficient documentation, and perhaps suggesting separate files
to be the canonical way of defining KFuzzTests, improve the situation?

For example something like:
For subsystem foo.c, define a KFuzzTest in foo_kfuzz.c, and then in
the Makfile add "obj-$(CONFIG_KFUZZTEST) += foo_kfuzz.o".
Alternatively, to test internal static functions, place the KFuzzTest
harness in a file foo_kfuzz.h, and include at the bottom of foo.c.

Alex, Ethan, and KUnit folks: What's your preference?

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Ignat Korchagin]

On Tue, Aug 19, 2025 at 11:08 AM Marco Elver <elver@google.com> wrote:
>
> On Fri, 15 Aug 2025 at 15:00, Ignat Korchagin <ignat@cloudflare.com> wrote:
> >
> > On Fri, Aug 15, 2025 at 2:18 AM Eric Biggers <ebiggers@kernel.org> wrote:
> > >
> > > On Thu, Aug 14, 2025 at 04:28:13PM +0100, Ignat Korchagin wrote:
> > > > Not sure if it has been mentioned elsewhere, but one thing I already
> > > > don't like about it is that these definitions "pollute" the actual
> > > > source files. Might not be such a big deal here, but kernel source
> > > > files for core subsystems tend to become quite large and complex
> > > > already, so not a great idea to make them even larger and harder to
> > > > follow with fuzz definitions.
> > > >
> > > > As far as I'm aware, for the same reason KUnit [1] is not that popular
> > > > (or at least less popular than other approaches, like selftests [2]).
> > > > Is it possible to make it that these definitions live in separate
> > > > files or even closer to selftests?
> > >
> > > That's not the impression I get.  KUnit suites are normally defined in
> > > separate files, and KUnit seems to be increasing in popularity.
> >
> > Great! Either I was wrong from the start or it changed and I haven't
> > looked there recently.
> >
> > > KFuzzTest can use separate files too, it looks like?
> > >
> > > Would it make any sense for fuzz tests to be a special type of KUnit
> > > test, instead of a separate framework?
> >
> > I think so, if possible. There is always some hurdles adopting new
> > framework, but if it would be a new feature of an existing one (either
> > KUnit or selftests - whatever fits better semantically), the existing
> > users of that framework are more likely to pick it up.
>
> The dependency would be in name only (i.e. "branding"). Right now it's
> possible to use KFuzzTest without the KUnit dependency. So there is
> technical merit to decouple.

Probably strong (Kbuild) dependency is not what I was thinking about,
rather just semantical similarity. That is, if I "learned" KUnit -
KFuzzTest is easy to pick up for me.

> Would sufficient documentation, and perhaps suggesting separate files
> to be the canonical way of defining KFuzzTests, improve the situation?

Probably.

> For example something like:
> For subsystem foo.c, define a KFuzzTest in foo_kfuzz.c, and then in
> the Makfile add "obj-$(CONFIG_KFUZZTEST) += foo_kfuzz.o".
> Alternatively, to test internal static functions, place the KFuzzTest
> harness in a file foo_kfuzz.h, and include at the bottom of foo.c.

Having includes at the bottom of the file feels weird and "leaks"
kfuzz tests into the sources. Perhaps we can somehow rely on the fact
that kernel is a flat address space and you can always get the address
of a symbol (even if static - similar to how eBPF kprobes do it)? Or
have a bit more complex Kbuild configuration: for example
"foo_kfuzz.c" would include "foo.c" (although including .c files also
feels weird). If CONFIG_KFUZZTEST is disabled, Kbuild just includes
"foo.o", if enabled we include "foo_kfuzz.o" (which includes foo.c as
a source).

Ignat

> Alex, Ethan, and KUnit folks: What's your preference?

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[Ethan Graham]

On Tue, Aug 19, 2025 at 12:08 PM Marco Elver <elver@google.com> wrote:
> For example something like:
> For subsystem foo.c, define a KFuzzTest in foo_kfuzz.c, and then in
> the Makfile add "obj-$(CONFIG_KFUZZTEST) += foo_kfuzz.o".

I agree that fuzz targets should only be built if CONFIG_KFUZZTEST is
enabled. Building a separate foo_kfuzz.o is probably ideal, but will
need to think about how to cleanly handle static functions.

> Alternatively, to test internal static functions, place the KFuzzTest
> harness in a file foo_kfuzz.h, and include at the bottom of foo.c.
>
> Alex, Ethan, and KUnit folks: What's your preference?

I think placing fuzz targets in separate files is a step in the right
direction. Including a foo_kfuzz.h file inside of the source does still
pollute the file to some extent but certainly less than having one or
more KFuzzTest targets defined alongside the code.

Re: [PATCH v1 RFC 6/6] crypto: implement KFuzzTest targets for PKCS7 and RSA parsing

[David Gow]

[-- Attachment #1: Type: text/plain, Size: 5833 bytes --]

On Tue, 19 Aug 2025 at 18:08, Marco Elver <elver@google.com> wrote:
>
> On Fri, 15 Aug 2025 at 15:00, Ignat Korchagin <ignat@cloudflare.com> wrote:
> >
> > On Fri, Aug 15, 2025 at 2:18 AM Eric Biggers <ebiggers@kernel.org> wrote:
> > >
> > > On Thu, Aug 14, 2025 at 04:28:13PM +0100, Ignat Korchagin wrote:
> > > > Not sure if it has been mentioned elsewhere, but one thing I already
> > > > don't like about it is that these definitions "pollute" the actual
> > > > source files. Might not be such a big deal here, but kernel source
> > > > files for core subsystems tend to become quite large and complex
> > > > already, so not a great idea to make them even larger and harder to
> > > > follow with fuzz definitions.
> > > >
> > > > As far as I'm aware, for the same reason KUnit [1] is not that popular
> > > > (or at least less popular than other approaches, like selftests [2]).
> > > > Is it possible to make it that these definitions live in separate
> > > > files or even closer to selftests?
> > >
> > > That's not the impression I get.  KUnit suites are normally defined in
> > > separate files, and KUnit seems to be increasing in popularity.
> >
> > Great! Either I was wrong from the start or it changed and I haven't
> > looked there recently.
> >
> > > KFuzzTest can use separate files too, it looks like?
> > >
> > > Would it make any sense for fuzz tests to be a special type of KUnit
> > > test, instead of a separate framework?
> >
> > I think so, if possible. There is always some hurdles adopting new
> > framework, but if it would be a new feature of an existing one (either
> > KUnit or selftests - whatever fits better semantically), the existing
> > users of that framework are more likely to pick it up.
>
> The dependency would be in name only (i.e. "branding"). Right now it's
> possible to use KFuzzTest without the KUnit dependency. So there is
> technical merit to decouple.
>

There's definitely some overlap between KFuzzTest and KUnit, from the
relatively superficial API similarities: both having similar
ASSERT/EXPECT macros; to the more specific: KUnit parameterised tests
allow running the same 'test' code against several different pieces of
input data.

Then again, there are definitely some differences, too: KUnit doesn't
have a way of describing complex binary data (though it's definitely a
feature we'd like one day), and the purpose of KUnit tests, while
having some overlap, is different than exposing fuzz targets.

If there's a bit of KUnit functionality you can reasonably re-use or
otherwise take advantage of, I'd not discourage you from doing so.
There's a balance to be struck between taking the extra dependency and
ending up with duplicate implementations of the same thing.

I also think that what Ignat says below around simply ensuring that
the API is familiar (i.e., not deviating from what KUnit or other
frameworks do without a good reason) is a good middle ground here.

So my gut feeling is that you could end up with one of three things:
- Make KFuzzTests a special case of (parameterised) KUnit tests. This
would probably involve adding a way to run the tests with a parameter
from debugfs or a kernel command-line argument using the metadata
format, and teaching the fuzzer to run KUnit tests. KUnit already has
an attributes mechanism that could be used to note which tests are
fuzz targets, and maybe even providing some of the annotation, but
there'd be some work needed to improve it. The big advantage here is
that you'd automatically gain the ability to use KUnit helpers to set
up things like memory regions, fake devices, etc.
- Share some of the implementation details between KUnit and
KFuzzTest, but keep them as separate things. We already have a bunch
of, e.g., work on assertions, logging, etc, which could possibly be
helpful. This could be done by depending on CONFIG_KUNIT or by
splitting those out into a shared test library.
- Keep them separate, but be careful to make the APIs similar enough
to be familiar. KFuzzTest already looks pretty similar to me, so I
think we're already in a good place here.

Personally, I'd quite like for there to be a bit more overlap/code
sharing -- at least eventually -- as I could see some benefits to
"borrowing" some KFuzzTest code to allow, e.g., providing custom
inputs/outputs for tests.

> Would sufficient documentation, and perhaps suggesting separate files
> to be the canonical way of defining KFuzzTests, improve the situation?
>
> For example something like:
> For subsystem foo.c, define a KFuzzTest in foo_kfuzz.c, and then in
> the Makfile add "obj-$(CONFIG_KFUZZTEST) += foo_kfuzz.o".
> Alternatively, to test internal static functions, place the KFuzzTest
> harness in a file foo_kfuzz.h, and include at the bottom of foo.c.
>
> Alex, Ethan, and KUnit folks: What's your preference?

I think that keeping tests in separate files by default is the right
way to go, but obviously either #including them or using a whole bunch
of conditional symbol exports (either with symbol namespaces or
something like EXPORT_SYMBOL_IF_KUNIT) will be necessary in some cases
to get coverage of internal functions.

I'd suggest being a little careful with the naming scheme, as Linus
was not happy with the foo_test.c names we were using as they make tab
completion more annoying; we ended up putting tests in a 'tests/'
subdirectory where appropriate:
https://docs.kernel.org/dev-tools/kunit/style.html

But ultimately, I think this is a style decision, not a critically
important technical one: provide some good practices to follow -- and
encourage people to be consistent -- but understand that occasionally
a maintainer will override it (sometimes even for good reason).

Cheers,
-- David

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Re: [PATCH v1 RFC 3/6] kfuzztest: implement core module and input processing

[David Gow]

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On Wed, 13 Aug 2025 at 21:38, Ethan Graham <ethan.w.s.graham@gmail.com> wrote:
>
> From: Ethan Graham <ethangraham@google.com>
>
> Add the core runtime implementation for KFuzzTest. This includes the
> module initialization, and the logic for receiving and processing
> user-provided inputs through debugfs.
>
> On module load, the framework discovers all test targets by iterating
> over the .kfuzztest_target section, creating a corresponding debugfs
> directory with a write-only 'input' file for each of them.
>
> Writing to an 'input' file triggers the main fuzzing sequence:
> 1. The serialized input is copied from userspace into a kernel buffer.
> 2. The buffer is parsed to validate the region array and relocation
>    table.
> 3. Pointers are patched based on the relocation entries, and in KASAN
>    builds the inter-region padding is poisoned.
> 4. The resulting struct is passed to the user-defined test logic.
>
> Signed-off-by: Ethan Graham <ethangraham@google.com>
> ---

I haven't had a chance to look over this in detail yet (though I
definitely love some of the ideas here), but I'd strongly encourage
you to taint the kernel (with TAINT_TEST) if this is enabled/used.

Cheers,
-- David


>  lib/Makefile           |   2 +
>  lib/kfuzztest/Makefile |   4 +
>  lib/kfuzztest/main.c   | 161 +++++++++++++++++++++++++++++++
>  lib/kfuzztest/parse.c  | 208 +++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 375 insertions(+)
>  create mode 100644 lib/kfuzztest/Makefile
>  create mode 100644 lib/kfuzztest/main.c
>  create mode 100644 lib/kfuzztest/parse.c
>
> diff --git a/lib/Makefile b/lib/Makefile
> index c38582f187dd..511c44ef4b19 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -354,6 +354,8 @@ obj-$(CONFIG_GENERIC_LIB_CMPDI2) += cmpdi2.o
>  obj-$(CONFIG_GENERIC_LIB_UCMPDI2) += ucmpdi2.o
>  obj-$(CONFIG_OBJAGG) += objagg.o
>
> +obj-$(CONFIG_KFUZZTEST) += kfuzztest/
> +
>  # pldmfw library
>  obj-$(CONFIG_PLDMFW) += pldmfw/
>
> diff --git a/lib/kfuzztest/Makefile b/lib/kfuzztest/Makefile
> new file mode 100644
> index 000000000000..142d16007eea
> --- /dev/null
> +++ b/lib/kfuzztest/Makefile
> @@ -0,0 +1,4 @@
> +# SPDX-License-Identifier: GPL-2.0
> +
> +obj-$(CONFIG_KFUZZTEST) += kfuzztest.o
> +kfuzztest-objs := main.o parse.o
> diff --git a/lib/kfuzztest/main.c b/lib/kfuzztest/main.c
> new file mode 100644
> index 000000000000..fccda1319fb0
> --- /dev/null
> +++ b/lib/kfuzztest/main.c
> @@ -0,0 +1,161 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * KFuzzTest core module initialization and debugfs interface.
> + *
> + * Copyright 2025 Google LLC
> + */
> +#include <linux/debugfs.h>
> +#include <linux/fs.h>
> +#include <linux/kfuzztest.h>
> +#include <linux/module.h>
> +#include <linux/printk.h>
> +
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Ethan Graham <ethangraham@google.com>");
> +MODULE_DESCRIPTION("Kernel Fuzz Testing Framework (KFuzzTest)");
> +
> +extern const struct kfuzztest_target __kfuzztest_targets_start[];
> +extern const struct kfuzztest_target __kfuzztest_targets_end[];
> +
> +/**
> + * struct kfuzztest_dentry - A container for a debugfs dentry and its fops.
> + * @dentry: Pointer to the created debugfs dentry.
> + * @fops: The file_operations struct associated with this dentry.
> + *
> + * This simplifies state management by keeping a file's dentry and its
> + * operations bundled together.
> + */
> +struct kfuzztest_dentry {
> +       struct dentry *dentry;
> +       struct file_operations fops;
> +};
> +
> +/**
> + * struct kfuzztest_debugfs_state - Per-test-case debugfs state.
> + * @test_dir: The top-level debugfs directory for a single test case, e.g.,
> + * /sys/kernel/debug/kfuzztest/<test-name>/.
> + * @input_dentry: The state for the "input" file, which is write-only.
> + *
> + * Wraps all debugfs components created for a single test case.
> + */
> +struct kfuzztest_debugfs_state {
> +       struct dentry *target_dir;
> +       struct kfuzztest_dentry input_dentry;
> +};
> +
> +/**
> + * struct kfuzztest_simple_fuzzer_state - Global state for the KFTF module.
> + * @kfuzztest_dir: The root debugfs directory, /sys/kernel/debug/kfuzztest/.
> + * @debugfs_state: A statically sized array holding the state for each
> + *     registered test case.
> + */
> +struct kfuzztest_state {
> +       struct file_operations fops;
> +       struct dentry *kfuzztest_dir;
> +       struct kfuzztest_debugfs_state *debugfs_state;
> +};
> +
> +/* Global static variable to hold all state for the module. */
> +static struct kfuzztest_state state;
> +
> +const umode_t KFUZZTEST_INPUT_PERMS = 0222;
> +
> +/**
> + * kfuzztest_init - Initializes the debug filesystem for KFuzzTest.
> + *
> + * Each registered test in the ".kfuzztest" section gets its own subdirectory
> + * under "/sys/kernel/debug/kfuzztest/<test-name>" with one files:
> + *     - input: write-only file to send input to the fuzz driver
> + *
> + * Returns:
> + *     0 on success.
> + *     -ENODEV or other error codes if debugfs creation fails.
> + */
> +static int __init kfuzztest_init(void)
> +{
> +       const struct kfuzztest_target *targ;
> +       int ret = 0;
> +       int i = 0;
> +       size_t num_test_cases;
> +
> +       num_test_cases = __kfuzztest_targets_end - __kfuzztest_targets_start;
> +
> +       state.debugfs_state =
> +               kzalloc(num_test_cases * sizeof(struct kfuzztest_debugfs_state),
> +                       GFP_KERNEL);
> +       if (!state.debugfs_state)
> +               return -ENOMEM;
> +
> +       /* Create the main "kfuzztest" directory in /sys/kernel/debug. */
> +       state.kfuzztest_dir = debugfs_create_dir("kfuzztest", NULL);
> +       if (!state.kfuzztest_dir) {
> +               pr_warn("KFuzzTest: could not create debugfs");
> +               return -ENODEV;
> +       }
> +
> +       if (IS_ERR(state.kfuzztest_dir)) {
> +               state.kfuzztest_dir = NULL;
> +               return PTR_ERR(state.kfuzztest_dir);
> +       }
> +
> +       for (targ = __kfuzztest_targets_start; targ < __kfuzztest_targets_end;
> +            targ++, i++) {
> +               /* Create debugfs directory for the target. */
> +               state.debugfs_state[i].target_dir =
> +                       debugfs_create_dir(targ->name, state.kfuzztest_dir);
> +
> +               if (!state.debugfs_state[i].target_dir) {
> +                       ret = -ENOMEM;
> +                       goto cleanup_failure;
> +               } else if (IS_ERR(state.debugfs_state[i].target_dir)) {
> +                       ret = PTR_ERR(state.debugfs_state[i].target_dir);
> +                       goto cleanup_failure;
> +               }
> +
> +               /* Create an input file under the target's directory. */
> +               state.debugfs_state[i].input_dentry.fops =
> +                       (struct file_operations){
> +                               .owner = THIS_MODULE,
> +                               .write = targ->write_input_cb,
> +                       };
> +               state.debugfs_state[i].input_dentry.dentry =
> +                       debugfs_create_file(
> +                               "input", KFUZZTEST_INPUT_PERMS,
> +                               state.debugfs_state[i].target_dir, NULL,
> +                               &state.debugfs_state[i].input_dentry.fops);
> +               if (!state.debugfs_state[i].input_dentry.dentry) {
> +                       ret = -ENOMEM;
> +                       goto cleanup_failure;
> +               } else if (IS_ERR(state.debugfs_state[i].input_dentry.dentry)) {
> +                       ret = PTR_ERR(
> +                               state.debugfs_state[i].input_dentry.dentry);
> +                       goto cleanup_failure;
> +               }
> +
> +               pr_info("KFuzzTest: registered target %s", targ->name);
> +       }
> +
> +       return 0;
> +
> +cleanup_failure:
> +       debugfs_remove_recursive(state.kfuzztest_dir);
> +       return ret;
> +}
> +
> +static void __exit kfuzztest_exit(void)
> +{
> +       pr_info("KFuzzTest: exiting");
> +       if (!state.kfuzztest_dir)
> +               return;
> +
> +       debugfs_remove_recursive(state.kfuzztest_dir);
> +       state.kfuzztest_dir = NULL;
> +
> +       if (state.debugfs_state) {
> +               kfree(state.debugfs_state);
> +               state.debugfs_state = NULL;
> +       }
> +}
> +
> +module_init(kfuzztest_init);
> +module_exit(kfuzztest_exit);
> diff --git a/lib/kfuzztest/parse.c b/lib/kfuzztest/parse.c
> new file mode 100644
> index 000000000000..6010171190ad
> --- /dev/null
> +++ b/lib/kfuzztest/parse.c
> @@ -0,0 +1,208 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * KFuzzTest input parsing and validation.
> + *
> + * Copyright 2025 Google LLC
> + */
> +#include <linux/kfuzztest.h>
> +#include <linux/kasan.h>
> +
> +/*
> + * Enforce a fixed struct size to ensure a consistent stride when iterating over
> + * the array of these structs in the dedicated ELF section.
> + */
> +static_assert(sizeof(struct kfuzztest_target) == 32, "struct kfuzztest_target should have size 32");
> +static_assert(sizeof(struct kfuzztest_constraint) == 64, "struct kfuzztest_constraint should have size 64");
> +static_assert(sizeof(struct kfuzztest_annotation) == 32, "struct kfuzztest_annotation should have size 32");
> +
> +static int kfuzztest_relocate_v0(struct reloc_region_array *regions, struct reloc_table *rt, void *payload_start,
> +                                void *payload_end)
> +{
> +       struct reloc_region reg, src, dst;
> +       void *poison_start, *poison_end;
> +       uintptr_t *ptr_location;
> +       struct reloc_entry re;
> +       size_t i;
> +
> +       /* Patch pointers. */
> +       for (i = 0; i < rt->num_entries; i++) {
> +               re = rt->entries[i];
> +               src = regions->regions[re.region_id];
> +               ptr_location = (uintptr_t *)((char *)payload_start + src.offset + re.region_offset);
> +               if (re.value == KFUZZTEST_REGIONID_NULL)
> +                       *ptr_location = (uintptr_t)NULL;
> +               else if (re.value < regions->num_regions) {
> +                       dst = regions->regions[re.value];
> +                       *ptr_location = (uintptr_t)((char *)payload_start + dst.offset);
> +               } else
> +                       return -EINVAL;
> +       }
> +
> +       /* Poison the padding between regions. */
> +       for (i = 0; i < regions->num_regions; i++) {
> +               reg = regions->regions[i];
> +
> +               /* Points to the beginning of the inter-region padding */
> +               poison_start = payload_start + reg.offset + reg.size;
> +               if (i < regions->num_regions - 1)
> +                       poison_end = payload_start + regions->regions[i + 1].offset;
> +               else
> +                       poison_end = payload_end;
> +
> +               if ((char *)poison_end > (char *)payload_end)
> +                       return -EINVAL;
> +
> +               kasan_poison_range(poison_start, poison_end - poison_start);
> +       }
> +
> +       /* Poison the padded area preceding the payload. */
> +       kasan_poison_range((char *)payload_start - rt->padding_size, rt->padding_size);
> +       return 0;
> +}
> +
> +static bool kfuzztest_input_is_valid(struct reloc_region_array *regions, struct reloc_table *rt, void *payload_start,
> +                                    void *payload_end)
> +{
> +       size_t payload_size = (char *)payload_end - (char *)payload_start;
> +       struct reloc_region reg, next_reg;
> +       size_t usable_payload_size;
> +       uint32_t region_end_offset;
> +       struct reloc_entry reloc;
> +       uint32_t i;
> +
> +       if ((char *)payload_start > (char *)payload_end)
> +               return false;
> +       if (payload_size < KFUZZTEST_POISON_SIZE)
> +               return false;
> +       usable_payload_size = payload_size - KFUZZTEST_POISON_SIZE;
> +
> +       for (i = 0; i < regions->num_regions; i++) {
> +               reg = regions->regions[i];
> +               if (check_add_overflow(reg.offset, reg.size, &region_end_offset))
> +                       return false;
> +               if ((size_t)region_end_offset > usable_payload_size)
> +                       return false;
> +
> +               if (i < regions->num_regions - 1) {
> +                       next_reg = regions->regions[i + 1];
> +                       if (reg.offset > next_reg.offset)
> +                               return false;
> +                       /*
> +                        * Enforce the minimum poisonable gap between
> +                        * consecutive regions.
> +                        */
> +                       if (reg.offset + reg.size + KFUZZTEST_POISON_SIZE > next_reg.offset)
> +                               return false;
> +               }
> +       }
> +
> +       if (rt->padding_size < KFUZZTEST_POISON_SIZE) {
> +               pr_info("validation failed because rt->padding_size = %u", rt->padding_size);
> +               return false;
> +       }
> +
> +       for (i = 0; i < rt->num_entries; i++) {
> +               reloc = rt->entries[i];
> +               if (reloc.region_id >= regions->num_regions)
> +                       return false;
> +               if (reloc.value != KFUZZTEST_REGIONID_NULL && reloc.value >= regions->num_regions)
> +                       return false;
> +
> +               reg = regions->regions[reloc.region_id];
> +               if (reloc.region_offset % (sizeof(uintptr_t)) || reloc.region_offset + sizeof(uintptr_t) > reg.size)
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +
> +static int kfuzztest_parse_input_v0(void *input, size_t input_size, struct reloc_region_array **ret_regions,
> +                                   struct reloc_table **ret_reloc_table, void **ret_payload_start,
> +                                   void **ret_payload_end)
> +{
> +       size_t reloc_entries_size, reloc_regions_size;
> +       size_t reloc_table_size, regions_size;
> +       struct reloc_region_array *regions;
> +       void *payload_end, *payload_start;
> +       struct reloc_table *rt;
> +       size_t curr_offset = 0;
> +
> +       if (input_size < sizeof(struct reloc_region_array) + sizeof(struct reloc_table))
> +               return -EINVAL;
> +
> +       regions = input;
> +       if (check_mul_overflow(regions->num_regions, sizeof(struct reloc_region), &reloc_regions_size))
> +               return -EINVAL;
> +       if (check_add_overflow(sizeof(*regions), reloc_regions_size, &regions_size))
> +               return -EINVAL;
> +
> +       curr_offset = regions_size;
> +       if (curr_offset > input_size)
> +               return -EINVAL;
> +       if (input_size - curr_offset < sizeof(struct reloc_table))
> +               return -EINVAL;
> +
> +       rt = (struct reloc_table *)((char *)input + curr_offset);
> +
> +       if (check_mul_overflow((size_t)rt->num_entries, sizeof(struct reloc_entry), &reloc_entries_size))
> +               return -EINVAL;
> +       if (check_add_overflow(sizeof(*rt), reloc_entries_size, &reloc_table_size))
> +               return -EINVAL;
> +       if (check_add_overflow(reloc_table_size, rt->padding_size, &reloc_table_size))
> +               return -EINVAL;
> +
> +       if (check_add_overflow(curr_offset, reloc_table_size, &curr_offset))
> +               return -EINVAL;
> +       if (curr_offset > input_size)
> +               return -EINVAL;
> +
> +       payload_start = (char *)input + curr_offset;
> +       payload_end = (char *)input + input_size;
> +
> +       if (!kfuzztest_input_is_valid(regions, rt, payload_start, payload_end))
> +               return -EINVAL;
> +
> +       *ret_regions = regions;
> +       *ret_reloc_table = rt;
> +       *ret_payload_start = payload_start;
> +       *ret_payload_end = payload_end;
> +       return 0;
> +}
> +
> +static int kfuzztest_parse_and_relocate_v0(void *input, size_t input_size, void **arg_ret)
> +{
> +       struct reloc_region_array *regions;
> +       void *payload_start, *payload_end;
> +       struct reloc_table *reloc_table;
> +       int ret;
> +
> +       ret = kfuzztest_parse_input_v0(input, input_size, &regions, &reloc_table, &payload_start, &payload_end);
> +       if (ret < 0)
> +               return ret;
> +
> +       ret = kfuzztest_relocate_v0(regions, reloc_table, payload_start, payload_end);
> +       if (ret < 0)
> +               return ret;
> +       *arg_ret = payload_start;
> +       return 0;
> +}
> +
> +int kfuzztest_parse_and_relocate(void *input, size_t input_size, void **arg_ret)
> +{
> +       u32 version, magic;
> +
> +       if (input_size < sizeof(u32) + sizeof(u32))
> +               return -EINVAL;
> +
> +       magic = *(u32 *)input;
> +       if (magic != KFUZZTEST_HEADER_MAGIC)
> +               return -EINVAL;
> +
> +       version = *(u32 *)((char *)input + sizeof(u32));
> +       switch (version) {
> +       case KFUZZTEST_V0:
> +               return kfuzztest_parse_and_relocate_v0(input + sizeof(u64), input_size - sizeof(u64), arg_ret);
> +       }
> +
> +       return -EINVAL;
> +}
> --
> 2.51.0.rc0.205.g4a044479a3-goog
>

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