[PATCH v1 06/10] kfuzztest: add KFuzzTest sample fuzz targets

[Ethan Graham <ethan.w.s.graham@gmail.com>]

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>
Acked-by: Alexander Potapenko <glider@google.com>
---
 samples/Kconfig                               |  7 ++
 samples/Makefile                              |  1 +
 samples/kfuzztest/Makefile                    |  3 +
 samples/kfuzztest/overflow_on_nested_buffer.c | 71 +++++++++++++++++++
 samples/kfuzztest/underflow_on_buffer.c       | 59 +++++++++++++++
 5 files changed, 141 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 6e072a5f1ed8..5209dd9d7a5c 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -320,6 +320,13 @@ config SAMPLE_HUNG_TASK
 	  Reading these files with multiple processes triggers hung task
 	  detection by holding locks for a long time (256 seconds).
 
+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..2f1c3ff9f750
--- /dev/null
+++ b/samples/kfuzztest/overflow_on_nested_buffer.c
@@ -0,0 +1,71 @@
+// 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
+ */
+
+/**
+ * DOC: test_overflow_on_nested_buffer
+ *
+ * This test uses a struct with two distinct dynamically allocated buffers.
+ * It checks that KFuzzTest's memory layout correctly poisons the memory
+ * regions and that KASAN can detect an overflow when reading one byte past the
+ * end of the first buffer (`a`).
+ *
+ * It can be invoked with kfuzztest-bridge using the following command:
+ *
+ * ./kfuzztest-bridge \
+ *   "nested_buffers { ptr[a] len[a, u64] ptr[b] len[b, u64] }; \
+ *   a { arr[u8, 64] }; b { arr[u8, 64] };" \
+ *   "test_overflow_on_nested_buffer" /dev/urandom
+ *
+ * The first argument describes the C struct `nested_buffers` and specifies that
+ * both `a` and `b` are pointers to arrays of 64 bytes.
+ */
+#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..02704a1bfebb
--- /dev/null
+++ b/samples/kfuzztest/underflow_on_buffer.c
@@ -0,0 +1,59 @@
+// 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
+ */
+
+/**
+ * DOC: test_underflow_on_buffer
+ *
+ * This test ensures that the region between the metadata struct and the
+ * dynamically allocated buffer is poisoned. It provokes a one-byte underflow
+ * on the buffer, which should be caught by KASAN.
+ *
+ * It can be invoked with kfuzztest-bridge using the following command:
+ *
+ * ./kfuzztest-bridge \
+ *   "some_buffer { ptr[buf] len[buf, u64]}; buf { arr[u8, 128] };" \
+ *   "test_underflow_on_buffer" /dev/urandom
+ *
+ * The first argument describes the C struct `some_buffer` and specifies that
+ * `buf` is a pointer to an array of 128 bytes. The second argument is the test
+ * name, and the third is a seed file.
+ */
+#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);
+}
-- 
2.51.0.384.g4c02a37b29-goog