[PATCH v3 0/5] slab: ZERO_SIZE_PTR alignment and ERR_PTR hardening

[PATCH v3 0/5] slab: ZERO_SIZE_PTR alignment and ERR_PTR hardening

[Karl Mehltretter]

The kmalloc entry points promise ARCH_KMALLOC_MINALIGN alignment but
return (void *)16 for zero-size requests. Patch 1 aligns the sentinel
to that promise.

Patch 2 limits ZERO_OR_NULL_PTR() to NULL and ZERO_SIZE_PTR. Patch 3
handles ERR_PTR explicitly in kfree() and hardened usercopy, without
giving ERR_PTR special meaning to other users such as krealloc(). The
old helper did not match ERR_PTR values either; patch 3 addresses the
adjacent hardening concern raised during review.

If backported, patches 1 and 2 should be taken together.

Patch 4 adds zero-size KUnit coverage; patch 5 adds ERR_PTR KUnit,
LKDTM, and kselftest coverage, which is why Arnd, Greg, Shuah, and
linux-kselftest@vger.kernel.org are newly copied on v3.

Tested on 940de590b839:

  - GCC 15.2 UML slub_test: 10 passed, 2 skipped, including
    test_zero_size_alloc and test_kfree_err_ptr; NULL and ZERO_SIZE_PTR
    produced no warning, and the expected kfree(ERR_PTR) warning was
    suppressed and counted
  - GCC 15.2 matched x86_64 builds immediately before and after patch 3:
    kfree grew by 15 bytes; __check_object_size gained a 13-byte
    comparison on its normal path and a 23-byte cold abort path
  - GCC 15.2 x86_64 defconfig with HARDENED_USERCOPY: build and normal
    QEMU boot passed; the committed LKDTM selftest matched the distinct
    ERR_PTR diagnostic after reaching usercopy_abort()
  - Clang 21.1.8 armv5 UBSAN_ALIGNMENT: no alignment-assumption or
    slab.h report; generated code retains the exact comparison and
    single evaluation
  - GCC 15.2 MIPS64 big-endian noncoherent build passed with
    ARCH_KMALLOC_MINALIGN equal to 128
  - Microchip SAM9X75 hardware (armv5, ARCH_KMALLOC_MINALIGN=32):
    ZERO_SIZE_PTR was 0x20; zero-size allocation, ksize(), and free
    passed with no boot splats
  - Raspberry Pi 400 hardware (arm64, Cortex-A72,
    ARCH_KMALLOC_MINALIGN=8): ZERO_SIZE_PTR remained 0x10; the ERR_PTR
    KUnit and hardened-usercopy rejection paths passed with no boot
    splats; the revised diagnostic was retested under x86_64 QEMU

---
v2 -> v3:

  - document why patch 2's sentinel-derived low-address window in hardened
    usercopy was incidental
  - add new patch 3 to warn and return when kfree() receives an ERR_PTR
    and reject ERR_PTR values in hardened usercopy
  - replace patch 4's pointer-cast static assertion with a run-time KUnit
    expectation
  - add new patch 5 with KUnit and LKDTM coverage for patch 3

v2: https://lore.kernel.org/r/20260811141240.62519-1-kmehltretter@gmail.com
v1: https://lore.kernel.org/r/20260808105622.62026-1-kmehltretter@gmail.com
RFC: https://lore.kernel.org/r/20260712120728.96628-1-kmehltretter@gmail.com

Karl Mehltretter (5):
  slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN
  slab: check for ZERO_SIZE_PTR by exact match
  slab: handle ERR_PTR values in kfree and hardened usercopy
  slab: test zero-size allocations in slub_kunit
  slab: test ERR_PTR handling in kfree and hardened usercopy

 drivers/misc/lkdtm/usercopy.c           | 27 +++++++++++
 include/linux/slab.h                    | 20 ++++++--
 lib/tests/slub_kunit.c                  | 61 +++++++++++++++++++++++++
 mm/slub.c                               |  3 ++
 mm/usercopy.c                           |  3 ++
 tools/testing/selftests/lkdtm/tests.txt |  1 +
 6 files changed, 112 insertions(+), 3 deletions(-)


base-commit: 940de590b839f71d6dc846160534bf202401b8b7
-- 
2.53.0

[PATCH v3 1/5] slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN

[Karl Mehltretter]

The kmalloc entry points are annotated with __assume_kmalloc_alignment
but return ZERO_SIZE_PTR, currently (void *)16, for zero-size requests.
This violates the annotation when ARCH_KMALLOC_MINALIGN exceeds 16.

This can mislead compiler optimizations. Clang's UBSAN_ALIGNMENT detects
the violation on armv5. GCC and Clang retain the ZERO_OR_NULL_PTR() range
check but eliminate an exact ZERO_SIZE_PTR comparison after an annotated
allocation.

Define ZERO_SIZE_PTR as the greater of 16 and ARCH_KMALLOC_MINALIGN,
retaining the existing value where it is already aligned. Assert that
ARCH_KMALLOC_MINALIGN remains below 0x100, the value of LIST_POISON1
when POISON_POINTER_DELTA is zero, so the sentinel remains distinct
from that poison pointer.

Fixes: 94a58c360a45 ("slab.h: sprinkle __assume_aligned attributes")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 include/linux/slab.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index cda126def67a..563dadc16d82 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -262,13 +262,16 @@ enum _slab_flag_bits {
 
 /*
  * ZERO_SIZE_PTR will be returned for zero sized kmalloc requests.
+ * It satisfies the alignment promised by __assume_kmalloc_alignment
+ * and keeps the historic value 16 where that is already aligned.
  *
  * Dereferencing ZERO_SIZE_PTR will lead to a distinct access fault.
  *
  * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can.
  * Both make kfree a no-op.
  */
-#define ZERO_SIZE_PTR ((void *)16)
+#define ZERO_SIZE_PTR ((void *)(ARCH_KMALLOC_MINALIGN > 16 ? \
+				ARCH_KMALLOC_MINALIGN : 16))
 
 #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
 				(unsigned long)ZERO_SIZE_PTR)
@@ -625,6 +628,13 @@ static inline bool kmem_dump_obj(void *object) { return false; }
 #define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE)
 #endif
 
+/*
+ * Keep ZERO_SIZE_PTR at most 128, i.e. below 0x100: LIST_POISON1 is
+ * 0x100 when POISON_POINTER_DELTA is 0, and no architecture currently
+ * has an ARCH_KMALLOC_MINALIGN above 128.
+ */
+static_assert(ARCH_KMALLOC_MINALIGN < 0x100);
+
 /*
  * Setting ARCH_SLAB_MINALIGN in arch headers allows a different alignment.
  * Intended for arches that get misalignment faults even for 64 bit integer
-- 
2.53.0

[PATCH v3 2/5] slab: check for ZERO_SIZE_PTR by exact match

[Karl Mehltretter]

ZERO_OR_NULL_PTR() returns true for every value less than or equal to
ZERO_SIZE_PTR. With the sentinel raised to ARCH_KMALLOC_MINALIGN, up
to 128 on some architectures, the helper matches additional values
that are neither NULL nor the sentinel.

Compare explicitly against NULL and ZERO_SIZE_PTR. Store the argument
in an unsigned long temporary to support both pointer and integer
address arguments while evaluating it only once.

This also changes check_bogus_address() in hardened usercopy: nonzero
addresses below ZERO_SIZE_PTR no longer cause its null-address abort.
The rejected window was incidental to the sentinel value: address 17
already passed the old check, including in s390's mapped lowcore, and a
principled first-page bound would be a separate usercopy change.

Apart from hardened usercopy, direct in-tree users apply the helper
to allocation results or explicit sentinel values.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 include/linux/slab.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 563dadc16d82..458c4229b1d7 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -273,8 +273,12 @@ enum _slab_flag_bits {
 #define ZERO_SIZE_PTR ((void *)(ARCH_KMALLOC_MINALIGN > 16 ? \
 				ARCH_KMALLOC_MINALIGN : 16))
 
-#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
-				(unsigned long)ZERO_SIZE_PTR)
+#define ZERO_OR_NULL_PTR(x)						\
+({									\
+	unsigned long __zon_ptr = (unsigned long)(x);			\
+	__zon_ptr == 0 ||						\
+	__zon_ptr == (unsigned long)ZERO_SIZE_PTR;			\
+})
 
 #include <linux/kasan.h>
 
-- 
2.53.0

[PATCH v3 3/5] slab: handle ERR_PTR values in kfree and hardened usercopy

[Karl Mehltretter]

Passing an ERR_PTR to kfree() currently reaches virt_to_page() and may
fault. Warn and return instead, leaving the bad caller visible without
using the pointer as allocator metadata.

Also reject ERR_PTR values in hardened usercopy. Keep both checks
separate from ZERO_OR_NULL_PTR(), whose exact matching is required by
krealloc().

Link: https://lore.kernel.org/r/CAG48ez05QVn6_gQ2TBrRa1a_DWQoaSSYubUsu5YMWxx-gqMijQ@mail.gmail.com
Link: https://lore.kernel.org/r/202608111716.0FA9DB17@keescook
Link: https://github.com/KSPP/linux/issues/93
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 mm/slub.c     | 3 +++
 mm/usercopy.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index f9b56cb439e7..027b44dd7f07 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -6780,6 +6780,9 @@ void kfree(const void *object)
 	if (unlikely(ZERO_OR_NULL_PTR(object)))
 		return;
 
+	if (WARN_ON(IS_ERR(object)))
+		return;
+
 	page = virt_to_page(object);
 	slab = page_slab(page);
 	if (!slab) {
diff --git a/mm/usercopy.c b/mm/usercopy.c
index 5de7a518b1b1..c8d8703544c6 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -157,6 +157,9 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 	/* Reject if NULL or ZERO-allocation. */
 	if (ZERO_OR_NULL_PTR(ptr))
 		usercopy_abort("null address", NULL, to_user, ptr, n);
+
+	if (IS_ERR_VALUE(ptr))
+		usercopy_abort("ERR_PTR", NULL, to_user, ptr, n);
 }
 
 static inline void check_heap_object(const void *ptr, unsigned long n,
-- 
2.53.0

[PATCH v3 4/5] slab: test zero-size allocations in slub_kunit

[Karl Mehltretter]

Add KUnit coverage for the zero-size allocation contract, including
ZERO_SIZE_PTR alignment and exact ZERO_OR_NULL_PTR() matching.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 lib/tests/slub_kunit.c | 43 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/lib/tests/slub_kunit.c b/lib/tests/slub_kunit.c
index e3b63f0338d5..923d8646bca1 100644
--- a/lib/tests/slub_kunit.c
+++ b/lib/tests/slub_kunit.c
@@ -465,6 +465,48 @@ static void test_kmalloc_nolock_and_friends_kprobe(struct kunit *test)
 }
 #endif
 
+static void test_zero_size_alloc(struct kunit *test)
+{
+	unsigned long zsp = (unsigned long)ZERO_SIZE_PTR;
+	void *p, *r;
+
+	KUNIT_EXPECT_EQ(test, zsp % ARCH_KMALLOC_MINALIGN, 0UL);
+
+	p = kmalloc(0, GFP_KERNEL);
+	KUNIT_EXPECT_PTR_EQ(test, p, ZERO_SIZE_PTR);
+	KUNIT_EXPECT_EQ(test, ksize(p), 0);
+	kfree(p);
+
+	KUNIT_EXPECT_PTR_EQ(test, kzalloc(0, GFP_KERNEL), ZERO_SIZE_PTR);
+	KUNIT_EXPECT_PTR_EQ(test, kmalloc_array(0, 8, GFP_KERNEL), ZERO_SIZE_PTR);
+	KUNIT_EXPECT_PTR_EQ(test, kcalloc(4, 0, GFP_KERNEL), ZERO_SIZE_PTR);
+
+	p = kvmalloc(0, GFP_KERNEL);
+	KUNIT_EXPECT_PTR_EQ(test, p, ZERO_SIZE_PTR);
+	kvfree(p);
+
+	p = krealloc(NULL, 0, GFP_KERNEL);
+	KUNIT_EXPECT_PTR_EQ(test, p, ZERO_SIZE_PTR);
+	r = krealloc(p, 64, GFP_KERNEL);
+	KUNIT_EXPECT_FALSE(test, ZERO_OR_NULL_PTR(r));
+	p = krealloc(r, 0, GFP_KERNEL);
+	KUNIT_EXPECT_PTR_EQ(test, p, ZERO_SIZE_PTR);
+	kfree(p);
+
+	/* Only NULL and the zero-size sentinel match. */
+	KUNIT_EXPECT_TRUE(test, ZERO_OR_NULL_PTR(NULL));
+	KUNIT_EXPECT_TRUE(test, ZERO_OR_NULL_PTR(ZERO_SIZE_PTR));
+	KUNIT_EXPECT_FALSE(test, ZERO_OR_NULL_PTR((void *)1));
+	KUNIT_EXPECT_FALSE(test, ZERO_OR_NULL_PTR((void *)(zsp - 1)));
+	KUNIT_EXPECT_FALSE(test, ZERO_OR_NULL_PTR((void *)(zsp + 1)));
+	KUNIT_EXPECT_FALSE(test, ZERO_OR_NULL_PTR((void *)(zsp * 2)));
+
+	/* freeing the sentinel must stay a no-op */
+	kfree(ZERO_SIZE_PTR);
+	kfree_sensitive(ZERO_SIZE_PTR);
+	kvfree(ZERO_SIZE_PTR);
+}
+
 static int test_init(struct kunit *test)
 {
 	slab_errors = 0;
@@ -489,6 +531,7 @@ static struct kunit_case test_cases[] = {
 	KUNIT_CASE(test_kfree_rcu_wq_destroy),
 	KUNIT_CASE(test_leak_destroy),
 	KUNIT_CASE(test_krealloc_redzone_zeroing),
+	KUNIT_CASE(test_zero_size_alloc),
 #ifdef CONFIG_PERF_EVENTS
 	KUNIT_CASE_SLOW(test_kmalloc_nolock_and_friends_perf),
 #endif
-- 
2.53.0

[PATCH v3 5/5] slab: test ERR_PTR handling in kfree and hardened usercopy

[Karl Mehltretter]

Add KUnit coverage that kfree() warns and returns for ERR_PTR values
while remaining silent for NULL and ZERO_SIZE_PTR.

Add an LKDTM test that verifies hardened usercopy rejects an ERR_PTR
before attempting the copy.

Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 drivers/misc/lkdtm/usercopy.c           | 27 +++++++++++++++++++++++++
 lib/tests/slub_kunit.c                  | 18 +++++++++++++++++
 tools/testing/selftests/lkdtm/tests.txt |  1 +
 3 files changed, 46 insertions(+)

diff --git a/drivers/misc/lkdtm/usercopy.c b/drivers/misc/lkdtm/usercopy.c
index 67db57249a34..10751e48211f 100644
--- a/drivers/misc/lkdtm/usercopy.c
+++ b/drivers/misc/lkdtm/usercopy.c
@@ -4,6 +4,7 @@
  * hardening.
  */
 #include "lkdtm.h"
+#include <linux/err.h>
 #include <linux/slab.h>
 #include <linux/highmem.h>
 #include <linux/vmalloc.h>
@@ -273,6 +274,31 @@ static void do_usercopy_slab_whitelist(bool to_user)
 }
 
 /* Callable tests. */
+static void lkdtm_USERCOPY_ERR_PTR(void)
+{
+	unsigned long user_addr;
+	size_t size = unconst + 1;
+
+	user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
+			    PROT_READ | PROT_WRITE,
+			    MAP_ANONYMOUS | MAP_PRIVATE, 0);
+	if (user_addr >= TASK_SIZE) {
+		pr_warn("Failed to allocate user memory\n");
+		return;
+	}
+
+	pr_info("attempting bad one-byte copy_to_user() from ERR_PTR\n");
+	if (copy_to_user((void __user *)user_addr, ERR_PTR(-EINVAL), size)) {
+		pr_warn("copy_to_user failed, but lacked Oops\n");
+		goto free_user;
+	}
+	pr_err("FAIL: ERR_PTR usercopy not detected!\n");
+	pr_expected_config_param(CONFIG_HARDENED_USERCOPY, "hardened_usercopy");
+
+free_user:
+	vm_munmap(user_addr, PAGE_SIZE);
+}
+
 static void lkdtm_USERCOPY_SLAB_SIZE_TO(void)
 {
 	do_usercopy_slab_size(true);
@@ -439,6 +465,7 @@ void __exit lkdtm_usercopy_exit(void)
 }
 
 static struct crashtype crashtypes[] = {
+	CRASHTYPE(USERCOPY_ERR_PTR),
 	CRASHTYPE(USERCOPY_SLAB_SIZE_TO),
 	CRASHTYPE(USERCOPY_SLAB_SIZE_FROM),
 	CRASHTYPE(USERCOPY_SLAB_WHITELIST_TO),
diff --git a/lib/tests/slub_kunit.c b/lib/tests/slub_kunit.c
index 923d8646bca1..079563387007 100644
--- a/lib/tests/slub_kunit.c
+++ b/lib/tests/slub_kunit.c
@@ -507,6 +507,23 @@ static void test_zero_size_alloc(struct kunit *test)
 	kvfree(ZERO_SIZE_PTR);
 }
 
+static void test_kfree_err_ptr(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_BUG))
+		kunit_skip(test, "requires CONFIG_BUG");
+
+	kunit_warning_suppress(test) {
+		kfree(NULL);
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 0);
+
+		kfree(ZERO_SIZE_PTR);
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 0);
+
+		kfree(ERR_PTR(-EINVAL));
+		KUNIT_EXPECT_SUPPRESSED_WARNING_COUNT(test, 1);
+	}
+}
+
 static int test_init(struct kunit *test)
 {
 	slab_errors = 0;
@@ -532,6 +549,7 @@ static struct kunit_case test_cases[] = {
 	KUNIT_CASE(test_leak_destroy),
 	KUNIT_CASE(test_krealloc_redzone_zeroing),
 	KUNIT_CASE(test_zero_size_alloc),
+	KUNIT_CASE(test_kfree_err_ptr),
 #ifdef CONFIG_PERF_EVENTS
 	KUNIT_CASE_SLOW(test_kmalloc_nolock_and_friends_perf),
 #endif
diff --git a/tools/testing/selftests/lkdtm/tests.txt b/tools/testing/selftests/lkdtm/tests.txt
index bec57a02913a..d3e38c1c75cd 100644
--- a/tools/testing/selftests/lkdtm/tests.txt
+++ b/tools/testing/selftests/lkdtm/tests.txt
@@ -70,6 +70,7 @@ REFCOUNT_DEC_AND_TEST_SATURATED Saturation detected: still saturated
 REFCOUNT_SUB_AND_TEST_SATURATED Saturation detected: still saturated
 #REFCOUNT_TIMING timing only
 #ATOMIC_TIMING timing only
+USERCOPY_ERR_PTR Kernel memory exposure attempt detected from ERR_PTR
 USERCOPY_SLAB_SIZE_TO
 USERCOPY_SLAB_SIZE_FROM
 USERCOPY_SLAB_WHITELIST_TO
-- 
2.53.0

Re: [PATCH v3 3/5] slab: handle ERR_PTR values in kfree and hardened usercopy

[Vlastimil Babka (SUSE)]

On 9/3/26 22:37, Karl Mehltretter wrote:
> Passing an ERR_PTR to kfree() currently reaches virt_to_page() and may
> fault. Warn and return instead, leaving the bad caller visible without
> using the pointer as allocator metadata.
> 
> Also reject ERR_PTR values in hardened usercopy. Keep both checks
> separate from ZERO_OR_NULL_PTR(), whose exact matching is required by
> krealloc().
> 
> Link: https://lore.kernel.org/r/CAG48ez05QVn6_gQ2TBrRa1a_DWQoaSSYubUsu5YMWxx-gqMijQ@mail.gmail.com
> Link: https://lore.kernel.org/r/202608111716.0FA9DB17@keescook
> Link: https://github.com/KSPP/linux/issues/93
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
>  mm/slub.c     | 3 +++
>  mm/usercopy.c | 3 +++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f9b56cb439e7..027b44dd7f07 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6780,6 +6780,9 @@ void kfree(const void *object)
>  	if (unlikely(ZERO_OR_NULL_PTR(object)))
>  		return;
>  
> +	if (WARN_ON(IS_ERR(object)))

Wonder if WARN_ON_ONCE() would be better.
Also wonder about the benefits for hardening (as opposed to debugging).
Without this check it would fault. Now it warns, but hardened setups often
use panic_on_warn anyway, so the result is the same?

> +		return;
> +
>  	page = virt_to_page(object);
>  	slab = page_slab(page);
>  	if (!slab) {
> diff --git a/mm/usercopy.c b/mm/usercopy.c
> index 5de7a518b1b1..c8d8703544c6 100644
> --- a/mm/usercopy.c
> +++ b/mm/usercopy.c
> @@ -157,6 +157,9 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
>  	/* Reject if NULL or ZERO-allocation. */
>  	if (ZERO_OR_NULL_PTR(ptr))
>  		usercopy_abort("null address", NULL, to_user, ptr, n);
> +
> +	if (IS_ERR_VALUE(ptr))
> +		usercopy_abort("ERR_PTR", NULL, to_user, ptr, n);
>  }
>  
>  static inline void check_heap_object(const void *ptr, unsigned long n,

Re: [PATCH v3 0/5] slab: ZERO_SIZE_PTR alignment and ERR_PTR hardening

[Harry Yoo]

On Thu, Sep 03, 2026 at 10:37:15PM +0200, Karl Mehltretter wrote:
> The kmalloc entry points promise ARCH_KMALLOC_MINALIGN alignment but
> return (void *)16 for zero-size requests. Patch 1 aligns the sentinel
> to that promise.
> 
> Patch 2 limits ZERO_OR_NULL_PTR() to NULL and ZERO_SIZE_PTR. Patch 3
> handles ERR_PTR explicitly in kfree() and hardened usercopy, without
> giving ERR_PTR special meaning to other users such as krealloc(). The
> old helper did not match ERR_PTR values either; patch 3 addresses the
> adjacent hardening concern raised during review.

I don't understand why v3 of this patchset suddenly implements
hardening for kfree() and usercopy to handle ERR_PTR().

The address range for 'zero or NULL PTR' range and ERR_PTR() do not
overlap. The bugfixes and hardening patches are irrelevant.
It should be a separate series?

Also usercopy.c changes go through the hardening tree, slub.c and
slub_kunit.c changes go through the slab tree, and LKDTM has its own
tree. Patch 3 and 5 touch files across multiple trees, which can be
avoided.

> If backported, patches 1 and 2 should be taken together.
>
> Patch 4 adds zero-size KUnit coverage; patch 5 adds ERR_PTR KUnit,
> LKDTM, and kselftest coverage, which is why Arnd, Greg, Shuah, and
> linux-kselftest@vger.kernel.org are newly copied on v3.
> 
> Tested on 940de590b839:
> 
>   - GCC 15.2 UML slub_test: 10 passed, 2 skipped, including
>     test_zero_size_alloc and test_kfree_err_ptr; NULL and ZERO_SIZE_PTR
>     produced no warning, and the expected kfree(ERR_PTR) warning was
>     suppressed and counted
>   - GCC 15.2 matched x86_64 builds immediately before and after patch 3:
>     kfree grew by 15 bytes; __check_object_size gained a 13-byte
>     comparison on its normal path and a 23-byte cold abort path
>   - GCC 15.2 x86_64 defconfig with HARDENED_USERCOPY: build and normal
>     QEMU boot passed; the committed LKDTM selftest matched the distinct
>     ERR_PTR diagnostic after reaching usercopy_abort()
>   - Clang 21.1.8 armv5 UBSAN_ALIGNMENT: no alignment-assumption or
>     slab.h report; generated code retains the exact comparison and
>     single evaluation
>   - GCC 15.2 MIPS64 big-endian noncoherent build passed with
>     ARCH_KMALLOC_MINALIGN equal to 128
>   - Microchip SAM9X75 hardware (armv5, ARCH_KMALLOC_MINALIGN=32):
>     ZERO_SIZE_PTR was 0x20; zero-size allocation, ksize(), and free
>     passed with no boot splats
>   - Raspberry Pi 400 hardware (arm64, Cortex-A72,
>     ARCH_KMALLOC_MINALIGN=8): ZERO_SIZE_PTR remained 0x10; the ERR_PTR
>     KUnit and hardened-usercopy rejection paths passed with no boot
>     splats; the revised diagnostic was retested under x86_64 QEMU
> 
> ---
> v2 -> v3:
> 
>   - document why patch 2's sentinel-derived low-address window in hardened
>     usercopy was incidental
>   - add new patch 3 to warn and return when kfree() receives an ERR_PTR
>     and reject ERR_PTR values in hardened usercopy
>   - replace patch 4's pointer-cast static assertion with a run-time KUnit
>     expectation
>   - add new patch 5 with KUnit and LKDTM coverage for patch 3
> 
> v2: https://lore.kernel.org/r/20260811141240.62519-1-kmehltretter@gmail.com
> v1: https://lore.kernel.org/r/20260808105622.62026-1-kmehltretter@gmail.com
> RFC: https://lore.kernel.org/r/20260712120728.96628-1-kmehltretter@gmail.com
> 
> Karl Mehltretter (5):
>   slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN
>   slab: check for ZERO_SIZE_PTR by exact match
>   slab: handle ERR_PTR values in kfree and hardened usercopy
>   slab: test zero-size allocations in slub_kunit
>   slab: test ERR_PTR handling in kfree and hardened usercopy
> 
>  drivers/misc/lkdtm/usercopy.c           | 27 +++++++++++
>  include/linux/slab.h                    | 20 ++++++--
>  lib/tests/slub_kunit.c                  | 61 +++++++++++++++++++++++++
>  mm/slub.c                               |  3 ++
>  mm/usercopy.c                           |  3 ++
>  tools/testing/selftests/lkdtm/tests.txt |  1 +
>  6 files changed, 112 insertions(+), 3 deletions(-)
> 
> 
> base-commit: 940de590b839f71d6dc846160534bf202401b8b7
> -- 
> 2.53.0