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[2a02:3100:a133:e401:bc8e:d36c:2d08:c5e0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4814a5be29fsm4464099f8f.12.2026.08.11.07.12.45 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 11 Aug 2026 07:12:45 -0700 (PDT) From: Karl Mehltretter To: Vlastimil Babka , Harry Yoo , Andrew Morton Cc: Karl Mehltretter , Rasmus Villemoes , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Catalin Marinas , Kees Cook , "Gustavo A. R. Silva" , linux-hardening@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: [PATCH v2 1/3] slab: align ZERO_SIZE_PTR to ARCH_KMALLOC_MINALIGN Date: Tue, 11 Aug 2026 16:12:38 +0200 Message-Id: <20260811141240.62519-2-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20260811141240.62519-1-kmehltretter@gmail.com> References: <20260811141240.62519-1-kmehltretter@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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. 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: Claude:claude-fable-5 Signed-off-by: Karl Mehltretter --- 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 32c9f8ed7ae20..3e012cc4f001e 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -259,13 +259,16 @@ enum _slab_flag_bits { =20 /* * 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 c= an. * 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)) =20 #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <=3D \ (unsigned long)ZERO_SIZE_PTR) @@ -622,6 +625,13 @@ static inline bool kmem_dump_obj(void *object) { retur= n false; } #define KMALLOC_SHIFT_LOW ilog2(KMALLOC_MIN_SIZE) #endif =20 +/* + * 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 --=20 2.53.0 From nobody Tue Sep 29 06:08:42 2026 Received: from mail-wm1-f41.google.com (mail-wm1-f41.google.com [209.85.128.41]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 3CC6A446BE8 for ; Tue, 11 Aug 2026 14:12:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.41 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786457572; cv=none; b=UhbR22+lLy9iv67wrb1D4bhQpvmptKJzjGRm99uTvSWzMnbPxCt7OFtPasPBJpoTtbNqI6F4iQCdocDiRgDdWA3nzxxAGREbw9hUFB0yfdGEt9pG0jOjQkkRquLgIkk36YJYs/pmlS2Ub92DHLZ1OTUvYJwpH3W4lxORg36Z6yE= ARC-Message-Signature: i=1; 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[2a02:3100:a133:e401:bc8e:d36c:2d08:c5e0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4814a5be29fsm4464099f8f.12.2026.08.11.07.12.47 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 11 Aug 2026 07:12:48 -0700 (PDT) From: Karl Mehltretter To: Vlastimil Babka , Harry Yoo , Andrew Morton Cc: Karl Mehltretter , Rasmus Villemoes , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Catalin Marinas , Kees Cook , "Gustavo A. R. Silva" , linux-hardening@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: [PATCH v2 2/3] slab: check for ZERO_SIZE_PTR by exact match Date: Tue, 11 Aug 2026 16:12:39 +0200 Message-Id: <20260811141240.62519-3-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20260811141240.62519-1-kmehltretter@gmail.com> References: <20260811141240.62519-1-kmehltretter@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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. Apart from hardened usercopy, direct in-tree users apply the helper to allocation results or explicit sentinel values. Assisted-by: Claude:claude-fable-5 Signed-off-by: Karl Mehltretter --- 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 3e012cc4f001e..023e10ab77f8d 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -270,8 +270,12 @@ enum _slab_flag_bits { #define ZERO_SIZE_PTR ((void *)(ARCH_KMALLOC_MINALIGN > 16 ? \ ARCH_KMALLOC_MINALIGN : 16)) =20 -#define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <=3D \ - (unsigned long)ZERO_SIZE_PTR) +#define ZERO_OR_NULL_PTR(x) \ +({ \ + unsigned long __zon_ptr =3D (unsigned long)(x); \ + __zon_ptr =3D=3D 0 || \ + __zon_ptr =3D=3D (unsigned long)ZERO_SIZE_PTR; \ +}) =20 #include =20 --=20 2.53.0 From nobody Tue Sep 29 06:08:42 2026 Received: from mail-wm1-f47.google.com (mail-wm1-f47.google.com [209.85.128.47]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 751F6448393 for ; 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[2a02:3100:a133:e401:bc8e:d36c:2d08:c5e0]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-4814a5be29fsm4464099f8f.12.2026.08.11.07.12.50 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 11 Aug 2026 07:12:51 -0700 (PDT) From: Karl Mehltretter To: Vlastimil Babka , Harry Yoo , Andrew Morton Cc: Karl Mehltretter , Rasmus Villemoes , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Catalin Marinas , Kees Cook , "Gustavo A. R. Silva" , linux-hardening@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: [PATCH v2 3/3] slab: test zero-size allocations in slub_kunit Date: Tue, 11 Aug 2026 16:12:40 +0200 Message-Id: <20260811141240.62519-4-kmehltretter@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20260811141240.62519-1-kmehltretter@gmail.com> References: <20260811141240.62519-1-kmehltretter@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" Add KUnit coverage for the zero-size allocation contract, including ZERO_SIZE_PTR alignment and exact ZERO_OR_NULL_PTR() matching. Assisted-by: Claude:claude-fable-5 Signed-off-by: Karl Mehltretter --- 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 fa6d31dbca166..a02fc6b401e3e 100644 --- a/lib/tests/slub_kunit.c +++ b/lib/tests/slub_kunit.c @@ -380,6 +380,48 @@ static void test_kmalloc_kfree_nolock(struct kunit *te= st) } #endif =20 +static void test_zero_size_alloc(struct kunit *test) +{ + unsigned long zsp =3D (unsigned long)ZERO_SIZE_PTR; + void *p, *r; + + static_assert((unsigned long)ZERO_SIZE_PTR % ARCH_KMALLOC_MINALIGN =3D=3D= 0); + + p =3D 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 =3D kvmalloc(0, GFP_KERNEL); + KUNIT_EXPECT_PTR_EQ(test, p, ZERO_SIZE_PTR); + kvfree(p); + + p =3D krealloc(NULL, 0, GFP_KERNEL); + KUNIT_EXPECT_PTR_EQ(test, p, ZERO_SIZE_PTR); + r =3D krealloc(p, 64, GFP_KERNEL); + KUNIT_EXPECT_FALSE(test, ZERO_OR_NULL_PTR(r)); + p =3D 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 =3D 0; @@ -404,6 +446,7 @@ static struct kunit_case test_cases[] =3D { 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_kfree_nolock), #endif --=20 2.53.0