From: Eric Biggers ebiggers@kernel.org
[ Upstream commit 85e1ff61060a765d91ee62dc5606d4d547d9d105 ]
Running sha224_kunit on a KMSAN-enabled kernel results in a crash in kmsan_internal_set_shadow_origin():
BUG: unable to handle page fault for address: ffffbc3840291000 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 1810067 P4D 1810067 PUD 192d067 PMD 3c17067 PTE 0 Oops: 0000 [#1] SMP NOPTI CPU: 0 UID: 0 PID: 81 Comm: kunit_try_catch Tainted: G N 6.17.0-rc3 #10 PREEMPT(voluntary) Tainted: [N]=TEST Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014 RIP: 0010:kmsan_internal_set_shadow_origin+0x91/0x100 [...] Call Trace: <TASK> __msan_memset+0xee/0x1a0 sha224_final+0x9e/0x350 test_hash_buffer_overruns+0x46f/0x5f0 ? kmsan_get_shadow_origin_ptr+0x46/0xa0 ? __pfx_test_hash_buffer_overruns+0x10/0x10 kunit_try_run_case+0x198/0xa00
This occurs when memset() is called on a buffer that is not 4-byte aligned and extends to the end of a guard page, i.e. the next page is unmapped.
The bug is that the loop at the end of kmsan_internal_set_shadow_origin() accesses the wrong shadow memory bytes when the address is not 4-byte aligned. Since each 4 bytes are associated with an origin, it rounds the address and size so that it can access all the origins that contain the buffer. However, when it checks the corresponding shadow bytes for a particular origin, it incorrectly uses the original unrounded shadow address. This results in reads from shadow memory beyond the end of the buffer's shadow memory, which crashes when that memory is not mapped.
To fix this, correctly align the shadow address before accessing the 4 shadow bytes corresponding to each origin.
Link: https://lkml.kernel.org/r/20250911195858.394235-1-ebiggers@kernel.org Fixes: 2ef3cec44c60 ("kmsan: do not wipe out origin when doing partial unpoisoning") Signed-off-by: Eric Biggers ebiggers@kernel.org Tested-by: Alexander Potapenko glider@google.com Reviewed-by: Alexander Potapenko glider@google.com Cc: Dmitriy Vyukov dvyukov@google.com Cc: Marco Elver elver@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org [ Adjust context in tests ] Signed-off-by: Sasha Levin sashal@kernel.org --- mm/kmsan/core.c | 10 +++++++--- mm/kmsan/kmsan_test.c | 16 ++++++++++++++++ 2 files changed, 23 insertions(+), 3 deletions(-)
diff --git a/mm/kmsan/core.c b/mm/kmsan/core.c index dff759f32bbb3..ea5a06fe94f99 100644 --- a/mm/kmsan/core.c +++ b/mm/kmsan/core.c @@ -258,7 +258,8 @@ void kmsan_internal_set_shadow_origin(void *addr, size_t size, int b, u32 origin, bool checked) { u64 address = (u64)addr; - u32 *shadow_start, *origin_start; + void *shadow_start; + u32 *aligned_shadow, *origin_start; size_t pad = 0;
KMSAN_WARN_ON(!kmsan_metadata_is_contiguous(addr, size)); @@ -277,9 +278,12 @@ void kmsan_internal_set_shadow_origin(void *addr, size_t size, int b, } __memset(shadow_start, b, size);
- if (!IS_ALIGNED(address, KMSAN_ORIGIN_SIZE)) { + if (IS_ALIGNED(address, KMSAN_ORIGIN_SIZE)) { + aligned_shadow = shadow_start; + } else { pad = address % KMSAN_ORIGIN_SIZE; address -= pad; + aligned_shadow = shadow_start - pad; size += pad; } size = ALIGN(size, KMSAN_ORIGIN_SIZE); @@ -293,7 +297,7 @@ void kmsan_internal_set_shadow_origin(void *addr, size_t size, int b, * corresponding shadow slot is zero. */ for (int i = 0; i < size / KMSAN_ORIGIN_SIZE; i++) { - if (origin || !shadow_start[i]) + if (origin || !aligned_shadow[i]) origin_start[i] = origin; } } diff --git a/mm/kmsan/kmsan_test.c b/mm/kmsan/kmsan_test.c index 1328636cbd6cd..bc2a460000ff6 100644 --- a/mm/kmsan/kmsan_test.c +++ b/mm/kmsan/kmsan_test.c @@ -470,6 +470,21 @@ static void test_memcpy_aligned_to_unaligned2(struct kunit *test) KUNIT_EXPECT_TRUE(test, report_matches(&expect)); }
+/* Test case: ensure that KMSAN does not access shadow memory out of bounds. */ +static void test_memset_on_guarded_buffer(struct kunit *test) +{ + void *buf = vmalloc(PAGE_SIZE); + + kunit_info(test, + "memset() on ends of guarded buffer should not crash\n"); + + for (size_t size = 0; size <= 128; size++) { + memset(buf, 0xff, size); + memset(buf + PAGE_SIZE - size, 0xff, size); + } + vfree(buf); +} + static noinline void fibonacci(int *array, int size, int start) { if (start < 2 || (start == size)) return; @@ -515,6 +530,7 @@ static struct kunit_case kmsan_test_cases[] = { KUNIT_CASE(test_memcpy_aligned_to_aligned), KUNIT_CASE(test_memcpy_aligned_to_unaligned), KUNIT_CASE(test_memcpy_aligned_to_unaligned2), + KUNIT_CASE(test_memset_on_guarded_buffer), KUNIT_CASE(test_long_origin_chain), {}, };