CVE-2025-40008 kmsan: fix out-of-bounds access to shadow memory
A bug in KMSAN shadow memory handling causes a kernel crash when processing unaligned buffers near guard pages, requiring a patched kernel.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Not stated
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-40008?
CVE-2025-40008 is a memory corruption vulnerability in Linux, published on 20 Oct 2025.
Official description
In the Linux kernel, the following vulnerability has been resolved:
kmsan: fix out-of-bounds access to shadow memory
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.
Which versions are affected by CVE-2025-40008?
How do I fix CVE-2025-40008?
Apply the vendor's patch.
Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.
Is CVE-2025-40008 being exploited?
There is no known exploitation of CVE-2025-40008 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-40008, DEBIAN-CVE-2025-40008, GHSA-jv2g-f3mc-8f2c, ROOT-OS-DEBIAN-12-CVE-2025-40008, ROOT-OS-DEBIAN-13-CVE-2025-40008, ROOT-OS-UBUNTU-2404-CVE-2025-40008 and 1 more.