mm/ksm: fix flag-dropping behavior in ksm_madvise

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2025-40040
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2025-10-28 12:15:37 UTC
Updated2026-07-30 06:24:07 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: mm/ksm: fix flag-dropping behavior in ksm_madvise syzkaller discovered the following crash: (kernel BUG) [ 44.607039] ------------[ cut here ]------------ [ 44.607422] kernel BUG at mm/userfaultfd.c:2067! [ 44.608148] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI [ 44.608814] CPU: 1 UID: 0 PID: 2475 Comm: reproducer Not tainted 6.16.0-rc6 #1 PREEMPT(none) [ 44.609635] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014 [ 44.610695] RIP: 0010:userfaultfd_release_all+0x3a8/0x460 <snip other registers, drop unreliable trace> [ 44.617726] Call Trace: [ 44.617926] <TASK> [ 44.619284] userfaultfd_release+0xef/0x1b0 [ 44.620976] __fput+0x3f9/0xb60 [ 44.621240] fput_close_sync+0x110/0x210 [ 44.622222] __x64_sys_close+0x8f/0x120 [ 44.622530] do_syscall_64+0x5b/0x2f0 [ 44.622840] entry_SYSCALL_64_after_hwframe+0x76/0x7e [ 44.623244] RIP: 0033:0x7f365bb3f227 Kernel panics because it detects UFFD inconsistency during userfaultfd_release_all(). Specifically, a VMA which has a valid pointer to vma->vm_userfaultfd_ctx, but no UFFD flags in vma->vm_flags. The inconsistency is caused in ksm_madvise(): when user calls madvise() with MADV_UNMEARGEABLE on a VMA that is registered for UFFD in MINOR mode, it accidentally clears all flags stored in the upper 32 bits of vma->vm_flags. Assuming x86_64 kernel build, unsigned long is 64-bit and unsigned int and int are 32-bit wide. This setup causes the following mishap during the &= ~VM_MERGEABLE assignment. VM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000'0000. After ~ is applied, it becomes 0x7fff'ffff unsigned int, which is then promoted to unsigned long before the & operation. This promotion fills upper 32 bits with leading 0s, as we're doing unsigned conversion (and even for a signed conversion, this wouldn't help as the leading bit is 0). & operation thus ends up AND-ing vm_flags with 0x0000'0000'7fff'ffff instead of intended 0xffff'ffff'7fff'ffff and hence accidentally clears the upper 32-bits of its value. Fix it by changing `VM_MERGEABLE` constant to unsigned long, using the BIT() macro. Note: other VM_* flags are not affected: This only happens to the VM_MERGEABLE flag, as the other VM_* flags are all constants of type int and after ~ operation, they end up with leading 1 and are thus converted to unsigned long with leading 1s. Note 2: After commit 31defc3b01d9 ("userfaultfd: remove (VM_)BUG_ON()s"), this is no longer a kernel BUG, but a WARNING at the same place: [ 45.595973] WARNING: CPU: 1 PID: 2474 at mm/userfaultfd.c:2067 but the root-cause (flag-drop) remains the same. [[email protected]: rust bindgen wasn't able to handle BIT(), from Miguel]

Risk And Classification

Primary CVSS: v3.1 5.5 MEDIUM from [email protected]

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Problem Types: NVD-CWE-noinfo

VersionSourceTypeScoreSeverityVector
3.1[email protected]Primary5.5MEDIUMCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
3.1416baaa9-dc9f-4396-8d5f-8c081fb06d67Secondary7.8HIGHCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
3.1CNADECLARED7.8HIGHCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Operating System Linux Linux Kernel All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 850f1ea245bdc0ce6a3fd36bfb80d8cf9647cb71 git Not specified
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 788e5385d0ff69cdba1cabccb9dab8d9647b9239 git Not specified
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 b69f19244c2b6475c8a6eb72f0fb0d53509e48cd git Not specified
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693 git Not specified
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 92b82e232b8d8b116ac6e57aeae7a6033db92c60 git Not specified
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 ac50c6e0a8f91a02b681af81abb2362fbb67cc18 git Not specified
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 76385629f45740b7888f8fcd83bde955b10f61fe git Not specified
CNA Linux Linux affected 63c17fb8e5a46a16e10e82005748837fd11a2024 f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93 git Not specified
CNA Linux Linux affected 4.6 Not specified
CNA Linux Linux unaffected 4.6 semver Not specified
CNA Linux Linux unaffected 5.4.302 5.4.* semver Not specified
CNA Linux Linux unaffected 5.10.247 5.10.* semver Not specified
CNA Linux Linux unaffected 5.15.197 5.15.* semver Not specified
CNA Linux Linux unaffected 6.1.158 6.1.* semver Not specified
CNA Linux Linux unaffected 6.6.114 6.6.* semver Not specified
CNA Linux Linux unaffected 6.12.55 6.12.* semver Not specified
CNA Linux Linux unaffected 6.17.3 6.17.* semver Not specified
CNA Linux Linux unaffected 6.18 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/92b82e232b8d8b116ac6e57aeae7a6033db92c60 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/850f1ea245bdc0ce6a3fd36bfb80d8cf9647cb71 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/b69f19244c2b6475c8a6eb72f0fb0d53509e48cd 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/ac50c6e0a8f91a02b681af81abb2362fbb67cc18 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/76385629f45740b7888f8fcd83bde955b10f61fe 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/788e5385d0ff69cdba1cabccb9dab8d9647b9239 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
git.kernel.org/stable/c/41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org Patch
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