---
id: CVE-2025-40040
title: 'mm/ksm: fix flag-dropping behavior in ksm_madvise'
summary: |-
  In 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 ]------------
  [   44.60…
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    850f1ea245bdc0ce6a3fd36bfb80d8cf9647cb71
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    788e5385d0ff69cdba1cabccb9dab8d9647b9239
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    b69f19244c2b6475c8a6eb72f0fb0d53509e48cd
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    92b82e232b8d8b116ac6e57aeae7a6033db92c60
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    ac50c6e0a8f91a02b681af81abb2362fbb67cc18
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    76385629f45740b7888f8fcd83bde955b10f61fe
  - >-
    Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 <
    f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93
  - Linux 4.6
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: total
  timestamp: '2026-10-15T21:56:21.086400Z'
exploitAvailable: true
published: '2025-10-28'
updated: '2026-10-15'
sourceUpdated: '2026-10-15T22:02:36.606Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2025-40040'
references:
  - url: 'https://git.kernel.org/stable/c/850f1ea245bdc0ce6a3fd36bfb80d8cf9647cb71'
  - url: 'https://git.kernel.org/stable/c/788e5385d0ff69cdba1cabccb9dab8d9647b9239'
  - url: 'https://git.kernel.org/stable/c/b69f19244c2b6475c8a6eb72f0fb0d53509e48cd'
  - url: 'https://git.kernel.org/stable/c/41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693'
  - url: 'https://git.kernel.org/stable/c/92b82e232b8d8b116ac6e57aeae7a6033db92c60'
  - url: 'https://git.kernel.org/stable/c/ac50c6e0a8f91a02b681af81abb2362fbb67cc18'
  - url: 'https://git.kernel.org/stable/c/76385629f45740b7888f8fcd83bde955b10f61fe'
  - url: 'https://git.kernel.org/stable/c/f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93'
tags:
  - cve.org
  - exploit-available
epss: 0.00376
epssPercentile: 0.31406
exploits:
  github: 1
  githubRepos:
    - 'https://github.com/SpiralBL0CK/CVE-2023-1206-CVE-2025-40040-CVE-2024-49882'
  checkedAt: '2026-10-27T16:46:32.571Z'
ingestedAt: '2026-10-15T22:48:25.538Z'
---

## Overview

In 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.

[akpm@linux-foundation.org: rust bindgen wasn't able to handle BIT(), from Miguel]

## Affected

- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 850f1ea245bdc0ce6a3fd36bfb80d8cf9647cb71`
- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 788e5385d0ff69cdba1cabccb9dab8d9647b9239`
- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < b69f19244c2b6475c8a6eb72f0fb0d53509e48cd`
- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693`
- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 92b82e232b8d8b116ac6e57aeae7a6033db92c60`
- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < ac50c6e0a8f91a02b681af81abb2362fbb67cc18`
- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 76385629f45740b7888f8fcd83bde955b10f61fe`
- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93`
- `Linux 4.6`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.