{"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:\n\nmm/ksm: fix flag-dropping behavior in ksm_madvise\n\nsyzkaller discovered the following crash: (kernel BUG)\n\n[   44.607039] ------------[ cut here ]------------\n[   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","slug":"CVE-2025-40040","body":"## Overview\n\nIn the Linux kernel, the following vulnerability has been resolved:\n\nmm/ksm: fix flag-dropping behavior in ksm_madvise\n\nsyzkaller discovered the following crash: (kernel BUG)\n\n[   44.607039] ------------[ cut here ]------------\n[   44.607422] kernel BUG at mm/userfaultfd.c:2067!\n[   44.608148] Oops: invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI\n[   44.608814] CPU: 1 UID: 0 PID: 2475 Comm: reproducer Not tainted 6.16.0-rc6 #1 PREEMPT(none)\n[   44.609635] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014\n[   44.610695] RIP: 0010:userfaultfd_release_all+0x3a8/0x460\n\n<snip other registers, drop unreliable trace>\n\n[   44.617726] Call Trace:\n[   44.617926]  <TASK>\n[   44.619284]  userfaultfd_release+0xef/0x1b0\n[   44.620976]  __fput+0x3f9/0xb60\n[   44.621240]  fput_close_sync+0x110/0x210\n[   44.622222]  __x64_sys_close+0x8f/0x120\n[   44.622530]  do_syscall_64+0x5b/0x2f0\n[   44.622840]  entry_SYSCALL_64_after_hwframe+0x76/0x7e\n[   44.623244] RIP: 0033:0x7f365bb3f227\n\nKernel panics because it detects UFFD inconsistency during\nuserfaultfd_release_all().  Specifically, a VMA which has a valid pointer\nto vma->vm_userfaultfd_ctx, but no UFFD flags in vma->vm_flags.\n\nThe inconsistency is caused in ksm_madvise(): when user calls madvise()\nwith MADV_UNMEARGEABLE on a VMA that is registered for UFFD in MINOR mode,\nit accidentally clears all flags stored in the upper 32 bits of\nvma->vm_flags.\n\nAssuming x86_64 kernel build, unsigned long is 64-bit and unsigned int and\nint are 32-bit wide.  This setup causes the following mishap during the &=\n~VM_MERGEABLE assignment.\n\nVM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000'0000. \nAfter ~ is applied, it becomes 0x7fff'ffff unsigned int, which is then\npromoted to unsigned long before the & operation.  This promotion fills\nupper 32 bits with leading 0s, as we're doing unsigned conversion (and\neven for a signed conversion, this wouldn't help as the leading bit is 0).\n& operation thus ends up AND-ing vm_flags with 0x0000'0000'7fff'ffff\ninstead of intended 0xffff'ffff'7fff'ffff and hence accidentally clears\nthe upper 32-bits of its value.\n\nFix it by changing `VM_MERGEABLE` constant to unsigned long, using the\nBIT() macro.\n\nNote: other VM_* flags are not affected: This only happens to the\nVM_MERGEABLE flag, as the other VM_* flags are all constants of type int\nand after ~ operation, they end up with leading 1 and are thus converted\nto unsigned long with leading 1s.\n\nNote 2:\nAfter commit 31defc3b01d9 (\"userfaultfd: remove (VM_)BUG_ON()s\"), this is\nno longer a kernel BUG, but a WARNING at the same place:\n\n[   45.595973] WARNING: CPU: 1 PID: 2474 at mm/userfaultfd.c:2067\n\nbut the root-cause (flag-drop) remains the same.\n\n[akpm@linux-foundation.org: rust bindgen wasn't able to handle BIT(), from Miguel]\n\n## Affected\n\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 850f1ea245bdc0ce6a3fd36bfb80d8cf9647cb71`\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 788e5385d0ff69cdba1cabccb9dab8d9647b9239`\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < b69f19244c2b6475c8a6eb72f0fb0d53509e48cd`\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693`\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 92b82e232b8d8b116ac6e57aeae7a6033db92c60`\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < ac50c6e0a8f91a02b681af81abb2362fbb67cc18`\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < 76385629f45740b7888f8fcd83bde955b10f61fe`\n- `Linux >= 63c17fb8e5a46a16e10e82005748837fd11a2024 < f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93`\n- `Linux 4.6`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"midnight","depthScore":55,"depthScoreParts":{"impact":42.9,"likelihood":0.1,"exploitation":12,"ransomware":0},"changes":[]}