fkie_cve-2025-40040
Vulnerability from fkie_nvd
Published
2025-10-28 12:15
Modified
2025-10-30 15:05
Severity ?
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 ]------------ [ 44.607422] 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]
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/76385629f45740b7888f8fcd83bde955b10f61fe
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/92b82e232b8d8b116ac6e57aeae7a6033db92c60
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/ac50c6e0a8f91a02b681af81abb2362fbb67cc18
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93
Impacted products
Vendor Product Version


To clipboard 

{
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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[   44.607422] 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\u003csnip other registers, drop unreliable trace\u003e\n\n[   44.617726] Call Trace:\n[   44.617926]  \u003cTASK\u003e\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-\u003evm_userfaultfd_ctx, but no UFFD flags in vma-\u003evm_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-\u003evm_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 \u0026=\n~VM_MERGEABLE assignment.\n\nVM_MERGEABLE is a 32-bit constant of type unsigned int, 0x8000\u00270000. \nAfter ~ is applied, it becomes 0x7fff\u0027ffff unsigned int, which is then\npromoted to unsigned long before the \u0026 operation.  This promotion fills\nupper 32 bits with leading 0s, as we\u0027re doing unsigned conversion (and\neven for a signed conversion, this wouldn\u0027t help as the leading bit is 0).\n\u0026 operation thus ends up AND-ing vm_flags with 0x0000\u00270000\u00277fff\u0027ffff\ninstead of intended 0xffff\u0027ffff\u00277fff\u0027ffff 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\u0027t able to handle BIT(), from Miguel]"
    }
  ],
  "id": "CVE-2025-40040",
  "lastModified": "2025-10-30T15:05:32.197",
  "metrics": {},
  "published": "2025-10-28T12:15:37.967",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/76385629f45740b7888f8fcd83bde955b10f61fe"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/92b82e232b8d8b116ac6e57aeae7a6033db92c60"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/ac50c6e0a8f91a02b681af81abb2362fbb67cc18"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Awaiting Analysis"
}


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