CVE-2025-40040 — linux / linux_kernel
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.
[[email protected]: rust bindgen wasn't able to handle BIT(), from Miguel]
| Software | From | Fixed in |
|---|---|---|
| linux / linux_kernel | 4.6 | 5.4.302 |
| linux / linux_kernel | 5.5 | 5.10.247 |
| linux / linux_kernel | 5.11 | 5.15.197 |
| linux / linux_kernel | 5.16 | 6.1.158 |
| linux / linux_kernel | 6.2 | 6.6.114 |
| linux / linux_kernel | 6.7 | 6.12.55 |
| linux / linux_kernel | 6.13 | 6.17.3 |
- https://git.kernel.org/stable/c/41cb9fd904fe0c39d52e82dd84dc3c96b7aa9693
- https://git.kernel.org/stable/c/76385629f45740b7888f8fcd83bde955b10f61fe
- https://git.kernel.org/stable/c/788e5385d0ff69cdba1cabccb9dab8d9647b9239
- https://git.kernel.org/stable/c/850f1ea245bdc0ce6a3fd36bfb80d8cf9647cb71
- https://git.kernel.org/stable/c/92b82e232b8d8b116ac6e57aeae7a6033db92c60
- https://git.kernel.org/stable/c/ac50c6e0a8f91a02b681af81abb2362fbb67cc18
- https://git.kernel.org/stable/c/b69f19244c2b6475c8a6eb72f0fb0d53509e48cd
- https://git.kernel.org/stable/c/f04aad36a07cc17b7a5d5b9a2d386ce6fae63e93
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