CVE-2025-40040

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]