On Thu, Nov 06, 2025 at 11:39:28AM +0100, Vlastimil Babka wrote:
On 10/1/25 11:03, Jakub Acs wrote:
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.
Fixes: 7677f7fd8be76 ("userfaultfd: add minor fault registration mode")
Late to the party, but it seems to me the correct Fixes: should be f8af4da3b4c1 ("ksm: the mm interface to ksm") which introduced the flag and the buggy clearing code, no?
Commit 7677f7fd8be76 is just one that notices it, right? But there are other flags in >32 bit area, including pkeys etc. Sounds rather dangerous if they can be cleared using a madvise.
So we can't amend the Fixes: now but maybe could advise stable to backport for even older versions than based on 7677f7fd8be76 ?
Good point. It was a bit tricky to determine the correct "fixes" tag, as there were more candidates: - the commit that initially introduced VM_MERGEABLE as a constant with different inferred type to other vm_flags constants - the commit that first started using upper 32 bits of vm_flags and did not make sure the constants are defined safely - f8af4da3b4c1 indeed, as the one that makes the drop actually possible - 7677f7fd8be76 that shows us a path where the drop manifests
Looking back, I agree f8af4da3b4c1 is the better option, but as you said, that won't be changed now.
Nevertheless, I'll send the backports after a round of kselftests, thanks for pointing this out.
Have a good day, Jakub
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