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")
Signed-off-by: Jakub Acs <acsjakub@amazon.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Xu Xin <xu.xin16@zte.com.cn>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org
---
include/linux/mm.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1ae97a0b8ec7..c6794d0e24eb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -296,7 +296,7 @@ extern unsigned int kobjsize(const void *objp);
#define VM_MIXEDMAP 0x10000000 /* Can contain "struct page" and pure PFN pages */
#define VM_HUGEPAGE 0x20000000 /* MADV_HUGEPAGE marked this vma */
#define VM_NOHUGEPAGE 0x40000000 /* MADV_NOHUGEPAGE marked this vma */
-#define VM_MERGEABLE 0x80000000 /* KSM may merge identical pages */
+#define VM_MERGEABLE BIT(31) /* KSM may merge identical pages */
#ifdef CONFIG_ARCH_USES_HIGH_VMA_FLAGS
#define VM_HIGH_ARCH_BIT_0 32 /* bit only usable on 64-bit architectures */
--
2.47.3
Amazon Web Services Development Center Germany GmbH
Tamara-Danz-Str. 13
10243 Berlin
Geschaeftsfuehrung: Christian Schlaeger
Eingetragen am Amtsgericht Charlottenburg unter HRB 257764 B
Sitz: Berlin
Ust-ID: DE 365 538 597On Wed, 1 Oct 2025 09:03:52 +0000 Jakub Acs <acsjakub@amazon.de> 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.
Nice!
>
> 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")
Nit. It is recommended [1] to use 12 characters of the SHA-1 ID, but you are
using 13 characters.
> Signed-off-by: Jakub Acs <acsjakub@amazon.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Xu Xin <xu.xin16@zte.com.cn>
> Cc: Chengming Zhou <chengming.zhou@linux.dev>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
Nit. This would be nice to be placed just after the 'Fixes:' tag.
Acked-by: SeongJae Park <sj@kernel.org>
[1] https://docs.kernel.org/process/submitting-patches.html#describe-your-changes
Thanks,
SJ
[...]
On 01.10.25 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")
Very Likely we want to CC stable.
> Signed-off-by: Jakub Acs <acsjakub@amazon.de>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Xu Xin <xu.xin16@zte.com.cn>
> Cc: Chengming Zhou <chengming.zhou@linux.dev>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Axel Rasmussen <axelrasmussen@google.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
> Cc: stable@vger.kernel.org
> ---
IMHO no need to resend this one if Andrew can just pick this one up.
Then, you can send out patch #2 separately as commented in reply to
patch #2.
Thanks!
Acked-by: David Hildenbrand <david@redhat.com>
--
Cheers
David / dhildenb
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