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. Modify all other VM_* flags constants for consistency.

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 --- v1 -> v2: - fix by adding ul to flag constants instead of explicit cast. - drop Mike Kravetz mike.kravetz@oracle.com from cc, as the mail returned

v1: https://lore.kernel.org/all/20250930063921.62354-1-acsjakub@amazon.de/

include/linux/mm.h | 72 +++++++++++++++++++++++----------------------- 1 file changed, 36 insertions(+), 36 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h index 1ae97a0b8ec7..26a5c0f78b36 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -246,57 +246,57 @@ extern unsigned int kobjsize(const void *objp); * vm_flags in vm_area_struct, see mm_types.h. * When changing, update also include/trace/events/mmflags.h */ -#define VM_NONE 0x00000000 +#define VM_NONE 0x00000000ul

-#define VM_READ 0x00000001 /* currently active flags */ -#define VM_WRITE 0x00000002 -#define VM_EXEC 0x00000004 -#define VM_SHARED 0x00000008 +#define VM_READ 0x00000001ul /* currently active flags */ +#define VM_WRITE 0x00000002ul +#define VM_EXEC 0x00000004ul +#define VM_SHARED 0x00000008ul

/* mprotect() hardcodes VM_MAYREAD >> 4 == VM_READ, and so for r/w/x bits. */ -#define VM_MAYREAD 0x00000010 /* limits for mprotect() etc */ -#define VM_MAYWRITE 0x00000020 -#define VM_MAYEXEC 0x00000040 -#define VM_MAYSHARE 0x00000080 +#define VM_MAYREAD 0x00000010ul /* limits for mprotect() etc */ +#define VM_MAYWRITE 0x00000020ul +#define VM_MAYEXEC 0x00000040ul +#define VM_MAYSHARE 0x00000080ul

-#define VM_GROWSDOWN 0x00000100 /* general info on the segment */ +#define VM_GROWSDOWN 0x00000100ul /* general info on the segment */ #ifdef CONFIG_MMU -#define VM_UFFD_MISSING 0x00000200 /* missing pages tracking */ +#define VM_UFFD_MISSING 0x00000200ul /* missing pages tracking */ #else /* CONFIG_MMU */ -#define VM_MAYOVERLAY 0x00000200 /* nommu: R/O MAP_PRIVATE mapping that might overlay a file mapping */ -#define VM_UFFD_MISSING 0 +#define VM_MAYOVERLAY 0x00000200ul /* nommu: R/O MAP_PRIVATE mapping that might overlay a file mapping */ +#define VM_UFFD_MISSING 0ul #endif /* CONFIG_MMU */ -#define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */ -#define VM_UFFD_WP 0x00001000 /* wrprotect pages tracking */ +#define VM_PFNMAP 0x00000400ul /* Page-ranges managed without "struct page", just pure PFN */ +#define VM_UFFD_WP 0x00001000ul /* wrprotect pages tracking */

-#define VM_LOCKED 0x00002000 -#define VM_IO 0x00004000 /* Memory mapped I/O or similar */ +#define VM_LOCKED 0x00002000ul +#define VM_IO 0x00004000ul /* Memory mapped I/O or similar */

/* Used by sys_madvise() */ -#define VM_SEQ_READ 0x00008000 /* App will access data sequentially */ -#define VM_RAND_READ 0x00010000 /* App will not benefit from clustered reads */ - -#define VM_DONTCOPY 0x00020000 /* Do not copy this vma on fork */ -#define VM_DONTEXPAND 0x00040000 /* Cannot expand with mremap() */ -#define VM_LOCKONFAULT 0x00080000 /* Lock the pages covered when they are faulted in */ -#define VM_ACCOUNT 0x00100000 /* Is a VM accounted object */ -#define VM_NORESERVE 0x00200000 /* should the VM suppress accounting */ -#define VM_HUGETLB 0x00400000 /* Huge TLB Page VM */ -#define VM_SYNC 0x00800000 /* Synchronous page faults */ -#define VM_ARCH_1 0x01000000 /* Architecture-specific flag */ -#define VM_WIPEONFORK 0x02000000 /* Wipe VMA contents in child. */ -#define VM_DONTDUMP 0x04000000 /* Do not include in the core dump */ +#define VM_SEQ_READ 0x00008000ul /* App will access data sequentially */ +#define VM_RAND_READ 0x00010000ul /* App will not benefit from clustered reads */ + +#define VM_DONTCOPY 0x00020000ul /* Do not copy this vma on fork */ +#define VM_DONTEXPAND 0x00040000ul /* Cannot expand with mremap() */ +#define VM_LOCKONFAULT 0x00080000ul /* Lock the pages covered when they are faulted in */ +#define VM_ACCOUNT 0x00100000ul /* Is a VM accounted object */ +#define VM_NORESERVE 0x00200000ul /* should the VM suppress accounting */ +#define VM_HUGETLB 0x00400000ul /* Huge TLB Page VM */ +#define VM_SYNC 0x00800000ul /* Synchronous page faults */ +#define VM_ARCH_1 0x01000000ul /* Architecture-specific flag */ +#define VM_WIPEONFORK 0x02000000ul /* Wipe VMA contents in child. */ +#define VM_DONTDUMP 0x04000000ul /* Do not include in the core dump */

#ifdef CONFIG_MEM_SOFT_DIRTY -# define VM_SOFTDIRTY 0x08000000 /* Not soft dirty clean area */ +# define VM_SOFTDIRTY 0x08000000ul /* Not soft dirty clean area */ #else -# define VM_SOFTDIRTY 0 +# define VM_SOFTDIRTY 0ul #endif

-#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_MIXEDMAP 0x10000000ul /* Can contain "struct page" and pure PFN pages */ +#define VM_HUGEPAGE 0x20000000ul /* MADV_HUGEPAGE marked this vma */ +#define VM_NOHUGEPAGE 0x40000000ul /* MADV_NOHUGEPAGE marked this vma */ +#define VM_MERGEABLE 0x80000000ul /* 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 */