[syzbot] [mm?] possible deadlock in mfill_get_vma

syzbot
2026-03-15T18:37:29Z

Hello,

syzbot found the following issue on:

HEAD commit: b84a0ebe421c Add linux-next specific files for 20260313
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=131ab8da580000
kernel config: https://syzkaller.appspot.com/x/.config?x=e7280ad1f68b2dce
dashboard link: https://syzkaller.appspot.com/bug?extid=c473aa669b5e8a6f48d2
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=126c98ba580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1644a2d6580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/09145161a8a9/disk-b84a0ebe.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/b64c254e474c/vmlinux-b84a0ebe.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a7c33f5f7f45/bzImage-b84a0ebe.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+c473aa669b5e8a6f48d2@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.0.17/5990 is trying to acquire lock:
ffff88802caef3b8 (&mm->mmap_lock){++++}-{4:4}, at: __might_fault+0xaf/0x130 mm/memory.c:7249

but task is already holding lock:
ffff88807cdbccf0 (&ctx->map_changing_lock){.+.+}-{4:4}, at: mfill_get_vma+0x162/0x660 mm/userfaultfd.c:226

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&ctx->map_changing_lock){.+.+}-{4:4}:
down_read+0x47/0x2e0 kernel/locking/rwsem.c:1568
mfill_get_vma+0x162/0x660 mm/userfaultfd.c:226
mfill_atomic mm/userfaultfd.c:900 [inline]
mfill_atomic_continue+0x189/0x12c0 mm/userfaultfd.c:974
userfaultfd_continue fs/userfaultfd.c:1806 [inline]
userfaultfd_ioctl+0x232d/0x4c70 fs/userfaultfd.c:2071
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (vm_lock){++++}-{0:0}:
__vma_start_exclude_readers+0x28a/0x940 mm/mmap_lock.c:125
__vma_start_write+0xdc/0x290 mm/mmap_lock.c:148
vma_start_write include/linux/mmap_lock.h:303 [inline]
mprotect_fixup+0x5eb/0xa80 mm/mprotect.c:768
setup_arg_pages+0x565/0xac0 fs/exec.c:670
load_elf_binary+0xc5e/0x2980 fs/binfmt_elf.c:1029
search_binary_handler fs/exec.c:1664 [inline]
exec_binprm fs/exec.c:1696 [inline]
bprm_execve+0x949/0x1470 fs/exec.c:1748
kernel_execve+0x844/0x930 fs/exec.c:1892
try_to_run_init_process+0x13/0x60 init/main.c:1514
kernel_init+0xad/0x1d0 init/main.c:1642
ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #0 (&mm->mmap_lock){++++}-{4:4}:
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
__might_fault+0xcb/0x130 mm/memory.c:7249
userfaultfd_continue fs/userfaultfd.c:1813 [inline]
userfaultfd_ioctl+0x2372/0x4c70 fs/userfaultfd.c:2071
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
&mm->mmap_lock --> vm_lock --> &ctx->map_changing_lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
rlock(&ctx->map_changing_lock);
lock(vm_lock);
lock(&ctx->map_changing_lock);
rlock(&mm->mmap_lock);

*** DEADLOCK ***

2 locks held by syz.0.17/5990:
#0: ffff88807c119d08 (vm_lock){++++}-{0:0}, at: lock_vma_under_rcu+0x1d1/0x500 mm/mmap_lock.c:310
#1: ffff88807cdbccf0 (&ctx->map_changing_lock){.+.+}-{4:4}, at: mfill_get_vma+0x162/0x660 mm/userfaultfd.c:226

stack backtrace:
CPU: 0 UID: 0 PID: 5990 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
__might_fault+0xcb/0x130 mm/memory.c:7249
userfaultfd_continue fs/userfaultfd.c:1813 [inline]
userfaultfd_ioctl+0x2372/0x4c70 fs/userfaultfd.c:2071
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f478759c799
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffcc2bbac28 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f4787815fa0 RCX: 00007f478759c799
RDX: 0000200000000080 RSI: 00000000c020aa07 RDI: 0000000000000003
RBP: 00007f4787632c99 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4787815fac R14: 00007f4787815fa0 R15: 00007f4787815fa0
</TASK>

---
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