syzbot

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Tainted: G             L     
------------------------------------------------------
syz.2.842/9892 is trying to acquire lock:
ffff88802f7f9460 (&sig->cred_guard_mutex){+.+.}-{4:4}, at: proc_pid_attr_write+0x547/0x630 fs/proc/base.c:2837

but task is already holding lock:
ffff888056f74420 (sb_writers#3){.+.+}-{0:0}, at: file_start_write include/linux/fs.h:2681 [inline]
ffff888056f74420 (sb_writers#3){.+.+}-{0:0}, at: vfs_write+0x211/0xb30 fs/read_write.c:682

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (sb_writers#3){.+.+}-{0:0}:
       percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
       percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
       __sb_start_write include/linux/fs/super.h:19 [inline]
       sb_start_write+0x4d/0x1c0 include/linux/fs/super.h:125
       mnt_want_write+0x41/0x90 fs/namespace.c:499
       open_last_lookups fs/namei.c:4529 [inline]
       path_openat+0xadd/0x3dd0 fs/namei.c:4784
       do_filp_open+0x1fa/0x410 fs/namei.c:4814
       io_openat2+0x3e0/0x5c0 io_uring/openclose.c:143
       __io_issue_sqe+0x181/0x4b0 io_uring/io_uring.c:1792
       io_issue_sqe+0x165/0x1060 io_uring/io_uring.c:1815
       io_queue_sqe io_uring/io_uring.c:2042 [inline]
       io_submit_sqe io_uring/io_uring.c:2320 [inline]
       io_submit_sqes+0xbf4/0x2140 io_uring/io_uring.c:2434
       __do_sys_io_uring_enter io_uring/io_uring.c:3280 [inline]
       __se_sys_io_uring_enter+0x2e0/0x2b60 io_uring/io_uring.c:3219
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&ctx->uring_lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:614 [inline]
       __mutex_lock+0x187/0x1350 kernel/locking/mutex.c:776
       io_uring_del_tctx_node+0xf0/0x2c0 io_uring/tctx.c:179
       io_uring_clean_tctx+0xd4/0x1a0 io_uring/tctx.c:195
       io_uring_cancel_generic+0x6ca/0x7d0 io_uring/cancel.c:646
       io_uring_task_cancel include/linux/io_uring.h:24 [inline]
       begin_new_exec+0x10ed/0x2440 fs/exec.c:1131
       load_elf_binary+0x9f8/0x2d70 fs/binfmt_elf.c:1010
       search_binary_handler fs/exec.c:1669 [inline]
       exec_binprm fs/exec.c:1701 [inline]
       bprm_execve+0x92e/0x1400 fs/exec.c:1753
       do_execveat_common+0x510/0x6a0 fs/exec.c:1859
       do_execveat fs/exec.c:1944 [inline]
       __do_sys_execveat fs/exec.c:2018 [inline]
       __se_sys_execveat fs/exec.c:2012 [inline]
       __x64_sys_execveat+0xc4/0xe0 fs/exec.c:2012
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&sig->cred_guard_mutex){+.+.}-{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+0x15a6/0x2cf0 kernel/locking/lockdep.c:5237
       lock_acquire+0x107/0x340 kernel/locking/lockdep.c:5868
       __mutex_lock_common kernel/locking/mutex.c:614 [inline]
       __mutex_lock+0x187/0x1350 kernel/locking/mutex.c:776
       proc_pid_attr_write+0x547/0x630 fs/proc/base.c:2837
       vfs_write+0x27e/0xb30 fs/read_write.c:684
       ksys_write+0x145/0x250 fs/read_write.c:738
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xec/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:
  &sig->cred_guard_mutex --> &ctx->uring_lock --> sb_writers#3

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(sb_writers#3);
                               lock(&ctx->uring_lock);
                               lock(sb_writers#3);
  lock(&sig->cred_guard_mutex);

 *** DEADLOCK ***

2 locks held by syz.2.842/9892:
 #0: ffff88807b3a3cf8 (&f->f_pos_lock){+.+.}-{4:4}, at: fdget_pos+0x247/0x320 fs/file.c:1255
 #1: ffff888056f74420 (sb_writers#3){.+.+}-{0:0}, at: file_start_write include/linux/fs.h:2681 [inline]
 #1: ffff888056f74420 (sb_writers#3){.+.+}-{0:0}, at: vfs_write+0x211/0xb30 fs/read_write.c:682

stack backtrace:
CPU: 1 UID: 0 PID: 9892 Comm: syz.2.842 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e2/0x300 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+0x15a6/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0x107/0x340 kernel/locking/lockdep.c:5868
 __mutex_lock_common kernel/locking/mutex.c:614 [inline]
 __mutex_lock+0x187/0x1350 kernel/locking/mutex.c:776
 proc_pid_attr_write+0x547/0x630 fs/proc/base.c:2837
 vfs_write+0x27e/0xb30 fs/read_write.c:684
 ksys_write+0x145/0x250 fs/read_write.c:738
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xec/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f23bc18f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f23ba3f6038 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f23bc3e5fa0 RCX: 00007f23bc18f749
RDX: 00000000000000f4 RSI: 0000200000000240 RDI: 0000000000000006
RBP: 00007f23bc213f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f23bc3e6038 R14: 00007f23bc3e5fa0 R15: 00007f23bc50fa28
 </TASK>