======================================================
[ INFO: possible circular locking dependency detected ]
4.9.141+ #1 Not tainted
-------------------------------------------------------
syz-executor.1/9546 is trying to acquire lock:
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff81526c0c>] __pipe_lock fs/pipe.c:87 [inline]
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff81526c0c>] fifo_open+0x15c/0x9e0 fs/pipe.c:921
but task is already holding lock:
 (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff81520d73>] prepare_bprm_creds+0x53/0x110 fs/exec.c:1369
which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       lock_acquire+0x130/0x3e0 kernel/locking/lockdep.c:3756
       __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       mutex_lock_interruptible_nested+0xcc/0x9c0 kernel/locking/mutex.c:650
       proc_pid_attr_write+0x150/0x260 fs/proc/base.c:2548
       __vfs_write+0x115/0x580 fs/read_write.c:507
       __kernel_write+0xf8/0x350 fs/read_write.c:529
       write_pipe_buf+0x15d/0x1f0 fs/splice.c:816
       splice_from_pipe_feed fs/splice.c:521 [inline]
       __splice_from_pipe+0x316/0x710 fs/splice.c:645
       splice_from_pipe+0xf9/0x170 fs/splice.c:680
       default_file_splice_write+0x3c/0x80 fs/splice.c:828
       do_splice_from fs/splice.c:870 [inline]
       do_splice fs/splice.c:1166 [inline]
       SYSC_splice fs/splice.c:1416 [inline]
       SyS_splice+0xe4d/0x14d0 fs/splice.c:1399
       do_syscall_64+0x19f/0x550 arch/x86/entry/common.c:285
       entry_SYSCALL_64_after_swapgs+0x5d/0xdb

       check_prev_add kernel/locking/lockdep.c:1828 [inline]
       check_prevs_add kernel/locking/lockdep.c:1938 [inline]
       validate_chain kernel/locking/lockdep.c:2265 [inline]
       __lock_acquire+0x3189/0x4a10 kernel/locking/lockdep.c:3345
       lock_acquire+0x130/0x3e0 kernel/locking/lockdep.c:3756
       __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       mutex_lock_nested+0xc0/0x900 kernel/locking/mutex.c:621
       __pipe_lock fs/pipe.c:87 [inline]
       fifo_open+0x15c/0x9e0 fs/pipe.c:921
       do_dentry_open+0x3ef/0xc90 fs/open.c:766
       vfs_open+0x11c/0x210 fs/open.c:879
       do_last fs/namei.c:3410 [inline]
       path_openat+0x542/0x2790 fs/namei.c:3534
       do_filp_open+0x197/0x270 fs/namei.c:3568
       do_open_execat+0x10f/0x640 fs/exec.c:844
       do_execveat_common.isra.14+0x687/0x1ed0 fs/exec.c:1723
       do_execve fs/exec.c:1829 [inline]
       SYSC_execve fs/exec.c:1910 [inline]
       SyS_execve+0x42/0x50 fs/exec.c:1905
       do_syscall_64+0x19f/0x550 arch/x86/entry/common.c:285
       entry_SYSCALL_64_after_swapgs+0x5d/0xdb

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&sig->cred_guard_mutex);
                               lock(&pipe->mutex/1);
                               lock(&sig->cred_guard_mutex);
  lock(&pipe->mutex/1);

 *** DEADLOCK ***

1 lock held by syz-executor.1/9546:
 #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff81520d73>] prepare_bprm_creds+0x53/0x110 fs/exec.c:1369

stack backtrace:
CPU: 0 PID: 9546 Comm: syz-executor.1 Not tainted 4.9.141+ #1
 ffff8801c7167518 ffffffff81b42e79 ffffffff83caa290 ffffffff83ca2fd0
 ffffffff83caa290 ffff8801d0205010 ffff8801d0204740 ffff8801c7167560
 ffffffff813fee40 0000000000000001 00000000d0204ff0 0000000000000001
Call Trace:
 [<ffffffff81b42e79>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81b42e79>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
 [<ffffffff813fee40>] print_circular_bug.cold.36+0x2f7/0x432 kernel/locking/lockdep.c:1202
 [<ffffffff8120a539>] check_prev_add kernel/locking/lockdep.c:1828 [inline]
 [<ffffffff8120a539>] check_prevs_add kernel/locking/lockdep.c:1938 [inline]
 [<ffffffff8120a539>] validate_chain kernel/locking/lockdep.c:2265 [inline]
 [<ffffffff8120a539>] __lock_acquire+0x3189/0x4a10 kernel/locking/lockdep.c:3345
 [<ffffffff8120c8d0>] lock_acquire+0x130/0x3e0 kernel/locking/lockdep.c:3756
 [<ffffffff8280b250>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff8280b250>] mutex_lock_nested+0xc0/0x900 kernel/locking/mutex.c:621
 [<ffffffff81526c0c>] __pipe_lock fs/pipe.c:87 [inline]
 [<ffffffff81526c0c>] fifo_open+0x15c/0x9e0 fs/pipe.c:921
 [<ffffffff81501e3f>] do_dentry_open+0x3ef/0xc90 fs/open.c:766
 [<ffffffff8150576c>] vfs_open+0x11c/0x210 fs/open.c:879
 [<ffffffff8153c542>] do_last fs/namei.c:3410 [inline]
 [<ffffffff8153c542>] path_openat+0x542/0x2790 fs/namei.c:3534
 [<ffffffff81541617>] do_filp_open+0x197/0x270 fs/namei.c:3568
 [<ffffffff8151b9cf>] do_open_execat+0x10f/0x640 fs/exec.c:844
 [<ffffffff815214b7>] do_execveat_common.isra.14+0x687/0x1ed0 fs/exec.c:1723
 [<ffffffff81523662>] do_execve fs/exec.c:1829 [inline]
 [<ffffffff81523662>] SYSC_execve fs/exec.c:1910 [inline]
 [<ffffffff81523662>] SyS_execve+0x42/0x50 fs/exec.c:1905
 [<ffffffff810056ef>] do_syscall_64+0x19f/0x550 arch/x86/entry/common.c:285
 [<ffffffff82817893>] entry_SYSCALL_64_after_swapgs+0x5d/0xdb
nla_parse: 36 callbacks suppressed
netlink: 4 bytes leftover after parsing attributes in process `syz-executor.3'.
netlink: 4 bytes leftover after parsing attributes in process `syz-executor.3'.
netlink: 4 bytes leftover after parsing attributes in process `syz-executor.3'.
netlink: 4 bytes leftover after parsing attributes in process `syz-executor.0'.
netlink: 4 bytes leftover after parsing attributes in process `syz-executor.0'.
netlink: 4 bytes leftover after parsserialport: VM disconnected.