======================================================
WARNING: possible circular locking dependency detected
4.14.105+ #29 Not tainted
------------------------------------------------------
syz-executor.0/16544 is trying to acquire lock:
 (&pipe->mutex/1){+.+.}, at: [<ffffffffa1973746>] __pipe_lock fs/pipe.c:88 [inline]
 (&pipe->mutex/1){+.+.}, at: [<ffffffffa1973746>] fifo_open+0x156/0x9b0 fs/pipe.c:921

but task is already holding lock:
 (&sig->cred_guard_mutex){+.+.}, at: [<ffffffffa196db61>] prepare_bprm_creds+0x51/0x110 fs/exec.c:1389

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (&sig->cred_guard_mutex){+.+.}:

-> #0 (&pipe->mutex/1){+.+.}:

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.0/16544:
 #0:  (&sig->cred_guard_mutex){+.+.}, at: [<ffffffffa196db61>] prepare_bprm_creds+0x51/0x110 fs/exec.c:1389

stack backtrace:
CPU: 0 PID: 16544 Comm: syz-executor.0 Not tainted 4.14.105+ #29
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0xb9/0x10e lib/dump_stack.c:53
 print_circular_bug.isra.0.cold+0x2dc/0x425 kernel/locking/lockdep.c:1258