======================================================
[ INFO: possible circular locking dependency detected ]
4.4.169+ #7 Not tainted
-------------------------------------------------------
syz-executor278/2067 is trying to acquire lock:
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814ad4ec>] __pipe_lock fs/pipe.c:86 [inline]
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814ad4ec>] fifo_open+0x15c/0x9e0 fs/pipe.c:896

but task is already holding lock:
 (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814a7823>] prepare_bprm_creds+0x53/0x110 fs/exec.c:1225

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       [<ffffffff8120384e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff8270be82>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270be82>] mutex_lock_interruptible_nested+0xd2/0xcc0 kernel/locking/mutex.c:650
       [<ffffffff815e115e>] proc_pid_attr_write+0x19e/0x290 fs/proc/base.c:2524
       [<ffffffff8149170c>] __vfs_write+0x11c/0x3e0 fs/read_write.c:489
       [<ffffffff81491ada>] __kernel_write+0x10a/0x350 fs/read_write.c:511
       [<ffffffff8152d12d>] write_pipe_buf+0x15d/0x1f0 fs/splice.c:1074
       [<ffffffff8152dfc4>] splice_from_pipe_feed fs/splice.c:776 [inline]
       [<ffffffff8152dfc4>] __splice_from_pipe+0x364/0x790 fs/splice.c:901
       [<ffffffff81531089>] splice_from_pipe+0xf9/0x170 fs/splice.c:936
       [<ffffffff8153118c>] default_file_splice_write+0x3c/0x80 fs/splice.c:1086
       [<ffffffff81532251>] do_splice_from fs/splice.c:1128 [inline]
       [<ffffffff81532251>] do_splice fs/splice.c:1404 [inline]
       [<ffffffff81532251>] SYSC_splice fs/splice.c:1707 [inline]
       [<ffffffff81532251>] SyS_splice+0xde1/0x1430 fs/splice.c:1690
       [<ffffffff827162e1>] entry_SYSCALL_64_fastpath+0x1e/0x9a

       [<ffffffff81200384>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
       [<ffffffff81200384>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
       [<ffffffff81200384>] validate_chain kernel/locking/lockdep.c:2144 [inline]
       [<ffffffff81200384>] __lock_acquire+0x3cd4/0x5530 kernel/locking/lockdep.c:3213
       [<ffffffff8120384e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff82709ad2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff82709ad2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
       [<ffffffff814ad4ec>] __pipe_lock fs/pipe.c:86 [inline]
       [<ffffffff814ad4ec>] fifo_open+0x15c/0x9e0 fs/pipe.c:896
       [<ffffffff8148c4fd>] do_dentry_open+0x38d/0xbd0 fs/open.c:749
       [<ffffffff8148fc6a>] vfs_open+0x12a/0x210 fs/open.c:862
       [<ffffffff814bff40>] do_last fs/namei.c:3269 [inline]
       [<ffffffff814bff40>] path_openat+0xc10/0x3f10 fs/namei.c:3406
       [<ffffffff814c6ed7>] do_filp_open+0x197/0x270 fs/namei.c:3440
       [<ffffffff814a29df>] do_open_execat+0x10f/0x6f0 fs/exec.c:805
       [<ffffffff814a7f81>] do_execveat_common.isra.14+0x6a1/0x1f00 fs/exec.c:1577
       [<ffffffff814aa152>] do_execve fs/exec.c:1683 [inline]
       [<ffffffff814aa152>] SYSC_execve fs/exec.c:1764 [inline]
       [<ffffffff814aa152>] SyS_execve+0x42/0x50 fs/exec.c:1759
       [<ffffffff82716635>] return_from_execve+0x0/0x23

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-executor278/2067:
 #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814a7823>] prepare_bprm_creds+0x53/0x110 fs/exec.c:1225

stack backtrace:
CPU: 1 PID: 2067 Comm: syz-executor278 Not tainted 4.4.169+ #7
 0000000000000000 ae1b562e92a6cdf1 ffff8800b6e474d0 ffffffff81aa635d
 ffffffff83ab9460 ffffffff83ab9460 ffff8801d5fe8000 ffffffff83ab2350
 ffff8801d5fe88e8 ffff8800b6e47520 ffffffff813a9589 ffff8801d5fe8000
Call Trace:
 [<ffffffff81aa635d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aa635d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff813a9589>] print_circular_bug.cold.31+0x2f6/0x435 kernel/locking/lockdep.c:1226
 [<ffffffff81200384>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
 [<ffffffff81200384>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
 [<ffffffff81200384>] validate_chain kernel/locking/lockdep.c:2144 [inline]
 [<ffffffff81200384>] __lock_acquire+0x3cd4/0x5530 kernel/locking/lockdep.c:3213
 [<ffffffff8120384e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
 [<ffffffff82709ad2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff82709ad2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
 [<ffffffff814ad4ec>] __pipe_lock fs/pipe.c:86 [inline]
 [<ffffffff814ad4ec>] fifo_open+0x15c/0x9e0 fs/pipe.c:896
 [<ffffffff8148c4fd>] do_dentry_open+0x38d/0xbd0 fs/open.c:749
 [<ffffffff8148fc6a>] vfs_open+0x12a/0x210 fs/open.c:862
 [<ffffffff814bff40>] do_last fs/namei.c:3269 [inline]
 [<ffffffff814bff40>] path_openat+0xc10/0x3f10 fs/namei.c:3406
 [<ffffffff8102e3e6>] ? save_stack_