======================================================
[ INFO: possible circular locking dependency detected ]
4.4.169+ #2 Not tainted
-------------------------------------------------------
syz-executor204/2075 is trying to acquire lock:
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814b226d>] __pipe_lock fs/pipe.c:86 [inline]
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814b226d>] fifo_open+0x15d/0xa00 fs/pipe.c:896

but task is already holding lock:
 (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814ac4b5>] prepare_bprm_creds+0x55/0x120 fs/exec.c:1225

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       [<ffffffff81205d7e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff8270b012>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270b012>] mutex_lock_interruptible_nested+0xd2/0xce0 kernel/locking/mutex.c:650
       [<ffffffff815e78e8>] proc_pid_attr_write+0x1a8/0x2a0 fs/proc/base.c:2524
       [<ffffffff814962e6>] __vfs_write+0x116/0x3d0 fs/read_write.c:489
       [<ffffffff814966b2>] __kernel_write+0x112/0x370 fs/read_write.c:511
       [<ffffffff815327dd>] write_pipe_buf+0x15d/0x1f0 fs/splice.c:1074
       [<ffffffff815334de>] splice_from_pipe_feed fs/splice.c:776 [inline]
       [<ffffffff815334de>] __splice_from_pipe+0x37e/0x7a0 fs/splice.c:901
       [<ffffffff81536558>] splice_from_pipe+0x108/0x170 fs/splice.c:936
       [<ffffffff8153664c>] default_file_splice_write+0x3c/0x80 fs/splice.c:1086
       [<ffffffff815376a1>] do_splice_from fs/splice.c:1128 [inline]
       [<ffffffff815376a1>] do_splice fs/splice.c:1404 [inline]
       [<ffffffff815376a1>] SYSC_splice fs/splice.c:1707 [inline]
       [<ffffffff815376a1>] SyS_splice+0xd71/0x13a0 fs/splice.c:1690
       [<ffffffff827153a1>] entry_SYSCALL_64_fastpath+0x1e/0x9a

       [<ffffffff81202b96>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
       [<ffffffff81202b96>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
       [<ffffffff81202b96>] validate_chain kernel/locking/lockdep.c:2144 [inline]
       [<ffffffff81202b96>] __lock_acquire+0x37d6/0x4f50 kernel/locking/lockdep.c:3213
       [<ffffffff81205d7e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff82708c01>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff82708c01>] mutex_lock_nested+0xc1/0xb80 kernel/locking/mutex.c:621
       [<ffffffff814b226d>] __pipe_lock fs/pipe.c:86 [inline]
       [<ffffffff814b226d>] fifo_open+0x15d/0xa00 fs/pipe.c:896
       [<ffffffff8149103f>] do_dentry_open+0x38f/0xbd0 fs/open.c:749
       [<ffffffff8149482b>] vfs_open+0x10b/0x210 fs/open.c:862
       [<ffffffff814c574f>] do_last fs/namei.c:3269 [inline]
       [<ffffffff814c574f>] path_openat+0x136f/0x4470 fs/namei.c:3406
       [<ffffffff814cc421>] do_filp_open+0x1a1/0x270 fs/namei.c:3440
       [<ffffffff814a75fc>] do_open_execat+0x10c/0x6e0 fs/exec.c:805
       [<ffffffff814acc76>] do_execveat_common.isra.0+0x6f6/0x1e90 fs/exec.c:1577
       [<ffffffff814aed92>] do_execve fs/exec.c:1683 [inline]
       [<ffffffff814aed92>] SYSC_execve fs/exec.c:1764 [inline]
       [<ffffffff814aed92>] SyS_execve+0x42/0x50 fs/exec.c:1759
       [<ffffffff827156f5>] 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-executor204/2075:
 #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814ac4b5>] prepare_bprm_creds+0x55/0x120 fs/exec.c:1225

stack backtrace:
CPU: 0 PID: 2075 Comm: syz-executor204 Not tainted 4.4.169+ #2
 0000000000000000 e5de6132897a38b7 ffff8800b6b2f530 ffffffff81aab9c1
 ffffffff84055ac0 ffff8801d48597c0 ffffffff83abb2b0 ffffffff83ab4860
 ffffffff83abb2b0 ffff8800b6b2f580 ffffffff813abaf4 ffff8800b6b2f660
Call Trace:
 [<ffffffff81aab9c1>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aab9c1>] dump_stack+0xc1/0x120 lib/dump_stack.c:51
 [<ffffffff813abaf4>] print_circular_bug.cold+0x2f7/0x44e kernel/locking/lockdep.c:1226
 [<ffffffff81202b96>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
 [<ffffffff81202b96>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
 [<ffffffff81202b96>] validate_chain kernel/locking/lockdep.c:2144 [inline]
 [<ffffffff81202b96>] __lock_acquire+0x37d6/0x4f50 kernel/locking/lockdep.c:3213
 [<ffffffff81205d7e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
 [<ffffffff82708c01>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff82708c01>] mutex_lock_nested+0xc1/0xb80 kernel/locking/mutex.c:621
 [<ffffffff814b226d>] __pipe_lock fs/pipe.c:86 [inline]
 [<ffffffff814b226d>] fifo_open+0x15d/0xa00 fs/pipe.c:896
 [<ffffffff8149103f>] do_dentry_open+0x38f/0xbd0 fs/open.c:749
 [<ffffffff8149482b>] vfs_open+0x10b/0x210 fs/open.c:862
 [<ffffffff814c574f>] do_last fs/namei.c:3269 [inline]
 [<ffffffff814c574f>] path_openat+0x136f/0x4470 fs/namei.c:3406
 [<ffffffff814cc421>] do_filp_open+0x1a1/0x270 fs/namei.c:3440
 [<ffffffff814a75fc>] do_open_execat+0x10c/0x6e0 fs/exec.c:805
 [<ffffffff814acc76>] do_execveat_common.isra.0+0x6f6/0x1e90 fs/exec.c:1577