======================================================
[ INFO: possible circular locking dependency detected ]
4.4.168+ #6 Not tainted
-------------------------------------------------------
syz-executor419/2092 is trying to acquire lock:
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814ad4fc>] __pipe_lock fs/pipe.c:86 [inline]
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814ad4fc>] fifo_open+0x15c/0x9e0 fs/pipe.c:896

but task is already holding lock:
 (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814a7833>] 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
       [<ffffffff8270be92>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270be92>] mutex_lock_interruptible_nested+0xd2/0xcc0 kernel/locking/mutex.c:650
       [<ffffffff815e116e>] proc_pid_attr_write+0x19e/0x290 fs/proc/base.c:2524
       [<ffffffff8149171c>] __vfs_write+0x11c/0x3e0 fs/read_write.c:489
       [<ffffffff81491aea>] __kernel_write+0x10a/0x350 fs/read_write.c:511
       [<ffffffff8152d13d>] write_pipe_buf+0x15d/0x1f0 fs/splice.c:1074
       [<ffffffff8152dfd4>] splice_from_pipe_feed fs/splice.c:776 [inline]
       [<ffffffff8152dfd4>] __splice_from_pipe+0x364/0x790 fs/splice.c:901
       [<ffffffff81531099>] splice_from_pipe+0xf9/0x170 fs/splice.c:936
       [<ffffffff8153119c>] default_file_splice_write+0x3c/0x80 fs/splice.c:1086
       [<ffffffff81532261>] do_splice_from fs/splice.c:1128 [inline]
       [<ffffffff81532261>] do_splice fs/splice.c:1404 [inline]
       [<ffffffff81532261>] SYSC_splice fs/splice.c:1707 [inline]
       [<ffffffff81532261>] 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
       [<ffffffff82709ae2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff82709ae2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
       [<ffffffff814ad4fc>] __pipe_lock fs/pipe.c:86 [inline]
       [<ffffffff814ad4fc>] fifo_open+0x15c/0x9e0 fs/pipe.c:896
       [<ffffffff8148c50d>] do_dentry_open+0x38d/0xbd0 fs/open.c:749
       [<ffffffff8148fc7a>] vfs_open+0x12a/0x210 fs/open.c:862
       [<ffffffff814bff50>] do_last fs/namei.c:3269 [inline]
       [<ffffffff814bff50>] path_openat+0xc10/0x3f10 fs/namei.c:3406
       [<ffffffff814c6ee7>] do_filp_open+0x197/0x270 fs/namei.c:3440
       [<ffffffff814a29ef>] do_open_execat+0x10f/0x6f0 fs/exec.c:805
       [<ffffffff814a7f91>] do_execveat_common.isra.14+0x6a1/0x1f00 fs/exec.c:1577
       [<ffffffff814aa162>] do_execve fs/exec.c:1683 [inline]
       [<ffffffff814aa162>] SYSC_execve fs/exec.c:1764 [inline]
       [<ffffffff814aa162>] 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-executor419/2092:
 #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814a7833>] prepare_bprm_creds+0x53/0x110 fs/exec.c:1225

stack backtrace:
CPU: 0 PID: 2092 Comm: syz-executor419 Not tainted 4.4.168+ #6
 0000000000000000 f826c5ec953c17f3 ffff8800b651f4d0 ffffffff81aa636d
 ffffffff83ab92b0 ffffffff83ab92b0 ffff8800b6ee5f00 ffffffff83ab2860
 ffff8800b6ee67e8 ffff8800b651f520 ffffffff813a9599 ffff8800b6ee5f00
Call Trace:
 [<ffffffff81aa636d>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aa636d>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff813a9599>] 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
 [<ffffffff82709ae2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff82709ae2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
 [<ffffffff814ad4fc>] __pipe_lock fs/pipe.c:86 [inline]
 [<ffffffff814ad4fc>] fifo_open+0x15c/0x9e0 fs/pipe.c:896
 [<ffffffff8148c50d>] do_dentry_open+0x38d/0xbd0 fs/open.c:749
 [<ffffffff8148fc7a>] vfs_open+0x12a/0x210 fs/open.c:862
 [<ffffffff814bff50>] do_last fs/namei.c:3269 [inline]
 [<ffffffff814bff50>] path_openat+0xc10/0x3f10 fs/namei.c:3406
 [<ffffffff814c6ee7>] do_filp_open+0x197/0x270 fs/namei.c:3440
 [<ffffffff814a29ef>] do_open_execat+0x10f/0x6f0 fs/exec.c:805
 [<ffffffff814a7f91>] do_execveat_common.isra.14+0x6a1/0x1f00 fs/exec.c:1577
 [<ffffffff814aa162>] do_execve fs/exec.c:1683 [inline]
 [<ffffffff814aa162>] SYSC_execve fs/exec.c:1764 [inline]
 [<ffffffff814aa162>] SyS_execve+0x42/0x50 fs/exec.c:1759
 [<ffffffff82716635>] stub_execve+0x5/0x5 arch/x86/entry/entry_64.S:440