======================================================
[ INFO: possible circular locking dependency detected ]
4.4.166+ #1 Not tainted
-------------------------------------------------------
syz-executor995/2071 is trying to acquire lock:
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814ad4cc>] __pipe_lock fs/pipe.c:86 [inline]
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814ad4cc>] fifo_open+0x15c/0x9e0 fs/pipe.c:896

but task is already holding lock:
 (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814a7803>] prepare_bprm_creds+0x53/0x110 fs/exec.c:1221

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       [<ffffffff812037de>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff8270b882>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270b882>] mutex_lock_interruptible_nested+0xd2/0xcc0 kernel/locking/mutex.c:650
       [<ffffffff815e10ee>] proc_pid_attr_write+0x19e/0x290 fs/proc/base.c:2519
       [<ffffffff814916dc>] __vfs_write+0x11c/0x3e0 fs/read_write.c:489
       [<ffffffff81491aaa>] __kernel_write+0x10a/0x350 fs/read_write.c:511
       [<ffffffff8152d11d>] write_pipe_buf+0x15d/0x1f0 fs/splice.c:1074
       [<ffffffff8152dfb4>] splice_from_pipe_feed fs/splice.c:776 [inline]
       [<ffffffff8152dfb4>] __splice_from_pipe+0x364/0x790 fs/splice.c:901
       [<ffffffff81531079>] splice_from_pipe+0xf9/0x170 fs/splice.c:936
       [<ffffffff8153117c>] default_file_splice_write+0x3c/0x80 fs/splice.c:1086
       [<ffffffff81532241>] do_splice_from fs/splice.c:1128 [inline]
       [<ffffffff81532241>] do_splice fs/splice.c:1404 [inline]
       [<ffffffff81532241>] SYSC_splice fs/splice.c:1707 [inline]
       [<ffffffff81532241>] SyS_splice+0xde1/0x1430 fs/splice.c:1690
       [<ffffffff82715ce1>] entry_SYSCALL_64_fastpath+0x1e/0x9a

       [<ffffffff81200314>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
       [<ffffffff81200314>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
       [<ffffffff81200314>] validate_chain kernel/locking/lockdep.c:2144 [inline]
       [<ffffffff81200314>] __lock_acquire+0x3cd4/0x5530 kernel/locking/lockdep.c:3213
       [<ffffffff812037de>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff827094d2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff827094d2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
       [<ffffffff814ad4cc>] __pipe_lock fs/pipe.c:86 [inline]
       [<ffffffff814ad4cc>] fifo_open+0x15c/0x9e0 fs/pipe.c:896
       [<ffffffff8148c4cd>] do_dentry_open+0x38d/0xbd0 fs/open.c:749
       [<ffffffff8148fc3a>] vfs_open+0x12a/0x210 fs/open.c:862
       [<ffffffff814bff20>] do_last fs/namei.c:3269 [inline]
       [<ffffffff814bff20>] path_openat+0xc10/0x3f10 fs/namei.c:3406
       [<ffffffff814c6eb7>] do_filp_open+0x197/0x270 fs/namei.c:3440
       [<ffffffff814a29af>] do_open_execat+0x10f/0x6f0 fs/exec.c:800
       [<ffffffff814a7f61>] do_execveat_common.isra.14+0x6a1/0x1f00 fs/exec.c:1573
       [<ffffffff814aa132>] do_execve fs/exec.c:1679 [inline]
       [<ffffffff814aa132>] SYSC_execve fs/exec.c:1760 [inline]
       [<ffffffff814aa132>] SyS_execve+0x42/0x50 fs/exec.c:1755
       [<ffffffff82716035>] 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-executor995/2071:
 #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814a7803>] prepare_bprm_creds+0x53/0x110 fs/exec.c:1221

stack backtrace:
CPU: 1 PID: 2071 Comm: syz-executor995 Not tainted 4.4.166+ #1
 0000000000000000 b758b28b139da398 ffff8801d413f4d0 ffffffff81aa62ad
 ffffffff83ab7610 ffffffff83ab7610 ffff8801d4f92f80 ffffffff83ab0860
 ffff8801d4f93868 ffff8801d413f520 ffffffff813a9549 ffff8801d4f92f80
Call Trace:
 [<ffffffff81aa62ad>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aa62ad>] dump_stack+0xc1/0x124 lib/dump_stack.c:51
 [<ffffffff813a9549>] print_circular_bug.cold.31+0x2f6/0x435 kernel/locking/lockdep.c:1226
 [<ffffffff81200314>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
 [<ffffffff81200314>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
 [<ffffffff81200314>] validate_chain kernel/locking/lockdep.c:2144 [inline]
 [<ffffffff81200314>] __lock_acquire+0x3cd4/0x5530 kernel/locking/lockdep.c:3213
 [<ffffffff812037de>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
 [<ffffffff827094d2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff827094d2>] mutex_lock_nested+0xc2/0xb60 kernel/locking/mutex.c:621
 [<ffffffff814ad4cc>] __pipe_lock fs/pipe.c:86 [inline]
 [<ffffffff814ad4cc>] fifo_open+0x15c/0x9e0 fs/pipe.c:896
 [<ffffffff8148c4cd>] do_dentry_open+0x38d/0xbd0 fs/open.c:749
 [<ffffffff8148fc3a>] vfs_open+0x12a/0x210 fs/open.c:862
 [<ffffffff814bff20>] do_last fs/namei.c:3269 [inline]
 [<ffffffff814bff20>] path_openat+0xc10/0x3f10 fs/namei.c:3406
 [<ffffffff814c6eb7>] do_filp_open+0x197/0x270 fs/namei.c:3440
 [<ffffffff814a29af>] do_open_execat+0x10f/0x6f0 fs/exec.c:800
 [<ffffffff814a7f61>] do_execveat_common.isra.14+0x6a1/0x1f00 fs/exec.c:1573
 [<ffffffff814aa132>] do_execve fs/exec.c:1679 [inline]
 [<ffffffff814aa132>] SYSC_execve fs/exec.c:1760 [inline]
 [<ffffffff814aa132>] SyS_execve+0x42/0x50 fs/exec.c:1755
 [<ffffffff82716035>] stub_execve+0x5/0x5 arch/x86/entry/entry_64.S:440