======================================================
[ INFO: possible circular locking dependency detected ]
4.4.170+ #4 Not tainted
-------------------------------------------------------
syz-executor174/2083 is trying to acquire lock:
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814b224d>] __pipe_lock fs/pipe.c:86 [inline]
 (&pipe->mutex/1){+.+.+.}, at: [<ffffffff814b224d>] fifo_open+0x15d/0xa00 fs/pipe.c:896

but task is already holding lock:
 (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814ac495>] 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:

       [<ffffffff81205d5e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff8270d5d2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270d5d2>] mutex_lock_interruptible_nested+0xd2/0xce0 kernel/locking/mutex.c:650
       [<ffffffff815e78c8>] proc_pid_attr_write+0x1a8/0x2a0 fs/proc/base.c:2524
       [<ffffffff814962c6>] __vfs_write+0x116/0x3d0 fs/read_write.c:489
       [<ffffffff81496692>] __kernel_write+0x112/0x370 fs/read_write.c:511
       [<ffffffff815327bd>] write_pipe_buf+0x15d/0x1f0 fs/splice.c:1074
       [<ffffffff815334be>] splice_from_pipe_feed fs/splice.c:776 [inline]
       [<ffffffff815334be>] __splice_from_pipe+0x37e/0x7a0 fs/splice.c:901
       [<ffffffff81536538>] splice_from_pipe+0x108/0x170 fs/splice.c:936
       [<ffffffff8153662c>] default_file_splice_write+0x3c/0x80 fs/splice.c:1086
       [<ffffffff81537681>] do_splice_from fs/splice.c:1128 [inline]
       [<ffffffff81537681>] do_splice fs/splice.c:1404 [inline]
       [<ffffffff81537681>] SYSC_splice fs/splice.c:1707 [inline]
       [<ffffffff81537681>] SyS_splice+0xd71/0x13a0 fs/splice.c:1690
       [<ffffffff8100603d>] do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
       [<ffffffff8100603d>] do_fast_syscall_32+0x32d/0xa90 arch/x86/entry/common.c:397
       [<ffffffff82719110>] sysenter_flags_fixed+0xd/0x1a

       [<ffffffff81202b76>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
       [<ffffffff81202b76>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
       [<ffffffff81202b76>] validate_chain kernel/locking/lockdep.c:2144 [inline]
       [<ffffffff81202b76>] __lock_acquire+0x37d6/0x4f50 kernel/locking/lockdep.c:3213
       [<ffffffff81205d5e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff8270b1c1>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270b1c1>] mutex_lock_nested+0xc1/0xb80 kernel/locking/mutex.c:621
       [<ffffffff814b224d>] __pipe_lock fs/pipe.c:86 [inline]
       [<ffffffff814b224d>] fifo_open+0x15d/0xa00 fs/pipe.c:896
       [<ffffffff8149101f>] do_dentry_open+0x38f/0xbd0 fs/open.c:749
       [<ffffffff8149480b>] vfs_open+0x10b/0x210 fs/open.c:862
       [<ffffffff814c572f>] do_last fs/namei.c:3269 [inline]
       [<ffffffff814c572f>] path_openat+0x136f/0x4470 fs/namei.c:3406
       [<ffffffff814cc401>] do_filp_open+0x1a1/0x270 fs/namei.c:3440
       [<ffffffff814a75dc>] do_open_execat+0x10c/0x6e0 fs/exec.c:805
       [<ffffffff814acc56>] do_execveat_common.isra.0+0x6f6/0x1e90 fs/exec.c:1577
       [<ffffffff814aee38>] compat_do_execve fs/exec.c:1710 [inline]
       [<ffffffff814aee38>] C_SYSC_execve fs/exec.c:1785 [inline]
       [<ffffffff814aee38>] compat_SyS_execve+0x48/0x60 fs/exec.c:1781
       [<ffffffff8100603d>] do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
       [<ffffffff8100603d>] do_fast_syscall_32+0x32d/0xa90 arch/x86/entry/common.c:397
       [<ffffffff82719110>] sysenter_flags_fixed+0xd/0x1a

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-executor174/2083:
 #0:  (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff814ac495>] prepare_bprm_creds+0x55/0x120 fs/exec.c:1225

stack backtrace:
CPU: 0 PID: 2083 Comm: syz-executor174 Not tainted 4.4.170+ #4
 0000000000000000 a387b8da4ee3c28a ffff8801d47d74c0 ffffffff81aaddc1
 ffffffff84055a80 ffff8800b7144740 ffffffff83abb100 ffffffff83ab4500
 ffffffff83abb100 ffff8801d47d7510 ffffffff813abad4 ffff8801d47d75f0
Call Trace:
 [<ffffffff81aaddc1>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aaddc1>] dump_stack+0xc1/0x120 lib/dump_stack.c:51
 [<ffffffff813abad4>] print_circular_bug.cold+0x2f7/0x44e kernel/locking/lockdep.c:1226
 [<ffffffff81202b76>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
 [<ffffffff81202b76>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
 [<ffffffff81202b76>] validate_chain kernel/locking/lockdep.c:2144 [inline]
 [<ffffffff81202b76>] __lock_acquire+0x37d6/0x4f50 kernel/locking/lockdep.c:3213
 [<ffffffff81205d5e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
 [<ffffffff8270b1c1>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff8270b1c1>] mutex_lock_nested+0xc1/0xb80 kernel/locking/mutex.c:621
 [<ffffffff814b224d>] __pipe_lock fs/pipe.c:86 [inline]
 [<ffffffff814b224d>] fifo_open+0x15d/0xa00 fs/pipe.c:896
 [<ffffffff8149101f>] do_dentry_open+0x38f/0xbd0 fs/open.c:749
 [<ffffffff8149480b>] vfs_open+0x10b/0x210 fs/open.c:862
 [<ffffffff814c572f>] do_last fs/namei.c:3269 [inline]
 [<ffffffff814c572f>] path_openat+0x136f/0x4470 fs/namei.c:3406
 [<ffffffff814cc401>] do_filp_open+0x1a1/0x270 fs/namei.c:3440
 [<ffffffff814a75dc>] do_open_execat+0x10c/0x6e0 fs/exec.c:805
 [<ffffffff814acc56>] do_execveat_common.isra.0+0x6f6/0x1e90 fs/exec.c:1577
 [<ffffffff814aee38>] compat_do_execve fs/exec.c:1710 [inline]
 [<ffffffff814aee38>] C_SYSC_execve fs/exec.c:1785 [inline]
 [<ffffffff814aee38>] compat_SyS_execve+0x48/0x60 fs/exec.c:1781
 [<ffffffff8100603d>] do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
 [<ffffffff8100603d>] do_fast_syscall_32+0x32d/0xa90 arch/x86/entry/common.c:397
 [<ffffffff82719110>] sysenter_flags_fixed+0xd/0x1a