syzbot

======================================================
[ INFO: possible circular locking dependency detected ]
4.4.174+ #17 Not tainted
-------------------------------------------------------
syz-executor.4/5691 is trying to acquire lock:
 (&sig->cred_guard_mutex){+.+.+.}, at: [<ffffffff810ca65f>] mm_access+0x4f/0x140 kernel/fork.c:857

but task is already holding lock:
 (&p->lock){+.+.+.}, at: [<ffffffff81507c66>] seq_read+0xd6/0x1240 fs/seq_file.c:178

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

       [<ffffffff81205f6e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff8270c191>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270c191>] mutex_lock_nested+0xc1/0xb80 kernel/locking/mutex.c:621
       [<ffffffff81507c66>] seq_read+0xd6/0x1240 fs/seq_file.c:178
       [<ffffffff81496556>] __vfs_read+0x116/0x3c0 fs/read_write.c:434
       [<ffffffff81498264>] vfs_read+0x134/0x360 fs/read_write.c:456
       [<ffffffff814a83ec>] kernel_read+0x11c/0x1f0 fs/exec.c:854
       [<ffffffff814ab6d7>] prepare_binprm fs/exec.c:1400 [inline]
       [<ffffffff814ab6d7>] prepare_binprm+0x577/0x770 fs/exec.c:1387
       [<ffffffff814ad996>] do_execveat_common.isra.0+0xd86/0x1e90 fs/exec.c:1620
       [<ffffffff814af55b>] compat_do_execveat fs/exec.c:1726 [inline]
       [<ffffffff814af55b>] C_SYSC_execveat fs/exec.c:1796 [inline]
       [<ffffffff814af55b>] compat_SyS_execveat+0x5b/0x70 fs/exec.c:1788
       [<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
       [<ffffffff8271a350>] sysenter_flags_fixed+0xd/0x1a

       [<ffffffff81202d86>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
       [<ffffffff81202d86>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
       [<ffffffff81202d86>] validate_chain kernel/locking/lockdep.c:2144 [inline]
       [<ffffffff81202d86>] __lock_acquire+0x37d6/0x4f50 kernel/locking/lockdep.c:3213
       [<ffffffff81205f6e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
       [<ffffffff8270d8a2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
       [<ffffffff8270d8a2>] mutex_lock_killable_nested+0xd2/0xd00 kernel/locking/mutex.c:641
       [<ffffffff810ca65f>] mm_access+0x4f/0x140 kernel/fork.c:857
       [<ffffffff815e9d55>] proc_pid_auxv+0x25/0xd0 fs/proc/base.c:409
       [<ffffffff815e2e66>] proc_single_show+0xf6/0x160 fs/proc/base.c:805
       [<ffffffff8150805d>] seq_read+0x4cd/0x1240 fs/seq_file.c:240
       [<ffffffff81497088>] do_loop_readv_writev+0x148/0x1e0 fs/read_write.c:682
       [<ffffffff81498ee3>] do_readv_writev+0x573/0x6e0 fs/read_write.c:812
       [<ffffffff814990ca>] vfs_readv+0x7a/0xb0 fs/read_write.c:836
       [<ffffffff8153601c>] kernel_readv fs/splice.c:586 [inline]
       [<ffffffff8153601c>] default_file_splice_read+0x3ac/0x8b0 fs/splice.c:662
       [<ffffffff815321ff>] do_splice_to+0xff/0x160 fs/splice.c:1154
       [<ffffffff815324a9>] splice_direct_to_actor+0x249/0x850 fs/splice.c:1226
       [<ffffffff81532c55>] do_splice_direct+0x1a5/0x260 fs/splice.c:1337
       [<ffffffff8149a2fd>] do_sendfile+0x4ed/0xba0 fs/read_write.c:1229
       [<ffffffff8149c474>] C_SYSC_sendfile fs/read_write.c:1311 [inline]
       [<ffffffff8149c474>] compat_SyS_sendfile+0x144/0x160 fs/read_write.c:1294
       [<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
       [<ffffffff8271a350>] sysenter_flags_fixed+0xd/0x1a

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&p->lock);
                               lock(&sig->cred_guard_mutex);
                               lock(&p->lock);
  lock(&sig->cred_guard_mutex);

 *** DEADLOCK ***

2 locks held by syz-executor.4/5691:
 #0:  (sb_writers#7){.+.+.+}, at: [<ffffffff8149a6b6>] file_start_write include/linux/fs.h:2543 [inline]
 #0:  (sb_writers#7){.+.+.+}, at: [<ffffffff8149a6b6>] do_sendfile+0x8a6/0xba0 fs/read_write.c:1228
 #1:  (&p->lock){+.+.+.}, at: [<ffffffff81507c66>] seq_read+0xd6/0x1240 fs/seq_file.c:178

stack backtrace:
CPU: 1 PID: 5691 Comm: syz-executor.4 Not tainted 4.4.174+ #17
 0000000000000000 82634e633d4c7e58 ffff8801d9046e70 ffffffff81aad1a1
 ffffffff84057a80 ffff8801c19a2f80 ffffffff83ab66b0 ffffffff83ab8000
 ffffffff83ab66b0 ffff8801d9046ec0 ffffffff813abcda ffff8801d9046fa0
Call Trace:
 [<ffffffff81aad1a1>] __dump_stack lib/dump_stack.c:15 [inline]
 [<ffffffff81aad1a1>] dump_stack+0xc1/0x120 lib/dump_stack.c:51
 [<ffffffff813abcda>] print_circular_bug.cold+0x2f7/0x44e kernel/locking/lockdep.c:1226
 [<ffffffff81202d86>] check_prev_add kernel/locking/lockdep.c:1853 [inline]
 [<ffffffff81202d86>] check_prevs_add kernel/locking/lockdep.c:1958 [inline]
 [<ffffffff81202d86>] validate_chain kernel/locking/lockdep.c:2144 [inline]
 [<ffffffff81202d86>] __lock_acquire+0x37d6/0x4f50 kernel/locking/lockdep.c:3213
 [<ffffffff81205f6e>] lock_acquire+0x15e/0x450 kernel/locking/lockdep.c:3592
 [<ffffffff8270d8a2>] __mutex_lock_common kernel/locking/mutex.c:521 [inline]
 [<ffffffff8270d8a2>] mutex_lock_killable_nested+0xd2/0xd00 kernel/locking/mutex.c:641
 [<ffffffff810ca65f>] mm_access+0x4f/0x140 kernel/fork.c:857
 [<ffffffff815e9d55>] proc_pid_auxv+0x25/0xd0 fs/proc/base.c:409
 [<ffffffff815e2e66>] proc_single_show+0xf6/0x160 fs/proc/base.c:805
 [<ffffffff8150805d>] seq_read+0x4cd/0x1240 fs/seq_file.c:240
 [<ffffffff81497088>] do_loop_readv_writev+0x148/0x1e0 fs/read_write.c:682
 [<ffffffff81498ee3>] do_readv_writev+0x573/0x6e0 fs/read_write.c:812
 [<ffffffff814990ca>] vfs_readv+0x7a/0xb0 fs/read_write.c:836
 [<ffffffff8153601c>] kernel_readv fs/splice.c:586 [inline]
 [<ffffffff8153601c>] default_file_splice_read+0x3ac/0x8b0 fs/splice.c:662
 [<ffffffff815321ff>] do_splice_to+0xff/0x160 fs/splice.c:1154
 [<ffffffff815324a9>] splice_direct_to_actor+0x249/0x850 fs/splice.c:1226
 [<ffffffff81532c55>] do_splice_direct+0x1a5/0x260 fs/splice.c:1337
 [<ffffffff8149a2fd>] do_sendfile+0x4ed/0xba0 fs/read_write.c:1229
 [<ffffffff8149c474>] C_SYSC_sendfile fs/read_write.c:1311 [inline]
 [<ffffffff8149c474>] compat_SyS_sendfile+0x144/0x160 fs/read_write.c:1294
 [<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
 [<ffffffff8271a350>] sysenter_flags_fixed+0xd/0x1a