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Subjectpossible deadlock in proc_pid_stack
From
Hello,

syzbot found the following crash on:

HEAD commit: e033e7d4 Merge branch 'dhowells' (patches from DavidH)
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1499533ee00000
kernel config: https://syzkaller.appspot.com/x/.config?x=cfbb8fa33f49f9f3
dashboard link: https://syzkaller.appspot.com/bug?extid=c84a45dcab486e41c053
compiler: clang version 10.0.0 (https://github.com/llvm/llvm-project/
c2443155a0fb245c8f17f2c1c72b6ea391e86e81)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c84a45dcab486e41c053@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
5.5.0-rc6-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor.5/21564 is trying to acquire lock:
ffff8880a29cdf10 (&sig->cred_guard_mutex){+.+.}, at: lock_trace
fs/proc/base.c:406 [inline]
ffff8880a29cdf10 (&sig->cred_guard_mutex){+.+.}, at:
proc_pid_stack+0xd9/0x200 fs/proc/base.c:450

but task is already holding lock:
ffff8880a8bb30a0 (&p->lock){+.+.}, at: seq_read+0x6b/0xdb0 fs/seq_file.c:161

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&p->lock){+.+.}:
lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4485
__mutex_lock_common+0x16e/0x2f30 kernel/locking/mutex.c:956
__mutex_lock kernel/locking/mutex.c:1103 [inline]
mutex_lock_nested+0x1b/0x30 kernel/locking/mutex.c:1118
seq_read+0x6b/0xdb0 fs/seq_file.c:161
proc_reg_read+0x1d5/0x2e0 fs/proc/inode.c:223
do_loop_readv_writev fs/read_write.c:714 [inline]
do_iter_read+0x4a2/0x5b0 fs/read_write.c:935
vfs_readv+0xc2/0x120 fs/read_write.c:997
kernel_readv fs/splice.c:365 [inline]
default_file_splice_read+0x579/0xa40 fs/splice.c:422
do_splice_to fs/splice.c:892 [inline]
splice_direct_to_actor+0x3c9/0xb90 fs/splice.c:971
do_splice_direct+0x200/0x330 fs/splice.c:1080
do_sendfile+0x7e4/0xfd0 fs/read_write.c:1464
__do_sys_sendfile64 fs/read_write.c:1525 [inline]
__se_sys_sendfile64 fs/read_write.c:1511 [inline]
__x64_sys_sendfile64+0x176/0x1b0 fs/read_write.c:1511
do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #2 (sb_writers#3){.+.+}:
lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4485
percpu_down_read include/linux/percpu-rwsem.h:40 [inline]
__sb_start_write+0x189/0x420 fs/super.c:1674
sb_start_write include/linux/fs.h:1650 [inline]
mnt_want_write+0x4a/0xa0 fs/namespace.c:354
ovl_want_write+0x77/0x80 fs/overlayfs/util.c:21
ovl_create_object+0xaf/0x2d0 fs/overlayfs/dir.c:596
ovl_create+0x29/0x30 fs/overlayfs/dir.c:627
lookup_open fs/namei.c:3241 [inline]
do_last fs/namei.c:3331 [inline]
path_openat+0x234d/0x4250 fs/namei.c:3537
do_filp_open+0x192/0x3d0 fs/namei.c:3567
do_sys_open+0x29f/0x560 fs/open.c:1097
ksys_open include/linux/syscalls.h:1383 [inline]
__do_sys_creat fs/open.c:1155 [inline]
__se_sys_creat fs/open.c:1153 [inline]
__x64_sys_creat+0x65/0x70 fs/open.c:1153
do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #1 (&ovl_i_mutex_dir_key[depth]#2){++++}:
lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4485
down_read+0x39/0x50 kernel/locking/rwsem.c:1495
inode_lock_shared include/linux/fs.h:801 [inline]
do_last fs/namei.c:3330 [inline]
path_openat+0xa7a/0x4250 fs/namei.c:3537
do_filp_open+0x192/0x3d0 fs/namei.c:3567
do_open_execat+0xff/0x620 fs/exec.c:856
__do_execve_file+0x758/0x1cc0 fs/exec.c:1761
do_execveat_common fs/exec.c:1867 [inline]
do_execve fs/exec.c:1884 [inline]
__do_sys_execve fs/exec.c:1960 [inline]
__se_sys_execve fs/exec.c:1955 [inline]
__x64_sys_execve+0x94/0xb0 fs/exec.c:1955
do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (&sig->cred_guard_mutex){+.+.}:
check_prev_add kernel/locking/lockdep.c:2476 [inline]
check_prevs_add kernel/locking/lockdep.c:2581 [inline]
validate_chain+0x1507/0x7be0 kernel/locking/lockdep.c:2971
__lock_acquire+0xc5a/0x1bc0 kernel/locking/lockdep.c:3955
lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4485
__mutex_lock_common+0x16e/0x2f30 kernel/locking/mutex.c:956
__mutex_lock kernel/locking/mutex.c:1103 [inline]
mutex_lock_killable_nested+0x1b/0x30 kernel/locking/mutex.c:1133
lock_trace fs/proc/base.c:406 [inline]
proc_pid_stack+0xd9/0x200 fs/proc/base.c:450
proc_single_show+0xe7/0x180 fs/proc/base.c:756
seq_read+0x4d8/0xdb0 fs/seq_file.c:229
do_loop_readv_writev fs/read_write.c:714 [inline]
do_iter_read+0x4a2/0x5b0 fs/read_write.c:935
vfs_readv fs/read_write.c:997 [inline]
do_preadv+0x178/0x290 fs/read_write.c:1089
__do_sys_preadv fs/read_write.c:1139 [inline]
__se_sys_preadv fs/read_write.c:1134 [inline]
__x64_sys_preadv+0x9e/0xb0 fs/read_write.c:1134
do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
&sig->cred_guard_mutex --> sb_writers#3 --> &p->lock

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&p->lock);
lock(sb_writers#3);
lock(&p->lock);
lock(&sig->cred_guard_mutex);

*** DEADLOCK ***

1 lock held by syz-executor.5/21564:
#0: ffff8880a8bb30a0 (&p->lock){+.+.}, at: seq_read+0x6b/0xdb0
fs/seq_file.c:161

stack backtrace:
CPU: 1 PID: 21564 Comm: syz-executor.5 Not tainted 5.5.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1fb/0x318 lib/dump_stack.c:118
print_circular_bug+0xc3f/0xe70 kernel/locking/lockdep.c:1685
check_noncircular+0x206/0x3a0 kernel/locking/lockdep.c:1809
check_prev_add kernel/locking/lockdep.c:2476 [inline]
check_prevs_add kernel/locking/lockdep.c:2581 [inline]
validate_chain+0x1507/0x7be0 kernel/locking/lockdep.c:2971
__lock_acquire+0xc5a/0x1bc0 kernel/locking/lockdep.c:3955
lock_acquire+0x154/0x250 kernel/locking/lockdep.c:4485
__mutex_lock_common+0x16e/0x2f30 kernel/locking/mutex.c:956
__mutex_lock kernel/locking/mutex.c:1103 [inline]
mutex_lock_killable_nested+0x1b/0x30 kernel/locking/mutex.c:1133
lock_trace fs/proc/base.c:406 [inline]
proc_pid_stack+0xd9/0x200 fs/proc/base.c:450
proc_single_show+0xe7/0x180 fs/proc/base.c:756
seq_read+0x4d8/0xdb0 fs/seq_file.c:229
do_loop_readv_writev fs/read_write.c:714 [inline]
do_iter_read+0x4a2/0x5b0 fs/read_write.c:935
vfs_readv fs/read_write.c:997 [inline]
do_preadv+0x178/0x290 fs/read_write.c:1089
__do_sys_preadv fs/read_write.c:1139 [inline]
__se_sys_preadv fs/read_write.c:1134 [inline]
__x64_sys_preadv+0x9e/0xb0 fs/read_write.c:1134
do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45af49
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f893d1e0c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000127
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 000000000045af49
RDX: 0000000000000332 RSI: 00000000200017c0 RDI: 0000000000000004
RBP: 000000000075bfc8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f893d1e16d4
R13: 00000000004c954e R14: 00000000004e2220 R15: 00000000ffffffff


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

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