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SubjectRe: possible deadlock in seq_read (2)
From
syzbot has found a reproducer for the following crash on:

HEAD commit: 7ddd09fc Add linux-next specific files for 20191220
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1648763ee00000
kernel config: https://syzkaller.appspot.com/x/.config?x=f183b01c3088afc6
dashboard link: https://syzkaller.appspot.com/bug?extid=5378878b09e052edef7f
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17e706e1e00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14478885e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+5378878b09e052edef7f@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
5.5.0-rc2-next-20191220-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor877/11363 is trying to acquire lock:
ffff8880920b8520 (&p->lock){+.+.}, at: seq_read+0x71/0x1170
fs/seq_file.c:161

but task is already holding lock:
ffff8880998ac428 (sb_writers#3){.+.+}, at: file_start_write
include/linux/fs.h:2880 [inline]
ffff8880998ac428 (sb_writers#3){.+.+}, at: do_sendfile+0x9b9/0xd00
fs/read_write.c:1463

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (sb_writers#3){.+.+}:
percpu_down_read include/linux/percpu-rwsem.h:40 [inline]
__sb_start_write+0x241/0x460 fs/super.c:1674
sb_start_write include/linux/fs.h:1649 [inline]
mnt_want_write+0x3f/0xc0 fs/namespace.c:354
ovl_want_write+0x76/0xa0 fs/overlayfs/util.c:21
ovl_create_object+0xb3/0x2c0 fs/overlayfs/dir.c:596
ovl_create+0x28/0x30 fs/overlayfs/dir.c:627
lookup_open+0x12d5/0x1a90 fs/namei.c:3374
do_last fs/namei.c:3464 [inline]
path_openat+0xf2a/0x34d0 fs/namei.c:3670
do_filp_open+0x192/0x260 fs/namei.c:3700
do_sys_openat2+0x633/0x840 fs/open.c:1152
do_sys_open+0xfc/0x190 fs/open.c:1168
ksys_open include/linux/syscalls.h:1385 [inline]
__do_sys_open fs/open.c:1174 [inline]
__se_sys_open fs/open.c:1172 [inline]
__x64_sys_open+0x7e/0xc0 fs/open.c:1172
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #2 (&ovl_i_mutex_dir_key[depth]){++++}:
down_read+0x95/0x440 kernel/locking/rwsem.c:1495
inode_lock_shared include/linux/fs.h:801 [inline]
do_last fs/namei.c:3463 [inline]
path_openat+0x1c60/0x34d0 fs/namei.c:3670
do_filp_open+0x192/0x260 fs/namei.c:3700
do_open_execat+0x13b/0x6d0 fs/exec.c:861
__do_execve_file.isra.0+0x1702/0x22b0 fs/exec.c:1766
do_execveat_common fs/exec.c:1872 [inline]
do_execve fs/exec.c:1889 [inline]
__do_sys_execve fs/exec.c:1965 [inline]
__se_sys_execve fs/exec.c:1960 [inline]
__x64_sys_execve+0x8f/0xc0 fs/exec.c:1960
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #1 (&sig->cred_guard_mutex){+.+.}:
__mutex_lock_common kernel/locking/mutex.c:956 [inline]
__mutex_lock+0x156/0x13c0 kernel/locking/mutex.c:1103
mutex_lock_killable_nested+0x16/0x20 kernel/locking/mutex.c:1133
do_io_accounting+0x1f4/0x820 fs/proc/base.c:2772
proc_tgid_io_accounting+0x23/0x30 fs/proc/base.c:2821
proc_single_show+0xfd/0x1c0 fs/proc/base.c:756
seq_read+0x4ca/0x1170 fs/seq_file.c:229
do_loop_readv_writev fs/read_write.c:714 [inline]
do_loop_readv_writev fs/read_write.c:701 [inline]
do_iter_read+0x4a4/0x660 fs/read_write.c:935
vfs_readv+0xf0/0x160 fs/read_write.c:997
kernel_readv fs/splice.c:365 [inline]
default_file_splice_read+0x4fb/0xa20 fs/splice.c:422
do_splice_to+0x127/0x180 fs/splice.c:892
splice_direct_to_actor+0x320/0xa30 fs/splice.c:971
do_splice_direct+0x1da/0x2a0 fs/splice.c:1080
do_sendfile+0x597/0xd00 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+0x1dd/0x220 fs/read_write.c:1511
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (&p->lock){+.+.}:
check_prev_add kernel/locking/lockdep.c:2476 [inline]
check_prevs_add kernel/locking/lockdep.c:2581 [inline]
validate_chain kernel/locking/lockdep.c:2971 [inline]
__lock_acquire+0x2596/0x4a00 kernel/locking/lockdep.c:3955
lock_acquire+0x190/0x410 kernel/locking/lockdep.c:4485
__mutex_lock_common kernel/locking/mutex.c:956 [inline]
__mutex_lock+0x156/0x13c0 kernel/locking/mutex.c:1103
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1118
seq_read+0x71/0x1170 fs/seq_file.c:161
do_loop_readv_writev fs/read_write.c:714 [inline]
do_loop_readv_writev fs/read_write.c:701 [inline]
do_iter_read+0x4a4/0x660 fs/read_write.c:935
vfs_readv+0xf0/0x160 fs/read_write.c:997
kernel_readv fs/splice.c:365 [inline]
default_file_splice_read+0x4fb/0xa20 fs/splice.c:422
do_splice_to+0x127/0x180 fs/splice.c:892
splice_direct_to_actor+0x320/0xa30 fs/splice.c:971
do_splice_direct+0x1da/0x2a0 fs/splice.c:1080
do_sendfile+0x597/0xd00 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+0x1dd/0x220 fs/read_write.c:1511
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
&p->lock --> &ovl_i_mutex_dir_key[depth] --> sb_writers#3

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(sb_writers#3);
lock(&ovl_i_mutex_dir_key[depth]);
lock(sb_writers#3);
lock(&p->lock);

*** DEADLOCK ***

1 lock held by syz-executor877/11363:
#0: ffff8880998ac428 (sb_writers#3){.+.+}, at: file_start_write
include/linux/fs.h:2880 [inline]
#0: ffff8880998ac428 (sb_writers#3){.+.+}, at: do_sendfile+0x9b9/0xd00
fs/read_write.c:1463

stack backtrace:
CPU: 0 PID: 11363 Comm: syz-executor877 Not tainted
5.5.0-rc2-next-20191220-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+0x197/0x210 lib/dump_stack.c:118
print_circular_bug.isra.0.cold+0x163/0x172 kernel/locking/lockdep.c:1685
check_noncircular+0x32e/0x3e0 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 kernel/locking/lockdep.c:2971 [inline]
__lock_acquire+0x2596/0x4a00 kernel/locking/lockdep.c:3955
lock_acquire+0x190/0x410 kernel/locking/lockdep.c:4485
__mutex_lock_common kernel/locking/mutex.c:956 [inline]
__mutex_lock+0x156/0x13c0 kernel/locking/mutex.c:1103
mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1118
seq_read+0x71/0x1170 fs/seq_file.c:161
do_loop_readv_writev fs/read_write.c:714 [inline]
do_loop_readv_writev fs/read_write.c:701 [inline]
do_iter_read+0x4a4/0x660 fs/read_write.c:935
vfs_readv+0xf0/0x160 fs/read_write.c:997
kernel_readv fs/splice.c:365 [inline]
default_file_splice_read+0x4fb/0xa20 fs/splice.c:422
do_splice_to+0x127/0x180 fs/splice.c:892
splice_direct_to_actor+0x320/0xa30 fs/splice.c:971
do_splice_direct+0x1da/0x2a0 fs/splice.c:1080
do_sendfile+0x597/0xd00 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+0x1dd/0x220 fs/read_write.c:1511
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x448dd9
Code: e8 9c e6 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 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 3b 05 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fc37ed06ce8 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
RAX: ffffffffffffffda RBX: 00000000006e4a08 RCX: 0000000000448dd9
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000005
RBP: 00000000006e4a00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000209 R11: 0000000000000246 R12: 00000000006e4a0c
R13: 00007ffecbcb224f R14: 00007fc37ed079c0 R15: 20c49ba5e353f7cf

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