possible deadlock in sk_diag_fill

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: c1c07416cdd4 Merge tag 'kbuild-fixes-v4.17' of git://git.k..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12164c97800000
kernel config: https://syzkaller.appspot.com/x/.config?x=5a1dc06635c10d27
dashboard link: https://syzkaller.appspot.com/bug?extid=c1872be62e587eae9669
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
userspace arch: i386

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+c1872b...@syzkaller.appspotmail.com


======================================================
WARNING: possible circular locking dependency detected
4.17.0-rc3+ #59 Not tainted
------------------------------------------------------
syz-executor1/25282 is trying to acquire lock:
000000004fddf743 (&(&u->lock)->rlock/1){+.+.}, at: sk_diag_dump_icons
net/unix/diag.c:82 [inline]
000000004fddf743 (&(&u->lock)->rlock/1){+.+.}, at:
sk_diag_fill.isra.5+0xa43/0x10d0 net/unix/diag.c:144

but task is already holding lock:
00000000b6895645 (rlock-AF_UNIX){+.+.}, at: spin_lock
include/linux/spinlock.h:310 [inline]
00000000b6895645 (rlock-AF_UNIX){+.+.}, at: sk_diag_dump_icons
net/unix/diag.c:64 [inline]
00000000b6895645 (rlock-AF_UNIX){+.+.}, at:
sk_diag_fill.isra.5+0x94e/0x10d0 net/unix/diag.c:144

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #1 (rlock-AF_UNIX){+.+.}:
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0x96/0xc0 kernel/locking/spinlock.c:152
skb_queue_tail+0x26/0x150 net/core/skbuff.c:2900
unix_dgram_sendmsg+0xf77/0x1730 net/unix/af_unix.c:1797
sock_sendmsg_nosec net/socket.c:629 [inline]
sock_sendmsg+0xd5/0x120 net/socket.c:639
___sys_sendmsg+0x525/0x940 net/socket.c:2117
__sys_sendmmsg+0x3bb/0x6f0 net/socket.c:2205
__compat_sys_sendmmsg net/compat.c:770 [inline]
__do_compat_sys_sendmmsg net/compat.c:777 [inline]
__se_compat_sys_sendmmsg net/compat.c:774 [inline]
__ia32_compat_sys_sendmmsg+0x9f/0x100 net/compat.c:774
do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139

-> #0 (&(&u->lock)->rlock/1){+.+.}:
lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
_raw_spin_lock_nested+0x28/0x40 kernel/locking/spinlock.c:354
sk_diag_dump_icons net/unix/diag.c:82 [inline]
sk_diag_fill.isra.5+0xa43/0x10d0 net/unix/diag.c:144
sk_diag_dump net/unix/diag.c:178 [inline]
unix_diag_dump+0x35f/0x550 net/unix/diag.c:206
netlink_dump+0x507/0xd20 net/netlink/af_netlink.c:2226
__netlink_dump_start+0x51a/0x780 net/netlink/af_netlink.c:2323
netlink_dump_start include/linux/netlink.h:214 [inline]
unix_diag_handler_dump+0x3f4/0x7b0 net/unix/diag.c:307
__sock_diag_cmd net/core/sock_diag.c:230 [inline]
sock_diag_rcv_msg+0x2e0/0x3d0 net/core/sock_diag.c:261
netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2448
sock_diag_rcv+0x2a/0x40 net/core/sock_diag.c:272
netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
netlink_unicast+0x58b/0x740 net/netlink/af_netlink.c:1336
netlink_sendmsg+0x9f0/0xfa0 net/netlink/af_netlink.c:1901
sock_sendmsg_nosec net/socket.c:629 [inline]
sock_sendmsg+0xd5/0x120 net/socket.c:639
sock_write_iter+0x35a/0x5a0 net/socket.c:908
call_write_iter include/linux/fs.h:1784 [inline]
new_sync_write fs/read_write.c:474 [inline]
__vfs_write+0x64d/0x960 fs/read_write.c:487
vfs_write+0x1f8/0x560 fs/read_write.c:549
ksys_write+0xf9/0x250 fs/read_write.c:598
__do_sys_write fs/read_write.c:610 [inline]
__se_sys_write fs/read_write.c:607 [inline]
__ia32_sys_write+0x71/0xb0 fs/read_write.c:607
do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(rlock-AF_UNIX);
lock(&(&u->lock)->rlock/1);
lock(rlock-AF_UNIX);
lock(&(&u->lock)->rlock/1);

*** DEADLOCK ***

5 locks held by syz-executor1/25282:
#0: 000000003919e1bd (sock_diag_mutex){+.+.}, at: sock_diag_rcv+0x1b/0x40
net/core/sock_diag.c:271
#1: 000000004f328d3e (sock_diag_table_mutex){+.+.}, at: __sock_diag_cmd
net/core/sock_diag.c:225 [inline]
#1: 000000004f328d3e (sock_diag_table_mutex){+.+.}, at:
sock_diag_rcv_msg+0x169/0x3d0 net/core/sock_diag.c:261
#2: 000000004cc04dbb (nlk_cb_mutex-SOCK_DIAG){+.+.}, at:
netlink_dump+0x98/0xd20 net/netlink/af_netlink.c:2182
#3: 00000000accdef41 (unix_table_lock){+.+.}, at: spin_lock
include/linux/spinlock.h:310 [inline]
#3: 00000000accdef41 (unix_table_lock){+.+.}, at:
unix_diag_dump+0x10a/0x550 net/unix/diag.c:192
#4: 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: spin_lock
include/linux/spinlock.h:310 [inline]
#4: 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: sk_diag_dump_icons
net/unix/diag.c:64 [inline]
#4: 00000000b6895645 (rlock-AF_UNIX){+.+.}, at:
sk_diag_fill.isra.5+0x94e/0x10d0 net/unix/diag.c:144

stack backtrace:
CPU: 1 PID: 25282 Comm: syz-executor1 Not tainted 4.17.0-rc3+ #59
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+0x1b9/0x294 lib/dump_stack.c:113
print_circular_bug.isra.36.cold.54+0x1bd/0x27d
kernel/locking/lockdep.c:1223
check_prev_add kernel/locking/lockdep.c:1863 [inline]
check_prevs_add kernel/locking/lockdep.c:1976 [inline]
validate_chain kernel/locking/lockdep.c:2417 [inline]
__lock_acquire+0x343e/0x5140 kernel/locking/lockdep.c:3431
lock_acquire+0x1dc/0x520 kernel/locking/lockdep.c:3920
_raw_spin_lock_nested+0x28/0x40 kernel/locking/spinlock.c:354
sk_diag_dump_icons net/unix/diag.c:82 [inline]
sk_diag_fill.isra.5+0xa43/0x10d0 net/unix/diag.c:144
sk_diag_dump net/unix/diag.c:178 [inline]
unix_diag_dump+0x35f/0x550 net/unix/diag.c:206
netlink_dump+0x507/0xd20 net/netlink/af_netlink.c:2226
__netlink_dump_start+0x51a/0x780 net/netlink/af_netlink.c:2323
netlink_dump_start include/linux/netlink.h:214 [inline]
unix_diag_handler_dump+0x3f4/0x7b0 net/unix/diag.c:307
__sock_diag_cmd net/core/sock_diag.c:230 [inline]
sock_diag_rcv_msg+0x2e0/0x3d0 net/core/sock_diag.c:261
netlink_rcv_skb+0x172/0x440 net/netlink/af_netlink.c:2448
sock_diag_rcv+0x2a/0x40 net/core/sock_diag.c:272
netlink_unicast_kernel net/netlink/af_netlink.c:1310 [inline]
netlink_unicast+0x58b/0x740 net/netlink/af_netlink.c:1336
netlink_sendmsg+0x9f0/0xfa0 net/netlink/af_netlink.c:1901
sock_sendmsg_nosec net/socket.c:629 [inline]
sock_sendmsg+0xd5/0x120 net/socket.c:639
sock_write_iter+0x35a/0x5a0 net/socket.c:908
call_write_iter include/linux/fs.h:1784 [inline]
new_sync_write fs/read_write.c:474 [inline]
__vfs_write+0x64d/0x960 fs/read_write.c:487
vfs_write+0x1f8/0x560 fs/read_write.c:549
ksys_write+0xf9/0x250 fs/read_write.c:598
__do_sys_write fs/read_write.c:610 [inline]
__se_sys_write fs/read_write.c:607 [inline]
__ia32_sys_write+0x71/0xb0 fs/read_write.c:607
do_syscall_32_irqs_on arch/x86/entry/common.c:323 [inline]
do_fast_syscall_32+0x345/0xf9b arch/x86/entry/common.c:394
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7f8ccb9
RSP: 002b:00000000f5f880ac EFLAGS: 00000282 ORIG_RAX: 0000000000000004
RAX: ffffffffffffffda RBX: 0000000000000017 RCX: 000000002058bfe4
RDX: 0000000000000029 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000296 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000


---
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 syzk...@googlegroups.com.

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is
merged
into any tree, please reply to this email with:
#syz fix: exact-commit-title
To mark this as a duplicate of another syzbot report, please reply with:
#syz dup: exact-subject-of-another-report
If it's a one-off invalid bug report, please reply with:
#syz invalid
Note: if the crash happens again, it will cause creation of a new bug
report.
Note: all commands must start from beginning of the line in the email body.

[Andrei Vagin]

In the code, we have a comment which explains why it is safe to take this lock

/*
* The state lock is outer for the same sk's
* queue lock. With the other's queue locked it's
* OK to lock the state.
*/
unix_state_lock_nested(req);

It is a question how to explain this to lockdep.

[Dmitry Vyukov]

Do I understand it correctly that (&u->lock)->rlock associated with
AF_UNIX is locked under rlock-AF_UNIX, and then rlock-AF_UNIX is
locked under (&u->lock)->rlock associated with AF_NETLINK? If so, I
think we need to split (&u->lock)->rlock by family too, so that we
have u->lock-AF_UNIX and u->lock-AF_NETLINK.

> --
> You received this message because you are subscribed to the Google Groups "syzkaller-bugs" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to syzkaller-bug...@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/syzkaller-bugs/20180511183358.GA1492%40outlook.office365.com.
> For more options, visit https://groups.google.com/d/optout.

[Andrei Vagin]

I think here is another problem. lockdep woried about
sk->sk_receive_queue vs unix_sk(s)->lock.

sk_diag_dump_icons() takes sk->sk_receive_queue and then
unix_sk(s)->lock.

unix_dgram_sendmsg takes unix_sk(sk)->lock and then sk->sk_receive_queue.

sk_diag_dump_icons() takes locks for two different sockets, but
unix_dgram_sendmsg() takes locks for one socket.

sk_diag_dump_icons
if (sk->sk_state == TCP_LISTEN) {
spin_lock(&sk->sk_receive_queue.lock);
skb_queue_walk(&sk->sk_receive_queue, skb) {
unix_state_lock_nested(req);
spin_lock_nested(&unix_sk(s)->lock,


unix_dgram_sendmsg
unix_state_lock(other)
spin_lock(&unix_sk(s)->lock)
skb_queue_tail(&other->sk_receive_queue, skb);
spin_lock_irqsave(&list->lock, flags);

[Dmitry Vyukov]

Do you mean the following?
There is socket 1 with state lock (S1) and queue lock (Q2), and socket
2 with state lock (S2) and queue lock (Q2). unix_dgram_sendmsg lock
S1->Q1. And sk_diag_dump_icons locks Q1->S2.
If yes, then this looks pretty much as deadlock. Consider that 2
unix_dgram_sendmsg in 2 different threads lock S1 and S2 respectively.
Now 2 sk_diag_dump_icons in 2 different threads lock Q1 and Q2
respectively. Now sk_diag_dump_icons want to lock S's, and
unix_dgram_sendmsg want to lock Q's. Nobody can proceed.

[Andrei Vagin]

Q1 and S1 belongs to a listen socket, so they can't be taken from
unix_dgram_sendmsg().

[Dmitry Vyukov]

Should we then split Q1/S1 for listening and data sockets? I don't
know it lockdep allows changing lock class on the fly, though. Always
wondered if there was a single reason to mix listening and data
sockets into a single thing on API level...

[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.