[syzbot] possible deadlock in mptcp_close

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: e30cd812dffa selftests: net: af_unix: Fix makefile to use ..
git tree: net
console output: https://syzkaller.appspot.com/x/log.txt?x=1689d3e7300000
kernel config: https://syzkaller.appspot.com/x/.config?x=6d93fe4341f98704
dashboard link: https://syzkaller.appspot.com/bug?extid=1dd53f7a89b299d59eaf
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11adc1d7300000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14a351ab300000

The issue was bisected to:

commit 2dcb96bacce36021c2f3eaae0cef607b5bb71ede
Author: Thomas Gleixner <tg...@linutronix.de>
Date: Sat Sep 18 12:42:35 2021 +0000

net: core: Correct the sock::sk_lock.owned lockdep annotations

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=15a511f3300000
final oops: https://syzkaller.appspot.com/x/report.txt?x=17a511f3300000
console output: https://syzkaller.appspot.com/x/log.txt?x=13a511f3300000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1dd53f...@syzkaller.appspotmail.com
Fixes: 2dcb96bacce3 ("net: core: Correct the sock::sk_lock.owned lockdep annotations")

MPTCP: kernel_bind error, err=-98
============================================
WARNING: possible recursive locking detected
5.15.0-rc1-syzkaller #0 Not tainted
--------------------------------------------
syz-executor998/6520 is trying to acquire lock:
ffff8880795718a0 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x267/0x7b0 net/mptcp/protocol.c:2738

but task is already holding lock:
ffff8880787c8c60 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1612 [inline]
ffff8880787c8c60 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x23/0x7b0 net/mptcp/protocol.c:2720

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(k-sk_lock-AF_INET);
lock(k-sk_lock-AF_INET);

*** DEADLOCK ***

May be due to missing lock nesting notation

3 locks held by syz-executor998/6520:
#0: ffffffff8d176c50 (cb_lock){++++}-{3:3}, at: genl_rcv+0x15/0x40 net/netlink/genetlink.c:802
#1: ffffffff8d176d08 (genl_mutex){+.+.}-{3:3}, at: genl_lock net/netlink/genetlink.c:33 [inline]
#1: ffffffff8d176d08 (genl_mutex){+.+.}-{3:3}, at: genl_rcv_msg+0x3e0/0x580 net/netlink/genetlink.c:790
#2: ffff8880787c8c60 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1612 [inline]
#2: ffff8880787c8c60 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x23/0x7b0 net/mptcp/protocol.c:2720

stack backtrace:
CPU: 1 PID: 6520 Comm: syz-executor998 Not tainted 5.15.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
print_deadlock_bug kernel/locking/lockdep.c:2944 [inline]
check_deadlock kernel/locking/lockdep.c:2987 [inline]
validate_chain kernel/locking/lockdep.c:3776 [inline]
__lock_acquire.cold+0x149/0x3ab kernel/locking/lockdep.c:5015
lock_acquire kernel/locking/lockdep.c:5625 [inline]
lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5590
lock_sock_fast+0x36/0x100 net/core/sock.c:3229
mptcp_close+0x267/0x7b0 net/mptcp/protocol.c:2738
inet_release+0x12e/0x280 net/ipv4/af_inet.c:431
__sock_release net/socket.c:649 [inline]
sock_release+0x87/0x1b0 net/socket.c:677
mptcp_pm_nl_create_listen_socket+0x238/0x2c0 net/mptcp/pm_netlink.c:900
mptcp_nl_cmd_add_addr+0x359/0x930 net/mptcp/pm_netlink.c:1170
genl_family_rcv_msg_doit+0x228/0x320 net/netlink/genetlink.c:731
genl_family_rcv_msg net/netlink/genetlink.c:775 [inline]
genl_rcv_msg+0x328/0x580 net/netlink/genetlink.c:792
netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2504
genl_rcv+0x24/0x40 net/netlink/genetlink.c:803
netlink_unicast_kernel net/netlink/af_netlink.c:1314 [inline]
netlink_unicast+0x533/0x7d0 net/netlink/af_netlink.c:1340
netlink_sendmsg+0x86d/0xdb0 net/netlink/af_netlink.c:1929
sock_sendmsg_nosec net/socket.c:704 [inline]
sock_sendmsg+0xcf/0x120 net/socket.c:724
sock_no_sendpage+0x101/0x150 net/core/sock.c:2980
kernel_sendpage.part.0+0x1a0/0x340 net/socket.c:3504
kernel_sendpage net/socket.c:3501 [inline]
sock_sendpage+0xe5/0x140 net/socket.c:1003
pipe_to_sendpage+0x2ad/0x380 fs/splice.c:364
splice_from_pipe_feed fs/splice.c:418 [inline]
__splice_from_pipe+0x43e/0x8a0 fs/splice.c:562
splice_from_pipe fs/splice.c:597 [inline]
generic_splice_sendpage+0xd4/0x140 fs/splice.c:746
do_splice_from fs/splice.c:767 [inline]
direct_splice_actor+0x110/0x180 fs/splice.c:936
splice_direct_to_actor+0x34b/0x8c0 fs/splice.c:891
do_splice_direct+0x1b3/0x280 fs/splice.c:979
do_sendfile+0xae9/0x1240 fs/read_write.c:1249
__do_sys_sendfile64 fs/read_write.c:1314 [inline]
__se_sys_sendfile64 fs/read_write.c:1300 [inline]
__x64_sys_sendfile64+0x1cc/0x210 fs/read_write.c:1300
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f215cb69969
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 e1 14 00 00 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 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc96bb3868 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
RAX: ffffffffffffffda RBX: 00007f215cbad072 RCX: 00007f215cb69969
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000005
RBP: 0000000000000000 R08: 00007ffc96bb3a08 R09: 00007ffc96bb3a08
R10: 0000000100000002 R11: 0000000000000246 R12: 00007ffc96bb387c
R13: 431bde82d7b634db R14: 0000000000000000 R15: 0000000000000000


---
This report 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 issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this issue, for details see:
https://goo.gl/tpsmEJ#testing-patches

[Thomas Gleixner]

On Mon, Sep 20 2021 at 15:04, syzbot wrote:
> The issue was bisected to:
>
> commit 2dcb96bacce36021c2f3eaae0cef607b5bb71ede
> Author: Thomas Gleixner <tg...@linutronix.de>
> Date: Sat Sep 18 12:42:35 2021 +0000
>
> net: core: Correct the sock::sk_lock.owned lockdep annotations

Shooting the messenger...

> MPTCP: kernel_bind error, err=-98
> ============================================
> WARNING: possible recursive locking detected
> 5.15.0-rc1-syzkaller #0 Not tainted
> --------------------------------------------
> syz-executor998/6520 is trying to acquire lock:
> ffff8880795718a0 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x267/0x7b0 net/mptcp/protocol.c:2738
>
> but task is already holding lock:
> ffff8880787c8c60 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1612 [inline]
> ffff8880787c8c60 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_close+0x23/0x7b0 net/mptcp/protocol.c:2720

So this is a lock nesting issue and looking at the stack trace this
comes from:

> lock_sock_fast+0x36/0x100 net/core/sock.c:3229

which does not support lockdep nesting. So from a lockdep POV this is
recursive locking the same lock class. And it's the case I was worried
about that lockdep testing never takes the slow path. The original
lockdep annotation would have produced exactly the same splat in the
slow path case.

So it's not a new problem. It's just visible by moving the lockdep
annotations to a place where they actually can detect issues which were
not reported before.

See also https://lore.kernel.org/lkml/874kacu248.ffs@tglx/

There are two ways to address this mptcp one:

1) Teach lock_sock_fast() about lock nesting

2) Use lock_sock_nested() in mptcp_close() as that should not be
really a hotpath. See patch below.

Thanks,

tglx
---

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 2602f1386160..27ea5d4dfdf6 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2735,10 +2735,10 @@ static void mptcp_close(struct sock *sk, long timeout)
inet_csk(sk)->icsk_mtup.probe_timestamp = tcp_jiffies32;
mptcp_for_each_subflow(mptcp_sk(sk), subflow) {
struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
- bool slow = lock_sock_fast(ssk);

+ lock_sock_nested(ssk, SINGLE_DEPTH_NESTING);
sock_orphan(ssk);
- unlock_sock_fast(ssk, slow);
+ unlock_sock(ssk);
}
sock_orphan(sk);

[Paolo Abeni]

Thank you for looking into this! I agree this specific case is not
fastpath, so definitely the proposed patch LGTM.

I fear there could be other similar cases in the MPTCP code, in more
time critical paths, and perhaps there are other relevant use-case, so
I'd like to experiment too with a lock_sock_fast_nested() variant - if
I find enough coffee ;)

Thanks,

Paolo

[Paolo Abeni]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git master

---
diff --git a/include/net/sock.h b/include/net/sock.h
index c005c3c750e8..3111b6f0882b 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1623,7 +1623,36 @@ void release_sock(struct sock *sk);
SINGLE_DEPTH_NESTING)
#define bh_unlock_sock(__sk) spin_unlock(&((__sk)->sk_lock.slock))

-bool lock_sock_fast(struct sock *sk) __acquires(&sk->sk_lock.slock);
+bool __lock_sock_fast(struct sock *sk) __acquires(&sk->sk_lock.slock);
+
+/**
+ * lock_sock_fast - fast version of lock_sock
+ * @sk: socket
+ *
+ * This version should be used for very small section, where process wont block
+ * return false if fast path is taken:
+ *
+ * sk_lock.slock locked, owned = 0, BH disabled
+ *
+ * return true if slow path is taken:
+ *
+ * sk_lock.slock unlocked, owned = 1, BH enabled
+ */
+static inline bool lock_sock_fast(struct sock *sk)
+{
+ /* The sk_lock has mutex_lock() semantics here. */
+ mutex_acquire(&sk->sk_lock.dep_map, 0, 0, _RET_IP_);
+
+ return __lock_sock_fast(sk);
+}
+
+static inline bool lock_sock_fast_nested(struct sock *sk)
+{
+ /* The sk_lock has mutex_lock() semantics here. */
+ mutex_acquire(&sk->sk_lock.dep_map, SINGLE_DEPTH_NESTING, 0, _RET_IP_);
+
+ return __lock_sock_fast(sk);
+}

/**
* unlock_sock_fast - complement of lock_sock_fast
diff --git a/net/core/sock.c b/net/core/sock.c
index 512e629f9780..ff924252986b 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3210,24 +3210,8 @@ void release_sock(struct sock *sk)
}
EXPORT_SYMBOL(release_sock);

-/**
- * lock_sock_fast - fast version of lock_sock
- * @sk: socket
- *
- * This version should be used for very small section, where process wont block
- * return false if fast path is taken:
- *
- * sk_lock.slock locked, owned = 0, BH disabled
- *
- * return true if slow path is taken:
- *
- * sk_lock.slock unlocked, owned = 1, BH enabled
- */
-bool lock_sock_fast(struct sock *sk) __acquires(&sk->sk_lock.slock)
+bool __lock_sock_fast(struct sock *sk) __acquires(&sk->sk_lock.slock)
{
- /* The sk_lock has mutex_lock() semantics here. */
- mutex_acquire(&sk->sk_lock.dep_map, 0, 0, _RET_IP_);
-
might_sleep();
spin_lock_bh(&sk->sk_lock.slock);

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 6b334f9b6242..dcfc2186a583 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2746,8 +2746,12 @@ static void mptcp_close(struct sock *sk, long timeout)

inet_csk(sk)->icsk_mtup.probe_timestamp = tcp_jiffies32;
mptcp_for_each_subflow(mptcp_sk(sk), subflow) {
struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
- bool slow = lock_sock_fast(ssk);

+ bool slow;

+ /* can be nested in netlink mutex, when invoked by
+ * the PM
+ */
+ slow = lock_sock_fast_nested(ssk);
sock_orphan(ssk);
unlock_sock_fast(ssk, slow);
}

[syzbot]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

ERROR: modpost: "lock_sock_fast" [vmlinux] is a static EXPORT_SYMBOL


Tested on:

commit: 977d293e mptcp: ensure tx skbs always have the MPTCP ext
git tree: net

dashboard link: https://syzkaller.appspot.com/bug?extid=1dd53f7a89b299d59eaf
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

patch: https://syzkaller.appspot.com/x/patch.diff?x=152aa7f3300000

[Paolo Abeni]

On Mon, 2021-09-20 at 15:04 -0700, syzbot wrote:

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git master

whoops, a chunk was lost in my previous attempt...
---
commit 189a9054edbe8be3fe312ac6f0e5ce9e13eee1b6
Author: Paolo Abeni <pab...@redhat.com>
Date: Wed Sep 22 19:36:47 2021 +0200

net: introduce and use lock_sock_fast_nested()

Signed-off-by: Paolo Abeni <pab...@redhat.com>

index 512e629f9780..7060d183216e 100644

--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3210,24 +3210,8 @@ void release_sock(struct sock *sk)
}
EXPORT_SYMBOL(release_sock);

-/**
- * lock_sock_fast - fast version of lock_sock
- * @sk: socket
- *
- * This version should be used for very small section, where process wont block
- * return false if fast path is taken:
- *
- * sk_lock.slock locked, owned = 0, BH disabled
- *
- * return true if slow path is taken:
- *
- * sk_lock.slock unlocked, owned = 1, BH enabled
- */
-bool lock_sock_fast(struct sock *sk) __acquires(&sk->sk_lock.slock)
+bool __lock_sock_fast(struct sock *sk) __acquires(&sk->sk_lock.slock)
{
- /* The sk_lock has mutex_lock() semantics here. */
- mutex_acquire(&sk->sk_lock.dep_map, 0, 0, _RET_IP_);
-
might_sleep();
spin_lock_bh(&sk->sk_lock.slock);

@@ -3256,7 +3240,7 @@ bool lock_sock_fast(struct sock *sk) __acquires(&sk->sk_lock.slock)
spin_unlock_bh(&sk->sk_lock.slock);
return true;
}
-EXPORT_SYMBOL(lock_sock_fast);
+EXPORT_SYMBOL(__lock_sock_fast);

int sock_gettstamp(struct socket *sock, void __user *userstamp,
bool timeval, bool time32)

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-and-tested-by: syzbot+1dd53f...@syzkaller.appspotmail.com

Tested on:

commit: 977d293e mptcp: ensure tx skbs always have the MPTCP ext
git tree: net

kernel config: https://syzkaller.appspot.com/x/.config?x=6d93fe4341f98704
dashboard link: https://syzkaller.appspot.com/bug?extid=1dd53f7a89b299d59eaf
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

patch: https://syzkaller.appspot.com/x/patch.diff?x=132bc1d7300000

Note: testing is done by a robot and is best-effort only.