Hello,

On Thu, 09 Jun 2022 15:33:02 +0200 Paolo wrote:

quoted

quoted

quoted

The skb_recv_datagram() in ax25_recvmsg() will hold lock_sock
and block until it receives a packet from the remote. If the client
doesn`t connect to server and calls read() directly, it will not
receive any packets forever. As a result, the deadlock will happen.

The fail log caused by deadlock is shown below:

[  369.606973] INFO: task ax25_deadlock:157 blocked for more than 245 seconds.
[  369.608919] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  369.613058] Call Trace:
[  369.613315]  <TASK>
[  369.614072]  __schedule+0x2f9/0xb20
[  369.615029]  schedule+0x49/0xb0
[  369.615734]  __lock_sock+0x92/0x100
[  369.616763]  ? destroy_sched_domains_rcu+0x20/0x20
[  369.617941]  lock_sock_nested+0x6e/0x70
[  369.618809]  ax25_bind+0xaa/0x210
[  369.619736]  __sys_bind+0xca/0xf0
[  369.620039]  ? do_futex+0xae/0x1b0
[  369.620387]  ? __x64_sys_futex+0x7c/0x1c0
[  369.620601]  ? fpregs_assert_state_consistent+0x19/0x40
[  369.620613]  __x64_sys_bind+0x11/0x20
[  369.621791]  do_syscall_64+0x3b/0x90
[  369.622423]  entry_SYSCALL_64_after_hwframe+0x46/0xb0
[  369.623319] RIP: 0033:0x7f43c8aa8af7
[  369.624301] RSP: 002b:00007f43c8197ef8 EFLAGS: 00000246 ORIG_RAX: 0000000000000031
[  369.625756] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f43c8aa8af7
[  369.626724] RDX: 0000000000000010 RSI: 000055768e2021d0 RDI: 0000000000000005
[  369.628569] RBP: 00007f43c8197f00 R08: 0000000000000011 R09: 00007f43c8198700
[  369.630208] R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff845e6afe
[  369.632240] R13: 00007fff845e6aff R14: 00007f43c8197fc0 R15: 00007f43c8198700

This patch moves the skb_recv_datagram() before lock_sock() in order that
other functions that need lock_sock could be executed. What`s more, we
add skb_free_datagram() before goto out in order to mitigate memory leak.

Suggested-by: Thomas Osterried <redacted>
Signed-off-by: Duoming Zhou <redacted>
Reported-by: Thomas Habets <thomas@@habets.se>
---
Changes in v3:
  - Add skb_free_datagram() before goto out in order to mitigate memory leak.

 net/ax25/af_ax25.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 95393bb2760..62aa5993093 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c

@@ -1665,6 +1665,11 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
 	int copied;
 	int err = 0;
 
+	/* Now we can treat all alike */
+	skb = skb_recv_datagram(sk, flags, &err);
+	if (!skb)
+		goto done;
+

Note that this causes a behavior change: before this patch, calling
recvmsg() on unconnected seqpacket sockets returned immediatelly with
an error (due to the the check below), now it blocks. 

The change may confuse (== break) user-space applications. I think it
would be better replacing skb_recv_datagram with an open-coded variant
of it releasing the socket lock before the
__skb_wait_for_more_packets() call and re-acquiring it after such call.
Somewhat alike __unix_dgram_recvmsg().

Thank you for your time and suggestions!
I think the following method may solve the problem.

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 95393bb2760..51b441c837c 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c

@@ -1675,8 +1675,10 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
                goto out;
        }

+       release_sock(sk);
        /* Now we can treat all alike */
        skb = skb_recv_datagram(sk, flags, &err);
+       lock_sock(sk);
        if (skb == NULL)
                goto out;

The skb_recv_datagram() is free of race conditions and could be re-entrant.
So calling skb_recv_datagram() without the protection of lock_sock() is ok.

What's more, releasing the lock_sock() before skb_recv_datagram() will not
cause UAF bugs. Because the sock will not be deallocated unless we call
ax25_release(), but ax25_release() and ax25_recvmsg() could not run in parallel.

Although the "sk->sk_state" may be changed due to the release of lock_sock(),
it will not influence the following operations in ax25_recvmsg().

One of the downside of the above is that recvmsg() will unconditionally
acquire and release the socket lock twice which can have non
trivial/nasty side effects on process scheduling.

With the suggested change the socket lock will be released only when
recvmsg will block and that should produce nicer overal behavior.

I test the following method, it runs well.

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index 95393bb2760..2888aee91a5 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c

@@ -1661,9 +1661,12 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
                        int flags)
 {
        struct sock *sk = sock->sk;
-       struct sk_buff *skb;
+       struct sk_buff *skb, *last;
+       struct sk_buff_head *sk_queue;
        int copied;
        int err = 0;
+       bool is_block = false;
+       long timeo;

        lock_sock(sk);
        /*

@@ -1676,9 +1679,29 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
        }

        /* Now we can treat all alike */
-       skb = skb_recv_datagram(sk, flags, &err);
-       if (skb == NULL)
-               goto out;
+       sk_queue = &sk->sk_receive_queue;
+       timeo = sock_rcvtimeo(sk, flags & MSG_DONTWAIT);
+       skb = __skb_try_recv_datagram(sk, sk_queue, flags, 0, &err,
+                                     &last);
+       if (!skb && (err == -EAGAIN)) {
+               is_block = true;
+               release_sock(sk);
+               do {
+                       skb = __skb_try_recv_datagram(sk, sk_queue, flags, 0, &err,
+                                                       &last);
+                       if (skb)
+                               break;
+
+                       if (err != -EAGAIN)
+                               goto done;
+               } while (timeo &&
+                       !__skb_wait_for_more_packets(sk, sk_queue, &err,
+                                                       &timeo, last));
+               if(!skb)
+                       goto done;
+       }
+       if (is_block)
+               lock_sock(sk);

        if (!sk_to_ax25(sk)->pidincl)
                skb_pull(skb, 1);               /* Remove PID */

@@ -1725,6 +1748,7 @@ static int ax25_recvmsg(struct socket *sock, struct msghdr *msg, size_t size,
 out:
        release_sock(sk);

+done:
        return err;
 }

I think this method could solve the problem. Welcome more advice.

Best regards,
Duoming Zhou