netdev - Re: [PATCH v2] net: ax25: Fix deadlock caused by skb_recv_datagram in ax25

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Message-ID: <3c463bfd.58476.18140d5501a.Coremail.duoming@zju.edu.cn>
Date:   Wed, 8 Jun 2022 09:02:10 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Eric Dumazet" <edumazet@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, jreuter@...na.de,
        "Ralf Baechle" <ralf@...ux-mips.org>,
        "David Miller" <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>,
        "Paolo Abeni" <pabeni@...hat.com>, netdev <netdev@...r.kernel.org>,
        linux-hams@...r.kernel.org, thomas@...erried.de
Subject: Re: [PATCH v2] net: ax25: Fix deadlock caused by skb_recv_datagram
 in ax25_recvmsg

Hello,

On Tue, 7 Jun 2022 10:06:27 -0700 Eric Dumazet wrote:

> On Tue, Jun 7, 2022 at 7:24 AM Duoming Zhou <duoming@....edu.cn> wrote:
> >
> > 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.
> >
> > Suggested-by: Thomas Osterried <thomas@...erried.de>
> > Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> > Reported-by: Thomas Habets <thomas@@habets.se>
> > ---
> > Changes in v2:
> >   - Make commit messages clearer.
> >
> >  net/ax25/af_ax25.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
> > index 95393bb2760..02cd6087512 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;
> > +
> 
> So at this point we have skb=something. This means that the following
> branch will leak it.
> 
> if (sk->sk_type == SOCK_SEQPACKET && sk->sk_state != TCP_ESTABLISHED) {
>     err =  -ENOTCONN;
>     goto out;    // skb will be leaked
> }
> 

Thank your for pointing out the problem!
I will add skb_free_datagram() before goto out in order to mitigate the memory leak.

        if (sk->sk_type == SOCK_SEQPACKET && sk->sk_state != TCP_ESTABLISHED) {
                err =  -ENOTCONN;
+               skb_free_datagram(sk, skb);
                goto out;
        }

Best regards,
Duoming Zhou