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Message-ID: <379f4146.5e94f.181493b8c18.Coremail.duoming@zju.edu.cn>
Date: Fri, 10 Jun 2022 00:10:48 +0800 (GMT+08:00)
From: duoming@....edu.cn
To: "Paolo Abeni" <pabeni@...hat.com>
Cc: linux-kernel@...r.kernel.org, jreuter@...na.de,
ralf@...ux-mips.org, davem@...emloft.net, edumazet@...gle.com,
kuba@...nel.org, netdev@...r.kernel.org,
linux-hams@...r.kernel.org, thomas@...erried.de
Subject: Re: [PATCH v3] net: ax25: Fix deadlock caused by skb_recv_datagram
in ax25_recvmsg
Hello,
On Thu, 09 Jun 2022 15:33:02 +0200 Paolo 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. What`s more, we
> > > > add skb_free_datagram() before goto out in order to mitigate memory leak.
> > > >
> > > > Suggested-by: Thomas Osterried <thomas@...erried.de>
> > > > Signed-off-by: Duoming Zhou <duoming@....edu.cn>
> > > > 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
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