Subject: Re: Recursive locking in sockmap

From: John Fastabend <[email-masked]>
Date: Thu, 13 Jun 2024 12:08:25 -0700

Cong Wang wrote:
> On Fri, Jun 07, 2024 at 02:09:59PM +0200, Vincent Whitchurch wrote:
> > On Thu, Jun 6, 2024 at 2:47 PM Jason Xing <[email-masked]> wrote:
> > > On Thu, Jun 6, 2024 at 6:00 PM Vincent Whitchurch
> > > <[email-masked]> wrote:
> > > > With a socket in the sockmap, if there's a parser callback installed
> > > > and the verdict callback returns SK_PASS, the kernel deadlocks
> > > > immediately after the verdict callback is run. This started at commit
> > > > 6648e613226e18897231ab5e42ffc29e63fa3365 ("bpf, skmsg: Fix NULL
> > > > pointer dereference in sk_psock_skb_ingress_enqueue").
> > > >
> > > > It can be reproduced by running ./test_sockmap -t ping
> > > > --txmsg_pass_skb.  The --txmsg_pass_skb command to test_sockmap is
> > > > available in this series:
> > > > https://lore.kernel.org/netdev/[email-masked]/.
> > >
> > > I don't have time right now to look into this issue carefully until
> > > this weekend. BTW, did you mean the patch [2/5] in the link that can
> > > solve the problem?
> > 
> > No.  That patch set addresses a different problem which occurs even if
> > only a verdict callback is used. But patch 4/5 in that patch set adds
> > the --txmsg_pass_skb option to the test_sockmap test program, and that
> > option can be used to reproduce this deadlock too.
> 
> I think we can remove that write_lock_bh(&sk->sk_callback_lock). Can you
> test the following patch?
> 
> ------------>
> 
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index fd20aae30be2..da64ded97f3a 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -1116,9 +1116,7 @@ static void sk_psock_strp_data_ready(struct sock *sk)
>  		if (tls_sw_has_ctx_rx(sk)) {
>  			psock->saved_data_ready(sk);
>  		} else {
> -			write_lock_bh(&sk->sk_callback_lock);
>  			strp_data_ready(&psock->strp);
> -			write_unlock_bh(&sk->sk_callback_lock);
>  		}
>  	}
>  	rcu_read_unlock();

Its not obvious to me that we can run the strp parser without the
sk_callback lock here. I believe below is the correct fix. It
fixes the splat above with test.

bpf: sockmap, fix introduced strparser recursive lock

Originally there was a race where removing a psock from the sock map while
it was also receiving an skb and calling sk_psock_data_ready(). It was
possible the removal code would NULL/set the data_ready callback while
concurrently calling the hook from receive path. The fix was to wrap the
access in sk_callback_lock to ensure the saved_data_ready pointer didn't
change under us. There was some discussion around doing a larger change
to ensure we could use READ_ONCE/WRITE_ONCE over the callback, but that
was for *next kernels not stable fixes.

But, we unfortunately introduced a regression with the fix because there
is another path into this code (that didn't have a test case) through
the stream parser. The stream parser runs with the lower lock which means
we get the following splat and lock up.

 ============================================
 WARNING: possible recursive locking detected
 6.10.0-rc2 #59 Not tainted
 --------------------------------------------
 test_sockmap/342 is trying to acquire lock:
 ffff888007a87228 (clock-AF_INET){++--}-{2:2}, at:
 sk_psock_skb_ingress_enqueue (./include/linux/skmsg.h:467
 net/core/skmsg.c:555)

 but task is already holding lock:
 ffff888007a87228 (clock-AF_INET){++--}-{2:2}, at:
 sk_psock_strp_data_ready (net/core/skmsg.c:1120)

To fix ensure we do not grap lock when we reach this code through the
strparser.

Fixes: 6648e613226e1 ("bpf, skmsg: Fix NULL pointer dereference in sk_psock_skb_ingress_enqueue")
Signed-off-by: John Fastabend <[email-masked]>
---
 include/linux/skmsg.h |    9 +++++++--
 net/core/skmsg.c      |    5 ++++-
 2 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index c9efda9df285..3659e9b514d0 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h
@@ -461,13 +461,18 @@ static inline void sk_psock_put(struct sock *sk, struct sk_psock *psock)
 		sk_psock_drop(sk, psock);
 }
 
-static inline void sk_psock_data_ready(struct sock *sk, struct sk_psock *psock)
+static inline void __sk_psock_data_ready(struct sock *sk, struct sk_psock *psock)
 {
-	read_lock_bh(&sk->sk_callback_lock);
 	if (psock->saved_data_ready)
 		psock->saved_data_ready(sk);
 	else
 		sk->sk_data_ready(sk);
+}
+
+static inline void sk_psock_data_ready(struct sock *sk, struct sk_psock *psock)
+{
+	read_lock_bh(&sk->sk_callback_lock);
+	__sk_psock_data_ready(sk, psock);
 	read_unlock_bh(&sk->sk_callback_lock);
 }
 
diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index fd20aae30be2..8429daecbbb6 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -552,7 +552,10 @@ static int sk_psock_skb_ingress_enqueue(struct sk_buff *skb,
 	msg->skb = skb;
 
 	sk_psock_queue_msg(psock, msg);
-	sk_psock_data_ready(sk, psock);
+	if (skb_bpf_strparser(skb))
+		__sk_psock_data_ready(sk, psock);
+	else
+		sk_psock_data_ready(sk, psock);
 	return copied;
 }