Re: [PATCH bpf 1/2] bpf: sockmap, fix introduced strparser recursive lock

[John Fastabend <john.fastabend@gmail.com>]

Jakub Sitnicki wrote:
> On Tue, Jun 25, 2024 at 01:16 PM -07, John Fastabend wrote:
> > 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")
> > Reported-by: Vincent Whitchurch <vincent.whitchurch@datadoghq.com>
> > Signed-off-by: John Fastabend <john.fastabend@gmail.com>
> > ---
> >  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;
> >  }
> 
> If I follow, this is the call chain that leads to the recursive lock:
> 
> sock::sk_data_ready → sk_psock_strp_data_ready
>     write_lock_bh(&sk->sk_callback_lock)
>     strp_data_ready
>       strp_read_sock
>         proto_ops::read_sock → tcp_read_sock
>           strp_recv
>             __strp_recv
>               strp_callbacks::rcv_msg → sk_psock_strp_read
>                   sk_psock_verdict_apply(verdict=__SK_PASS)
>                     sk_psock_skb_ingress_self
>                       sk_psock_skb_ingress_enqueue
>                         sk_psock_data_ready
>                           read_lock_bh(&sk->sk_callback_lock) !!!
> 
> What I don't get, though, is why strp_data_ready has to be called with a
> _writer_ lock? Maybe that should just be a reader lock, and then it can
> be recursive.

Agree read lock should be fine we just want to ensure the strp
is not changing during the callchain there. Let me do that
fix instead.