Re: [PATCH bpf] bpf: Check sk_fullsock() before returning from bpf_sk_lookup()

[Martin Lau <kafai@fb.com>]

On Fri, May 17, 2019 at 02:51:48PM -0700, Eric Dumazet wrote:
> 
> 
> On 5/17/19 2:21 PM, Martin KaFai Lau wrote:
> > The BPF_FUNC_sk_lookup_xxx helpers return RET_PTR_TO_SOCKET_OR_NULL.
> > Meaning a fullsock ptr and its fullsock's fields in bpf_sock can be
> > accessed, e.g. type, protocol, mark and priority.
> > Some new helper, like bpf_sk_storage_get(), also expects
> > ARG_PTR_TO_SOCKET is a fullsock.
> > 
> > bpf_sk_lookup() currently calls sk_to_full_sk() before returning.
> > However, the ptr returned from sk_to_full_sk() is not guaranteed
> > to be a fullsock.  For example, it cannot get a fullsock if sk
> > is in TCP_TIME_WAIT.
> > 
> > This patch checks for sk_fullsock() before returning. If it is not
> > a fullsock, sock_gen_put() is called if needed and then returns NULL.
> > 
> > Fixes: 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
> > Cc: Joe Stringer <joe@isovalent.com>
> > Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> > ---
> >  net/core/filter.c | 16 ++++++++++++++--
> >  1 file changed, 14 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 55bfc941d17a..85def5a20aaf 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -5337,8 +5337,14 @@ __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
> >  	struct sock *sk = __bpf_skc_lookup(skb, tuple, len, caller_net,
> >  					   ifindex, proto, netns_id, flags);
> >  
> > -	if (sk)
> > +	if (sk) {
> >  		sk = sk_to_full_sk(sk);
> > +		if (!sk_fullsock(sk)) {
> > +			if (!sock_flag(sk, SOCK_RCU_FREE))
> > +				sock_gen_put(sk);
> 
> This looks a bit convoluted/weird.
> 
> What about telling/asking __bpf_skc_lookup() to not return a non fullsock instead ?
It is becausee some other helpers, like BPF_FUNC_skc_lookup_tcp,
can return non fullsock.

> 
> > +			return NULL;
> > +		}
> > +	}
> >  
> >  	return sk;
> >  }
> > @@ -5369,8 +5375,14 @@ bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
> >  	struct sock *sk = bpf_skc_lookup(skb, tuple, len, proto, netns_id,
> >  					 flags);
> >  
> > -	if (sk)
> > +	if (sk) {
> >  		sk = sk_to_full_sk(sk);
> > +		if (!sk_fullsock(sk)) {
> > +			if (!sock_flag(sk, SOCK_RCU_FREE))
> > +				sock_gen_put(sk);
> > +			return NULL;
> > +		}
> > +	}
> >  
> >  	return sk;
> >  }
> >