Re: [PATCH bpf 1/1] net: bpf: don't leak time wait and request sockets

[Martin Lau]

On Thu, Jan 09, 2020 at 11:57:48AM +0000, Lorenz Bauer wrote:
> It's possible to leak time wait and request sockets via the following
> BPF pseudo code:
>  
>   sk = bpf_skc_lookup_tcp(...)
>   if (sk)
>     bpf_sk_release(sk)
> 
> If sk->sk_state is TCP_NEW_SYN_RECV or TCP_TIME_WAIT the refcount taken
> by bpf_skc_lookup_tcp is not undone by bpf_sk_release. This is because
> sk_flags is re-used for other data in both kinds of sockets. The check
Thanks for the report.

> 
>   !sock_flag(sk, SOCK_RCU_FREE)
> 
> therefore returns a bogus result.
> 
> Introduce a helper to account for this complication, and call it from
> the necessary places.
> 
> Fixes: edbf8c01de5a ("bpf: add skc_lookup_tcp helper")
> Fixes: f7355a6c0497 ("bpf: Check sk_fullsock() before returning from bpf_sk_lookup()")
> Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxxx>
> ---
>  net/core/filter.c | 17 +++++++++++------
>  1 file changed, 11 insertions(+), 6 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 42fd17c48c5f..d98dc4526d82 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -5266,6 +5266,14 @@ __bpf_skc_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
>  	return sk;
>  }
>  
> +static void __bpf_sk_release(struct sock *sk)
> +{
> +	/* time wait and request socks don't have sk_flags. */
> +	if (sk->sk_state == TCP_TIME_WAIT || sk->sk_state == TCP_NEW_SYN_RECV ||
> +	    !sock_flag(sk, SOCK_RCU_FREE))
Would this work too?
	if (!sk_fullsock(sk) || !sock_flag(sk, SOCK_RCU_FREE))

> +		sock_gen_put(sk);
> +}
> +
>  static struct sock *
>  __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
>  		struct net *caller_net, u32 ifindex, u8 proto, u64 netns_id,
> @@ -5277,8 +5285,7 @@ __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
>  	if (sk) {
>  		sk = sk_to_full_sk(sk);
>  		if (!sk_fullsock(sk)) {
> -			if (!sock_flag(sk, SOCK_RCU_FREE))
> -				sock_gen_put(sk);
> +			__bpf_sk_release(sk);
>  			return NULL;
>  		}
>  	}
> @@ -5315,8 +5322,7 @@ bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
>  	if (sk) {
>  		sk = sk_to_full_sk(sk);
>  		if (!sk_fullsock(sk)) {
> -			if (!sock_flag(sk, SOCK_RCU_FREE))
> -				sock_gen_put(sk);
> +			__bpf_sk_release(sk);
>  			return NULL;
>  		}
>  	}
> @@ -5383,8 +5389,7 @@ static const struct bpf_func_proto bpf_sk_lookup_udp_proto = {
>  
>  BPF_CALL_1(bpf_sk_release, struct sock *, sk)
>  {
> -	if (!sock_flag(sk, SOCK_RCU_FREE))
> -		sock_gen_put(sk);
> +	__bpf_sk_release(sk);
>  	return 0;
>  }
>  
> -- 
> 2.20.1
>