netdev - Re: [PATCH bpf] bpf: Check sk_fullsock() before returning from bpf_sk

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Date:   Fri, 17 May 2019 14:51:48 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Martin KaFai Lau <kafai@...com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Cc:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, kernel-team@...com,
        Joe Stringer <joe@...valent.com>
Subject: Re: [PATCH bpf] bpf: Check sk_fullsock() before returning from
 bpf_sk_lookup()



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@...valent.com>
> Signed-off-by: Martin KaFai Lau <kafai@...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 ?

> +			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;
>  }
>