Re: [PATCH v2] bpf: Fix sk_psock refcnt leak when receiving message

[Jakub Sitnicki]

On Sun, Apr 26, 2020 at 05:35 AM CEST, Xiyu Yang wrote:
> tcp_bpf_recvmsg() invokes sk_psock_get(), which returns a reference of
> the specified sk_psock object to "psock" with increased refcnt.
>
> When tcp_bpf_recvmsg() returns, local variable "psock" becomes invalid,
> so the refcount should be decreased to keep refcount balanced.
>
> The reference counting issue happens in several exception handling paths
> of tcp_bpf_recvmsg(). When those error scenarios occur such as "flags"
> includes MSG_ERRQUEUE, the function forgets to decrease the refcnt
> increased by sk_psock_get(), causing a refcnt leak.
>
> Fix this issue by calling sk_psock_put() or pulling up the error queue
> read handling when those error scenarios occur.
>
> Fixes: e7a5f1f1cd000 ("bpf/sockmap: Read psock ingress_msg before sk_receive_queue")
> Signed-off-by: Xiyu Yang <xiyuyang19@xxxxxxxxxxxx>
> Signed-off-by: Xin Tan <tanxin.ctf@xxxxxxxxx>
> ---
> Changes in v2:
> - Add Fixes tag
> - Pull up the error queue read handling
> ---
>  net/ipv4/tcp_bpf.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index 5a05327f97c1..ff96466ea6da 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
> @@ -262,14 +262,17 @@ static int tcp_bpf_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>  	struct sk_psock *psock;
>  	int copied, ret;
>  
> +	if (unlikely(flags & MSG_ERRQUEUE))
> +		return inet_recv_error(sk, msg, len, addr_len);
> +
>  	psock = sk_psock_get(sk);
>  	if (unlikely(!psock))
>  		return tcp_recvmsg(sk, msg, len, nonblock, flags, addr_len);
> -	if (unlikely(flags & MSG_ERRQUEUE))
> -		return inet_recv_error(sk, msg, len, addr_len);
>  	if (!skb_queue_empty(&sk->sk_receive_queue) &&
> -	    sk_psock_queue_empty(psock))
> +	    sk_psock_queue_empty(psock)) {
> +		sk_psock_put(sk, psock);
>  		return tcp_recvmsg(sk, msg, len, nonblock, flags, addr_len);
> +	}
>  	lock_sock(sk);
>  msg_bytes_ready:
>  	copied = __tcp_bpf_recvmsg(sk, psock, msg, len, flags);

Reviewed-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>