Re: [PATCH bpf] bpf: sockmap: check update requirements after locking

[Jakub Sitnicki <jakub@cloudflare.com>]

On Fri, Feb 07, 2020 at 11:37 AM CET, Lorenz Bauer wrote:
> It's currently possible to insert sockets in unexpected states into
> a sockmap, due to a TOCTTOU when updating the map from a syscall.
> sock_map_update_elem checks that sk->sk_state == TCP_ESTABLISHED,
> locks the socket and then calls sock_map_update_common. At this
> point, the socket may have transitioned into another state, and
> the earlier assumptions don't hold anymore. Crucially, it's
> conceivable (though very unlikely) that a socket has become unhashed.
> This breaks the sockmap's assumption that it will get a callback
> via sk->sk_prot->unhash.
>
> Fix this by checking the (fixed) sk_type and sk_protocol without the
> lock, followed by a locked check of sk_state.
>
> Unfortunately it's not possible to push the check down into
> sock_(map|hash)_update_common, since BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB
> run before the socket has transitioned from TCP_SYN_RECV into
> TCP_ESTABLISHED.
>
> Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> ---
>  net/core/sock_map.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 8998e356f423..36a2433e183f 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -416,14 +416,16 @@ static int sock_map_update_elem(struct bpf_map *map, void *key,
>  		ret = -EINVAL;
>  		goto out;
>  	}
> -	if (!sock_map_sk_is_suitable(sk) ||
> -	    sk->sk_state != TCP_ESTABLISHED) {
> +	if (!sock_map_sk_is_suitable(sk)) {
>  		ret = -EOPNOTSUPP;
>  		goto out;
>  	}
>
>  	sock_map_sk_acquire(sk);
> -	ret = sock_map_update_common(map, idx, sk, flags);
> +	if (sk->sk_state != TCP_ESTABLISHED)
> +		ret = -EOPNOTSUPP;
> +	else
> +		ret = sock_map_update_common(map, idx, sk, flags);
>  	sock_map_sk_release(sk);
>  out:
>  	fput(sock->file);
> @@ -739,14 +741,16 @@ static int sock_hash_update_elem(struct bpf_map *map, void *key,
>  		ret = -EINVAL;
>  		goto out;
>  	}
> -	if (!sock_map_sk_is_suitable(sk) ||
> -	    sk->sk_state != TCP_ESTABLISHED) {
> +	if (!sock_map_sk_is_suitable(sk)) {
>  		ret = -EOPNOTSUPP;
>  		goto out;
>  	}
>
>  	sock_map_sk_acquire(sk);
> -	ret = sock_hash_update_common(map, key, sk, flags);
> +	if (sk->sk_state != TCP_ESTABLISHED)
> +		ret = -EOPNOTSUPP;
> +	else
> +		ret = sock_hash_update_common(map, key, sk, flags);
>  	sock_map_sk_release(sk);
>  out:
>  	fput(sock->file);
> --
> 2.20.1

Thanks for fixing this, Lorenz. I'll adapt socket state checks on update
in "Extend SOCKMAP to store listening sockets" series accordingly.

Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>