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Date:   Mon, 13 Jan 2020 23:45:45 +0000
From:   Martin Lau <kafai@...com>
To:     Jakub Sitnicki <jakub@...udflare.com>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kernel-team@...udflare.com" <kernel-team@...udflare.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "John Fastabend" <john.fastabend@...il.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [PATCH bpf-next v2 09/11] bpf: Allow selecting reuseport socket
 from a SOCKMAP

On Fri, Jan 10, 2020 at 11:50:25AM +0100, Jakub Sitnicki wrote:
> SOCKMAP now supports storing references to listening sockets. Nothing keeps
> us from using it as an array of sockets to select from in SK_REUSEPORT
> programs.
> 
> Whitelist the map type with the BPF helper for selecting socket.
> 
> The restriction that the socket has to be a member of a reuseport group
> still applies. Socket from a SOCKMAP that does not have sk_reuseport_cb set
> is not a valid target and we signal it with -EINVAL.
> 
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---
>  kernel/bpf/verifier.c |  6 ++++--
>  net/core/filter.c     | 15 ++++++++++-----
>  2 files changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index f5af759a8a5f..0ee5f1594b5c 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3697,7 +3697,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
>  		if (func_id != BPF_FUNC_sk_redirect_map &&
>  		    func_id != BPF_FUNC_sock_map_update &&
>  		    func_id != BPF_FUNC_map_delete_elem &&
> -		    func_id != BPF_FUNC_msg_redirect_map)
> +		    func_id != BPF_FUNC_msg_redirect_map &&
> +		    func_id != BPF_FUNC_sk_select_reuseport)
>  			goto error;
>  		break;
>  	case BPF_MAP_TYPE_SOCKHASH:
> @@ -3778,7 +3779,8 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
>  			goto error;
>  		break;
>  	case BPF_FUNC_sk_select_reuseport:
> -		if (map->map_type != BPF_MAP_TYPE_REUSEPORT_SOCKARRAY)
> +		if (map->map_type != BPF_MAP_TYPE_REUSEPORT_SOCKARRAY &&
> +		    map->map_type != BPF_MAP_TYPE_SOCKMAP)
>  			goto error;
>  		break;
>  	case BPF_FUNC_map_peek_elem:
> diff --git a/net/core/filter.c b/net/core/filter.c
> index a702761ef369..c79c62a54167 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -8677,6 +8677,7 @@ struct sock *bpf_run_sk_reuseport(struct sock_reuseport *reuse, struct sock *sk,
>  BPF_CALL_4(sk_select_reuseport, struct sk_reuseport_kern *, reuse_kern,
>  	   struct bpf_map *, map, void *, key, u32, flags)
>  {
> +	bool is_sockarray = map->map_type == BPF_MAP_TYPE_REUSEPORT_SOCKARRAY;
>  	struct sock_reuseport *reuse;
>  	struct sock *selected_sk;
>  
> @@ -8685,12 +8686,16 @@ BPF_CALL_4(sk_select_reuseport, struct sk_reuseport_kern *, reuse_kern,
>  		return -ENOENT;
>  
>  	reuse = rcu_dereference(selected_sk->sk_reuseport_cb);
> -	if (!reuse)
> -		/* selected_sk is unhashed (e.g. by close()) after the
> -		 * above map_lookup_elem().  Treat selected_sk has already
> -		 * been removed from the map.
> +	if (!reuse) {
> +		/* reuseport_array has only sk with non NULL sk_reuseport_cb.
> +		 * The only (!reuse) case here is - the sk has already been
> +		 * unhashed (e.g. by close()), so treat it as -ENOENT.
> +		 *
> +		 * Other maps (e.g. sock_map) do not provide this guarantee and
> +		 * the sk may never be in the reuseport group to begin with.
>  		 */
> -		return -ENOENT;
> +		return is_sockarray ? -ENOENT : -EINVAL;
> +	}
>  
>  	if (unlikely(reuse->reuseport_id != reuse_kern->reuseport_id)) {
I guess the later testing patch passed is because reuseport_id is init to 0.

Note that in reuseport_array, reuseport_get_id() is called at update_elem() to
init the reuse->reuseport_id.  It was done there because reuseport_array
was the only one requiring reuseport_id.  It is to ensure the bpf_prog
cannot accidentally use a sk from another reuseport-group.

The same has to be done in patch 5 or may be considering to
move it to reuseport_alloc() itself.

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