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

[Daniel Borkmann]

On 2/7/20 11:37 AM, 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@xxxxxxxxxxxxxx>
> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")

Applied, thanks!