Re: [PATCH bpf v2 1/2] bpf: sockmap, af_unix stream sockets need to hold ref for pair sock

[Cong Wang <xiyou.wangcong@gmail.com>]

On Wed, Nov 22, 2023 at 11:24:51AM -0800, John Fastabend wrote:
> AF_UNIX stream sockets are a paired socket. So sending on one of the pairs
> will lookup the paired socket as part of the send operation. It is possible
> however to put just one of the pairs in a BPF map. This currently
> increments the refcnt on the sock in the sockmap to ensure it is not
> free'd by the stack before sockmap cleans up its state and stops any
> skbs being sent/recv'd to that socket.
> 
> But we missed a case. If the peer socket is closed it will be
> free'd by the stack. However, the paired socket can still be
> referenced from BPF sockmap side because we hold a reference
> there. Then if we are sending traffic through BPF sockmap to
> that socket it will try to dereference the free'd pair in its
> send logic creating a use after free.  And following splat,

Hmm, how could it pass the SOCK_DEAD test in unix_stream_sendmsg()?

2285                 unix_state_lock(other);
2286
2287                 if (sock_flag(other, SOCK_DEAD) ||
2288                     (other->sk_shutdown & RCV_SHUTDOWN))
2289                         goto pipe_err_free;


Thanks.