Re: [PATCH bpf-next v4 2/3] bpf, sockmap: avoid using sk_socket after free when reading

Alexei Starovoitov
2025-04-10T03:02:14Z

On Tue, Apr 8, 2025 at 12:31 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>
> There are potential concurrency issues, as shown below.
> '''
> CPU0 CPU1
> sk_psock_verdict_data_ready:
> socket *sock = sk->sk_socket
> if (!sock) return
> close(fd):
> ...
> ops->release()
> if (!sock->ops) return
> sock->ops = NULL
> rcu_call(sock)
> free(sock)
> READ_ONCE(sock->ops)
> ^
> use 'sock' after free
> '''
>
> RCU is not applicable to Unix sockets read path, because the Unix socket
> implementation itself assumes it's always in process context and heavily
> uses mutex_lock, so, we can't call read_skb within rcu lock.
>
> Incrementing the psock reference count would not help either, since
> sock_map_close() does not wait for data_ready() to complete its execution.
>
> While we don't utilize sk_socket here, implementing read_skb at the sock
> layer instead of socket layer might be architecturally preferable ?
> However, deferring this optimization as current fix adequately addresses
> the immediate issue.
>
> Fixes: c63829182c37 ("af_unix: Implement ->psock_update_sk_prot()")
> Reported-by: syzbot+dd90a702f518e0eac072@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/bpf/6734c033.050a0220.2a2fcc.0015.GAE@google.com/
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
> net/core/skmsg.c | 13 ++++++++++---
> 1 file changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 6101c1bb279a..5e913b62929e 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -1231,17 +1231,24 @@ static int sk_psock_verdict_recv(struct sock *sk, struct sk_buff *skb)
>
> static void sk_psock_verdict_data_ready(struct sock *sk)
> {
> - struct socket *sock = sk->sk_socket;
> + struct socket *sock;
> const struct proto_ops *ops;
> int copied;
>
> trace_sk_data_ready(sk);
>
> - if (unlikely(!sock))
> + rcu_read_lock();
> + sock = sk->sk_socket;
> + if (unlikely(!sock)) {
> + rcu_read_unlock();
> return;
> + }
> ops = READ_ONCE(sock->ops);
> - if (!ops || !ops->read_skb)
> + if (!ops || !ops->read_skb) {
> + rcu_read_unlock();
> return;
> + }
> + rcu_read_unlock();

This makes no sense to me. RCU doesn't work this way.

pw-bot: cr