[PATCH bpf-next v3 2/3] bpf, sockmap: avoid using sk_socket after free when reading

[Jiayuan Chen <jiayuan.chen@linux.dev>]

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();
 	copied = ops->read_skb(sk, sk_psock_verdict_recv);
 	if (copied >= 0) {
 		struct sk_psock *psock;
-- 
2.47.1