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

On Mon, Mar 17, 2025 at 05:22:55PM +0800, Jiayuan Chen 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.

Hm, I guess the RCU work in sk_psock_drop() does not work for Unix
domain sockets either?

Thanks.

March 20, 2025 at 08:34, "Cong Wang" <xiyou.wangcong@gmail.com> wrote:

> 
> On Mon, Mar 17, 2025 at 05:22:55PM +0800, Jiayuan Chen 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.
> > 
> 
> Hm, I guess the RCU work in sk_psock_drop() does not work for Unix
> 
> domain sockets either?
> 
> Thanks.
>

Although the Unix domain socket framework does not use RCU locks, the
entire sockmap process protects access to psock via RCU:
'''
rcu_read_lock();
psock = sk_psock(sk_other);
if (psock) {
 ...
}
rcu_read_unlock(); // `sk_psock_drop` will not execute until the unlock
'''

Therefore, I believe there are no issues with the psock operations here.

Thanks~