On 1/30/26 3:00 AM, Michal Luczaj wrote:

quoted

quoted

Follow-up to discussion at
https://lore.kernel.org/netdev/20240610174906.32921-1-kuniyu@amazon.com/ (local).

It is a long thread to dig. Please summarize the discussion in the
commit message.

OK, there we go:

The root cause of the null-ptr-deref is that unix_stream_connect() sets
sk_state (`WRITE_ONCE(sk->sk_state, TCP_ESTABLISHED)`) _before_ it assigns
a peer (`unix_peer(sk) = newsk`). sk_state == TCP_ESTABLISHED makes
sock_map_sk_state_allowed() believe that socket is properly set up, which
would include having a defined peer.

In other words, there's a window when you can call
unix_stream_bpf_update_proto() on socket which still has unix_peer(sk) == NULL.

My initial idea was to simply move peer assignment _before_ the sk_state
update, but the maintainer wasn't interested in changing the
unix_stream_connect() hot path. He suggested taking care of it in the
sockmap code.

My understanding is that users are not supposed to put sockets in a sockmap
when said socket is only half-way through connect() call. Hence `return
-EINVAL` on a missing peer. Now, if users should be allowed to legally race
connect() vs. sockmap update, then I guess we can wait for connect() to
"finalize" e.g. by taking the unix_state_lock(), as discussed below.

quoted

  From looking at this commit message, if the existing lock_sock held by
update_elem is not useful for af_unix,

Right, the existing lock_sock is not useful. update's lock_sock holds
sock::sk_lock, while unix_state_lock() holds unix_sock::lock.

It sounds like lock_sock is the incorrect lock to hold for af_unix. Is 
taking lock_sock in sock_map doing anything useful for af_unix? Should 
sock_map hold the unix_state_lock instead of lock_sock?

Other than update_elem, do other lock_sock() usages in sock_map have a 
similar issue for af_unix?

quoted

it is not clear why a new test
"!sk_pair" on top of the existing WRITE_ONCE(sk->sk_state...) is a fix.

"On top"? Just to make sure we're looking at the same thing: above I was
trying to show two parallel flows with unix_peer() fetch in thread-0 and
WRITE_ONCE(sk->sk_state...) and `unix_peer(sk) = newsk` in thread-1.

It fixes the problem because now update_proto won't call sock_hold(NULL).

quoted

A minor thing is sock_map_sk_state_allowed doesn't have
READ_ONCE(sk->sk_state) for sk_is_stream_unix also.

Ok, I'll add this as a separate patch in v2. Along with the !tcp case of
sock_map_redirect_allowed()?

sgtm. thanks.

quoted

If unix_stream_connect does not hold lock_sock, can unix_state_lock be
used here? lock_sock has already been taken, update_elem should not be
the hot path.

Yes, it can be used, it was proposed in the old thread. In fact, critical
section can be empty; only used to wait for unix_stream_connect() to
release the lock, which would guarantee unix_peer(sk) != NULL by then.

         if (!psock->sk_pair) {
+               unix_state_lock(sk);
+               unix_state_unlock(sk);
                 sk_pair = unix_peer(sk);
                 sock_hold(sk_pair);

I don't have a strong opinion on waiting or checking NULL. imo, both are 
not easy to understand. One is sk_state had already been checked earlier 
under a lock_sock but still needs to check NULL on unix_peer(). Another 
one is an empty unix_state_[un]lock(). If taking unix_state_lock, may as 
well just use the existing unix_peer_get(sk). If its return value cannot 
(?) be NULL, WARN_ON_ONCE() instead of having a special empty 
lock/unlock pattern here. If the correct lock (unix_state_lock) was held 
earlier in update_elem, all these would go away.

Also, it is not immediately clear why a non-NULL unix_peer(sk) is safe 
here. From looking around af_unix.c, is it because the sk refcnt is held 
earlier in update_elem? For unix_stream, unix_peer(sk) will stay valid 
until unix_release_sock(sk). Am I reading it correctly?