[PATCH v1 net 3/4] af_unix: Don't set -ECONNRESET for consumed OOB skb.

[Kuniyuki Iwashima <kuni1840@gmail.com>]

From: Kuniyuki Iwashima <kuniyu@google.com>

Christian Brauner reported that even after MSG_OOB data is consumed,
calling close() on the receiver socket causes the peer's recv() to
return -ECONNRESET:

  1. send() and recv() an OOB data.

    >>> from socket import *
    >>> s1, s2 = socketpair(AF_UNIX, SOCK_STREAM)
    >>> s1.send(b'x', MSG_OOB)
    1
    >>> s2.recv(1, MSG_OOB)
    b'x'

  2. close() for s2 sets ECONNRESET to s1->sk_err even though
     s2 consumed the OOB data

    >>> s2.close()
    >>> s1.recv(10, MSG_DONTWAIT)
    ...
    ConnectionResetError: [Errno 104] Connection reset by peer

Even after being consumed, the skb holding the OOB 1-byte data stays in
the recv queue to mark the OOB boundary and break recv() at that point.

This must be considered while close()ing a socket.

Let's free the leading consumed OOB skb before checking the -ECONNRESET
condition in unix_release_sock().

Fixes: 314001f0bf92 ("af_unix: Add OOB support")
Reported-by: Christian Brauner <brauner@kernel.org>
Closes: https://lore.kernel.org/netdev/20250529-sinkt-abfeuern-e7b08200c6b0@brauner/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 net/unix/af_unix.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 5392aa53cbc8..50e56365f4f6 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -660,6 +660,11 @@ static void unix_sock_destructor(struct sock *sk)
 #endif
 }
 
+static unsigned int unix_skb_len(const struct sk_buff *skb)
+{
+	return skb->len - UNIXCB(skb).consumed;
+}
+
 static void unix_release_sock(struct sock *sk, int embrion)
 {
 	struct unix_sock *u = unix_sk(sk);
@@ -687,6 +692,12 @@ static void unix_release_sock(struct sock *sk, int embrion)
 	unix_state_unlock(sk);
 
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
+	skb = skb_peek(&sk->sk_receive_queue);
+	if (skb && !unix_skb_len(skb)) {
+		__skb_unlink(skb, &sk->sk_receive_queue);
+		consume_skb(skb);
+	}
+
 	u->oob_skb = NULL;
 #endif
 
@@ -2661,11 +2672,6 @@ static long unix_stream_data_wait(struct sock *sk, long timeo,
 	return timeo;
 }
 
-static unsigned int unix_skb_len(const struct sk_buff *skb)
-{
-	return skb->len - UNIXCB(skb).consumed;
-}
-
 struct unix_stream_read_state {
 	int (*recv_actor)(struct sk_buff *, int, int,
 			  struct unix_stream_read_state *);
-- 
2.49.0