[PATCH net v2 1/2] af_unix: decrement u->inq_len when skipping unread OOB skb in manage_oob()

When a normal in-band read on an AF_UNIX stream socket (without MSG_PEEK
and without SO_OOBINLINE) encounters an unread u->oob_skb in manage_oob(),
manage_oob() clears u->oob_skb, unlinks the 1-byte skb from
sk->sk_receive_queue, and drops it with SKB_DROP_REASON_UNIX_SKIP_OOB
without decrementing u->inq_len. Because queue_oob() incremented
u->inq_len by 1 when queuing the OOB skb, u->inq_len remains permanently
inflated by 1 byte for each skipped unread OOB skb, causing SIOCINQ /
FIONREAD to report a stale positive byte count on an empty socket.

Decrement u->inq_len by 1 when unlinking the unread OOB skb in
manage_oob().

Tested in QEMU against Linux 7.3.0-rc3 by sending three 1-byte MSG_OOB
packets interleaved with normal stream data on an AF_UNIX SOCK_STREAM
socketpair and draining all in-band data via recv(): on the unfixed
kernel, ioctl(SIOCINQ) reports 3 on the empty socket; with this patch
applied, ioctl(SIOCINQ) reports 0.

Fixes: f4e1fb04c123 ("af_unix: Use cached value for SOCK_STREAM in unix_inq_len().")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Hui Peng <redacted>
---
Changes in v2:
- Split the manage_oob() and unix_stream_read_skb() fixes into a 2-patch
  series as requested by Kuniyuki Iwashima.
- Update Fixes: tag to f4e1fb04c123 ("af_unix: Use cached value for
  SOCK_STREAM in unix_inq_len().") and clarify the commit message as
  noted by Sashiko.

 net/unix/af_unix.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 42cffeafc8c1..1770af3c2684 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c

@@ -2849,6 +2849,7 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sk_buff **last,
 		WRITE_ONCE(u->oob_skb, NULL);
 
 		if (!sock_flag(sk, SOCK_URGINLINE)) {
+			WRITE_ONCE(u->inq_len, u->inq_len - 1);
 			__skb_unlink(skb, &sk->sk_receive_queue);
 			unread_skb = skb;
 			skb = skb_peek(&sk->sk_receive_queue);

-- 
2.49.0