af_unix: Fix UAF read of tail->len in unix_stream_data_wait()

unix_stream_data_wait() does skb_peek_tail(&sk->sk_receive_queue) without
holding any lock that prevents SKBs on that queue from being dequeued and
freed.
This has been the case since commit 79f632c71b ("unix/stream: fix
peeking with an offset larger than data in queue").
The first consequence of this is that the pointer comparison
`tail != last` can be false even if `last` semantically refers to an
already-freed SKB while `tail` is a new SKB allocated at the same address;
which can cause unix_stream_data_wait() to wrongly keep blocking after new
data has arrived, but only in a weird scenario where a peeking recv() and
a normal recv() on the same socket are racing, which is probably not a
real problem.

But since commit 2b514574f7 ("net: af_unix: implement splice for stream
af_unix sockets"), `tail` is actually dereferenced, which can cause UAF in
the following race scenario (where test_setup() runs single-threaded,
and afterwards, test_thread1() and test_thread2() run concurrently in
two threads:
```
static int socks[2];
void test_setup(void) {
  socketpair(AF_UNIX, SOCK_STREAM, 0, socks);
  send(socks[1], "A", 1, 0);
  int peekoff = 1;
  setsockopt(socks[0], SOL_SOCKET, SO_PEEK_OFF, &peekoff, sizeof(peekoff));
}
void test_thread1(void) {
  char dummy;
  recv(socks[0], &dummy, 1, MSG_PEEK);
}
void test_thread2(void) {
  char dummy;
  recv(socks[0], &dummy, 1, 0);
  shutdown(socks[1], SHUT_WR);
}
```

when racing like this:
```
thread1                       thread2
unix_stream_read_generic
  mutex_lock(&u->iolock)
  skb_peek(&sk->sk_receive_queue)
  skb_peek_next(skb, &sk->sk_receive_queue)
  mutex_unlock(&u->iolock)
                              unix_stream_read_generic
                                unix_state_lock(sk)
                                skb_peek(&sk->sk_receive_queue)
                                unix_state_unlock(sk)
  unix_stream_data_wait
    unix_state_lock(sk)
    tail = skb_peek_tail(&sk->sk_receive_queue)
                                spin_lock(&sk->sk_receive_queue.lock)
                                __skb_unlink(skb, &sk->sk_receive_queue)
                                spin_unlock(&sk->sk_receive_queue.lock)
                                consume_skb(skb) [frees the SKB]
    `tail != last`: false
    `tail`: true
    `tail->len != last_len` ***UAF***
```

Fix the UAF by removing the read of tail->len; checking tail->len would
only make sense if SKBs in the receive queue of a UNIX socket could grow,
which can no longer happen.

Kuniyuki explained:

> When commit 869e7c6248 ("net: af_unix: implement stream sendpage
> support") added sendpage() support, data could be appended to the last
> skb in the receiver's queue.
>
> That's why we needed to check if the length of the last skb was changed
> while waiting for new data in unix_stream_data_wait().
>
> However, commit a0dbf5f818 ("af_unix: Support MSG_SPLICE_PAGES") and
> commit 57d44a354a ("unix: Convert unix_stream_sendpage() to use
> MSG_SPLICE_PAGES") refactored sendmsg(), and now data is always added
> to a new skb.

That means this fix is not suitable for kernels before 6.5.

Fixes: 2b514574f7 ("net: af_unix: implement splice for stream af_unix sockets")
Cc: stable@vger.kernel.org # 6.5.x
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260518-b4-unix-recv-wait-hotfix-v2-1-83e29ce8ad31@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -2711,8 +2711,7 @@ static int unix_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
  *	Sleep until more data has arrived. But check for races..
  */
 static long unix_stream_data_wait(struct sock *sk, long timeo,
-				  struct sk_buff *last, unsigned int last_len,
+				  struct sk_buff *last, bool freezable)
-				  bool freezable)
 {
 	unsigned int state = TASK_INTERRUPTIBLE | freezable * TASK_FREEZABLE;
 	struct sk_buff *tail;
@@ -2725,7 +2724,6 @@ static long unix_stream_data_wait(struct sock *sk, long timeo,
 
 		tail = skb_peek_tail(&sk->sk_receive_queue);
 		if (tail != last ||
-		    (tail && tail->len != last_len) ||
 		    sk->sk_err ||
 		    (sk->sk_shutdown & RCV_SHUTDOWN) ||
 		    signal_pending(current) ||
@@ -2921,7 +2919,6 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 	int flags = state->flags;
 	bool check_creds = false;
 	struct scm_cookie scm;
-	unsigned int last_len;
 	struct unix_sock *u;
 	int copied = 0;
 	int err = 0;
@@ -2967,7 +2964,6 @@ redo:
 			goto unlock;
 		}
 		last = skb = skb_peek(&sk->sk_receive_queue);
-		last_len = last ? last->len : 0;
 
 again:
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
@@ -3001,8 +2997,7 @@ again:
 
 			mutex_unlock(&u->iolock);
 
-			timeo = unix_stream_data_wait(sk, timeo, last,
+			timeo = unix_stream_data_wait(sk, timeo, last, freezable);
-						      last_len, freezable);
 
 			if (signal_pending(current)) {
 				err = sock_intr_errno(timeo);
@@ -3019,7 +3014,6 @@ unlock:
 		while (skip >= unix_skb_len(skb)) {
 			skip -= unix_skb_len(skb);
 			last = skb;
-			last_len = skb->len;
 			skb = skb_peek_next(skb, &sk->sk_receive_queue);
 			if (!skb)
 				goto again;
@@ -3094,7 +3088,6 @@ unlock:
 
 			skip = 0;
 			last = skb;
-			last_len = skb->len;
 			unix_state_lock(sk);
 			skb = skb_peek_next(skb, &sk->sk_receive_queue);
 			if (skb)