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 79f632c71bea ("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 2b514574f7e8 ("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 AFAIK is not supposed to happen. Fixes: 2b514574f7e8 ("net: af_unix: implement splice for stream af_unix sockets") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn --- net/unix/af_unix.c | 11 ++--------- 1 file changed, 2 insertions(+), 9 deletions(-) diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c index 1cbf36ea043b..dc71ed79be4a 100644 --- a/net/unix/af_unix.c +++ b/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, - bool freezable) + struct sk_buff *last, 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 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state, 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 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state, mutex_unlock(&u->iolock); - timeo = unix_stream_data_wait(sk, timeo, last, - last_len, freezable); + timeo = unix_stream_data_wait(sk, timeo, last, freezable); if (signal_pending(current)) { err = sock_intr_errno(timeo); @@ -3019,7 +3014,6 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state, 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 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state, skip = 0; last = skb; - last_len = skb->len; unix_state_lock(sk); skb = skb_peek_next(skb, &sk->sk_receive_queue); if (skb) -- 2.54.0.563.g4f69b47b94-goog