[PATCH net] af-unix: fix use-after-free with concurrent readers while splicing

[PATCH net] af-unix: fix use-after-free with concurrent readers while splicing

[Hannes Frederic Sowa]

During splicing an af-unix socket to a pipe we have to drop all
af-unix socket locks. While doing so we allow another reader to enter
unix_stream_read_generic which can read, copy and finally free another
skb. If exactly this skb is just in process of being spliced we get a
use-after-free report by kasan.

First, we must make sure to not have a free while the skb is used during
the splice operation. We simply increment its use counter before unlocking
the reader lock.

Stream sockets have the nice characteristic that we don't care about
zero length writes and they never reach the peer socket's queue. That
said, we can take the UNIXCB.consumed field as the indicator if the
skb was already freed from the socket's receive queue. If the skb was
fully consumed after we locked the reader side again we know it has been
dropped by a second reader. We indicate a short read to user space and
abort the current splice operation.

This bug has been found with syzkaller
(http://github.com/google/syzkaller) by Dmitry Vyukov.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
---
 net/unix/af_unix.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index aaa0b58..b2c4131 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -441,6 +441,7 @@ static void unix_release_sock(struct sock *sk, int embrion)
 		if (state == TCP_LISTEN)
 			unix_release_sock(skb->sk, 1);
 		/* passed fds are erased in the kfree_skb hook	      */
+		UNIXCB(skb).consumed = skb->len;
 		kfree_skb(skb);
 	}
 
@@ -2152,7 +2153,7 @@ unlock:
 		}
 
 		chunk = min_t(unsigned int, unix_skb_len(skb) - skip, size);
-		chunk = state->recv_actor(skb, skip, chunk, state);
+		chunk = state->recv_actor(skb_get(skb), skip, chunk, state);
 		if (chunk < 0) {
 			if (copied == 0)
 				copied = -EFAULT;
@@ -2161,6 +2162,21 @@ unlock:
 		copied += chunk;
 		size -= chunk;
 
+		if (!unix_skb_len(skb)) {
+			/* the skb was touched by a concurrent reader;
+			 * we should not expect anything from this skb
+			 * anymore and assume it invalid - we can be
+			 * sure it was dropped from the socket queue
+			 *
+			 * let's report a short read
+			 */
+			consume_skb(skb);
+			err = 0;
+			break;
+		}
+
+		consume_skb(skb);
+
 		/* Mark read part of skb as used */
 		if (!(flags & MSG_PEEK)) {
 			UNIXCB(skb).consumed += chunk;
-- 
2.5.0

Re: [PATCH net] af-unix: fix use-after-free with concurrent readers while splicing

[Hannes Frederic Sowa]

On Tue, Nov 10, 2015, at 15:26, Hannes Frederic Sowa wrote:
>  net/unix/af_unix.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index aaa0b58..b2c4131 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -441,6 +441,7 @@ static void unix_release_sock(struct sock *sk, int
> embrion)
>  		if (state == TCP_LISTEN)
>  			unix_release_sock(skb->sk, 1);
>  		/* passed fds are erased in the kfree_skb hook	      */
> +               UNIXCB(skb).consumed = skb->len;
>  		kfree_skb(skb);
>  	}
>  
> @@ -2152,7 +2153,7 @@ unlock:
>  		}
>  
>  		chunk = min_t(unsigned int, unix_skb_len(skb) - skip, size);
> -               chunk = state->recv_actor(skb, skip, chunk, state);
> +               chunk = state->recv_actor(skb_get(skb), skip, chunk,
> state);
>  		if (chunk < 0) {
>  			if (copied == 0)
>  				copied = -EFAULT;

I forgot a consume_skb here. Will send v2. Seems like I can review
patches much better in a mail client. :}

Bye,
Hannes