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authorHannes Frederic Sowa <hannes@stressinduktion.org>2015-11-10 16:23:15 +0100
committerDavid S. Miller <davem@davemloft.net>2015-11-15 13:16:34 -0500
commit73ed5d25dce0354ea381d6dc93005c3085fae03d (patch)
tree38dbb1ef110ad0c1edfa88e331051d328adc6197
parent49e4a2293035b420e807e739999d59c8ec1488e9 (diff)
af-unix: fix use-after-free with concurrent readers while splicing
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. Fixes: 2b514574f7e8 ("net: af_unix: implement splice for stream af_unix sockets") Reported-by: Dmitry Vyukov <dvyukov@google.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Eric Dumazet <eric.dumazet@gmail.com> Acked-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org> Signed-off-by: David S. Miller <davem@davemloft.net>
-rw-r--r--net/unix/af_unix.c18
1 files changed, 18 insertions, 0 deletions
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index aaa0b58d6aba..12b886f07982 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);
}
@@ -2072,6 +2073,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state)
do {
int chunk;
+ bool drop_skb;
struct sk_buff *skb, *last;
unix_state_lock(sk);
@@ -2152,7 +2154,11 @@ unlock:
}
chunk = min_t(unsigned int, unix_skb_len(skb) - skip, size);
+ skb_get(skb);
chunk = state->recv_actor(skb, skip, chunk, state);
+ drop_skb = !unix_skb_len(skb);
+ /* skb is only safe to use if !drop_skb */
+ consume_skb(skb);
if (chunk < 0) {
if (copied == 0)
copied = -EFAULT;
@@ -2161,6 +2167,18 @@ unlock:
copied += chunk;
size -= chunk;
+ if (drop_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
+ */
+ err = 0;
+ break;
+ }
+
/* Mark read part of skb as used */
if (!(flags & MSG_PEEK)) {
UNIXCB(skb).consumed += chunk;