[PATCH net] rxrpc: Don't move a peeked OOB message onto the pending queue

[Hyunwoo Kim]

rxrpc_recvmsg_oob() takes a received oob message off recvmsg_oobq and,
if a response is needed, moves it onto the pending_oobq tree. However,
only the unlink from recvmsg_oobq is guarded by MSG_PEEK; the move onto
pending_oobq always runs.

As a result, reading a challenge with MSG_PEEK leaves the skb on
recvmsg_oobq while also adding it to pending_oobq. Since struct
sk_buff's rbnode shares storage with its next and prev pointers,
rb_insert_color() overwrites the list linkage, and the skb, which holds
a single reference, becomes reachable from both queues at once.

When the socket is closed both queues are drained in turn. While
draining recvmsg_oobq, __skb_unlink() follows the next and prev
pointers that rbnode has overwritten and writes to a bad address. Also,
as the skb holds a single reference but is freed from each queue, both
the skb and the connection reference it holds are released twice. This
leads to memory corruption and to a use-after-free caused by the
connection refcount underflow.

MSG_PEEK does not consume the message from the queue, so only unlink it
from recvmsg_oobq and then move it onto pending_oobq or free it when
the message is actually consumed.

Fixes: 5800b1cf3fd8 ("rxrpc: Allow CHALLENGEs to the passed to the app for a RESPONSE")
Signed-off-by: Hyunwoo Kim <imv4bel at gmail.com>
---
 net/rxrpc/recvmsg.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/net/rxrpc/recvmsg.c b/net/rxrpc/recvmsg.c
index c940600117a4..bc7f8a505595 100644
--- a/net/rxrpc/recvmsg.c
+++ b/net/rxrpc/recvmsg.c
@@ -262,12 +262,13 @@ static int rxrpc_recvmsg_oob(struct socket *sock, struct msghdr *msg,
 		break;
 	}
 
-	if (!(flags & MSG_PEEK))
+	if (!(flags & MSG_PEEK)) {
 		skb_unlink(skb, &rx->recvmsg_oobq);
-	if (need_response)
-		rxrpc_add_pending_oob(rx, skb);
-	else
-		rxrpc_free_skb(skb, rxrpc_skb_put_oob);
+		if (need_response)
+			rxrpc_add_pending_oob(rx, skb);
+		else
+			rxrpc_free_skb(skb, rxrpc_skb_put_oob);
+	}
 	return ret;
 }
 
-- 
2.43.0