[PATCH net] udp: revalidate socket family before publishing an IPv6 cork

[Daehyeon Ko <4ncienth@gmail.com>]

udpv6_sendmsg() prepares the IPv6 flow and route before taking the socket
lock when a datagram is corked.  IPV6_ADDRFORM takes the same lock, but it
can convert the socket to AF_INET while the send path is doing that
lockless preparation because no cork has been published yet.

If the conversion wins the race, udpv6_sendmsg() later publishes an
AF_INET6 cork on an AF_INET socket.  Uncorking through the IPv4 socket
operations then interprets the IPv6 cork as IPv4 state.  The IPv4
finalizer writes a 20-byte IPv4 header into the 40-byte IPv6 header
reservation while the retained IPv6 dst routes the skb through
ip6_output().  ip6_finish_output2() consequently consumes the unwritten
20-byte tail.

An unprivileged reproducer triggered the mixed state on 12 of 10,000
sockets.  KMSAN reported an uninitialized-value read in
ip6_finish_output2() on three fresh boots, with the allocation origin in
__alloc_skb() through __ip6_append_data().  The same process recovered the
20-byte region from the TX timestamp error queue; one of three fresh boots
contained recognizable stale heap data.

After taking the lock, revalidate that IPV6_ADDRFORM has not changed the
socket family before publishing the cork.  The existing error path releases
the prepared dst, flowlabel, and transmit-option references.  With this
change, the serialized controls retain their existing results and the
forbidden mixed state occurred zero times across 20,000 sockets.

Fixes: 03485f2adcde ("udpv6: Add lockless sendmsg() support")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
 net/ipv6/udp.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index fd875908ac0c66..566c634a5a5945 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1716,6 +1716,11 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	}
 
 	lock_sock(sk);
+	if (unlikely(sk->sk_family != AF_INET6)) {
+		release_sock(sk);
+		err = -EAFNOSUPPORT;
+		goto out;
+	}
 	if (unlikely(up->pending)) {
 		/* The socket is already corked while preparing it. */
 		/* ... which is an evident application bug. --ANK */
-- 
2.54.0