[PATCH net] udp6: fix potential access to stale information

[Eric Dumazet <edumazet@google.com>]

lena wang reported an issue caused by udpv6_sendmsg()
mangling msg->msg_name and msg->msg_namelen, which
are later read from ____sys_sendmsg() :

	/*
	 * If this is sendmmsg() and sending to current destination address was
	 * successful, remember it.
	 */
	if (used_address && err >= 0) {
		used_address->name_len = msg_sys->msg_namelen;
		if (msg_sys->msg_name)
			memcpy(&used_address->name, msg_sys->msg_name,
			       used_address->name_len);
	}

udpv6_sendmsg() wants to pretend the remote address family
is AF_INET in order to call udp_sendmsg().

A fix would be to modify the address in-place, instead
of using a local variable, but this could have other side effects.

Instead, restore initial values before we return from udpv6_sendmsg().

Fixes: c71d8ebe7a44 ("net: Fix security_socket_sendmsg() bypass problem.")
Reported-by: lena wang <lena.wang@mediatek.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
---
 net/ipv6/udp.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 9fb2f33ee3a76a09bbe15a9aaf1371a804f91ee2..a675acfb901d102ce56563b1d50ae827d9e04859 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1395,9 +1395,11 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 			msg->msg_name = &sin;
 			msg->msg_namelen = sizeof(sin);
 do_udp_sendmsg:
-			if (ipv6_only_sock(sk))
-				return -ENETUNREACH;
-			return udp_sendmsg(sk, msg, len);
+			err = ipv6_only_sock(sk) ?
+				-ENETUNREACH : udp_sendmsg(sk, msg, len);
+			msg->msg_name = sin6;
+			msg->msg_namelen = addr_len;
+			return err;
 		}
 	}
 
-- 
2.40.0.577.gac1e443424-goog