broken ping to ipv6 linklocal addresses on debian buster

I am seeing ping failures to IPv6 linklocal addresses with Debian
buster. Easiest example to reproduce is:

$ ping -c1 -w1 ff02::1%eth1
connect: Invalid argument

$ ping -c1 -w1 ff02::1%eth1
PING ff02::01%eth1(ff02::1%eth1) 56 data bytes
64 bytes from fe80::e0:f9ff:fe0c:37%eth1: icmp_seq=1 ttl=64 time=0.059 ms

git bisect traced the failure to
commit b9ef5513c99b ("smack: Check address length before reading address family")

Arguably ping is being stupid since the buster version is not setting
the address family properly (ping on stretch for example does):

$ strace -e connect ping6 -c1 -w1 ff02::1%eth1
connect(5, {sa_family=AF_UNSPEC,
sa_data="\4\1\0\0\0\0\377\2\0\0\0\0\0\0\0\0\0\0\0\0\0\1\3\0\0\0"}, 28)
= -1 EINVAL (Invalid argument)

but the command works fine on kernels prior to this commit, so this is
breakage which goes against the Linux paradigm of "don't break userspace"

Cc: stable@vger.kernel.org
Reported-by: David Ahern <dsahern@gmail.com>
Suggested-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

 security/smack/smack_lsm.c | 41 +++++++++++++++++++----------------------
 1 file changed, 19 insertions(+), 22 deletions(-)

1 files changed, 19 insertions, 22 deletions

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index ecea41ce919b..8bc7b04769a8 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -2831,42 +2831,39 @@ static int smack_socket_connect(struct socket *sock, struct sockaddr *sap,
 				int addrlen)
 {
 	int rc = 0;
-#if IS_ENABLED(CONFIG_IPV6)
-	struct sockaddr_in6 *sip = (struct sockaddr_in6 *)sap;
-#endif
-#ifdef SMACK_IPV6_SECMARK_LABELING
-	struct smack_known *rsp;
-	struct socket_smack *ssp;
-#endif
 
 	if (sock->sk == NULL)
 		return 0;
-
+	if (sock->sk->sk_family != PF_INET &&
+	    (!IS_ENABLED(CONFIG_IPV6) || sock->sk->sk_family != PF_INET6))
+		return 0;
+	if (addrlen < offsetofend(struct sockaddr, sa_family))
+		return 0;
+	if (IS_ENABLED(CONFIG_IPV6) && sap->sa_family == AF_INET6) {
+		struct sockaddr_in6 *sip = (struct sockaddr_in6 *)sap;
 #ifdef SMACK_IPV6_SECMARK_LABELING
-	ssp = sock->sk->sk_security;
+		struct smack_known *rsp;
 #endif
 
-	switch (sock->sk->sk_family) {
-	case PF_INET:
-		if (addrlen < sizeof(struct sockaddr_in) ||
-		    sap->sa_family != AF_INET)
-			return -EINVAL;
-		rc = smack_netlabel_send(sock->sk, (struct sockaddr_in *)sap);
-		break;
-	case PF_INET6:
-		if (addrlen < SIN6_LEN_RFC2133 || sap->sa_family != AF_INET6)
-			return -EINVAL;
+		if (addrlen < SIN6_LEN_RFC2133)
+			return 0;
 #ifdef SMACK_IPV6_SECMARK_LABELING
 		rsp = smack_ipv6host_label(sip);
-		if (rsp != NULL)
+		if (rsp != NULL) {
+			struct socket_smack *ssp = sock->sk->sk_security;
+
 			rc = smk_ipv6_check(ssp->smk_out, rsp, sip,
-						SMK_CONNECTING);
+					    SMK_CONNECTING);
+		}
 #endif
 #ifdef SMACK_IPV6_PORT_LABELING
 		rc = smk_ipv6_port_check(sock->sk, sip, SMK_CONNECTING);
 #endif
-		break;
+		return rc;
 	}
+	if (sap->sa_family != AF_INET || addrlen < sizeof(struct sockaddr_in))
+		return 0;
+	rc = smack_netlabel_send(sock->sk, (struct sockaddr_in *)sap);
 	return rc;
 }