macsec: fix NULL deref in macsec_add_rxsa

Commit 48ef50f added a test on tb_sa[MACSEC_SA_ATTR_PN], but
nothing guarantees that it's not NULL at this point. The same code was
added to macsec_add_txsa, but there it's not a problem because
validate_add_txsa checks that the MACSEC_SA_ATTR_PN attribute is
present.

Note: it's not possible to reproduce with iproute, because iproute
doesn't allow creating an SA without specifying the PN.

Fixes: 48ef50f ("macsec: Netlink support of XPN cipher suites (IEEE 802.1AEbw)")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=208315
Reported-by: Frantisek Sumsal <fsumsal@redhat.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: qsn

drivers/net/macsec.c

@@ -1753,7 +1753,8 @@ static int macsec_add_rxsa(struct sk_buff *skb, struct genl_info *info)
 	}
 
 	pn_len = secy->xpn ? MACSEC_XPN_PN_LEN : MACSEC_DEFAULT_PN_LEN;
-	if (nla_len(tb_sa[MACSEC_SA_ATTR_PN]) != pn_len) {
+	if (tb_sa[MACSEC_SA_ATTR_PN] &&
+	    nla_len(tb_sa[MACSEC_SA_ATTR_PN]) != pn_len) {
 		pr_notice("macsec: nl: add_rxsa: bad pn length: %d != %d\n",
 			  nla_len(tb_sa[MACSEC_SA_ATTR_PN]), pn_len);
 		rtnl_unlock();