sctp: fix possibly using a bad saddr with a given dst

Under certain circumstances, depending on the order of addresses on the
interfaces, it could be that sctp_v[46]_get_dst() would return a dst
with a mismatched struct flowi.

For example, if when walking through the bind addresses and the first
one is not a match, it saves the dst as a fallback (added in
410f03831c07), but not the flowi. Then if the next one is also not a
match, the previous dst will be returned but with the flowi information
for the 2nd address, which is wrong.

The fix is to use a locally stored flowi that can be used for such
attempts, and copy it to the parameter only in case it is a possible
match, together with the corresponding dst entry.

The patch updates IPv6 code mostly just to be in sync. Even though the issue
is also present there, it fallback is not expected to work with IPv6.

Fixes: 410f03831c07 ("sctp: add routing output fallback")
Reported-by: Jin Meng <[e-mail]>
Signed-off-by: Marcelo Ricardo Leitner <[e-mail]>
Tested-by: Xin Long <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/sctp/ipv6.c

···
 {
 	struct sctp_association *asoc = t->asoc;
 	struct dst_entry *dst = NULL;
-	struct flowi6 *fl6 = &fl->u.ip6;
+	struct flowi _fl;
+	struct flowi6 *fl6 = &_fl.u.ip6;
 	struct sctp_bind_addr *bp;
 	struct ipv6_pinfo *np = inet6_sk(sk);
 	struct sctp_sockaddr_entry *laddr;
···
 	enum sctp_scope scope;
 	__u8 matchlen = 0;
 
-	memset(fl6, 0, sizeof(struct flowi6));
+	memset(&_fl, 0, sizeof(_fl));
 	fl6->daddr = daddr->v6.sin6_addr;
 	fl6->fl6_dport = daddr->v6.sin6_port;
 	fl6->flowi6_proto = IPPROTO_SCTP;
···
 	rcu_read_unlock();
 
 	dst = ip6_dst_lookup_flow(sock_net(sk), sk, fl6, final_p);
-	if (!asoc || saddr)
+	if (!asoc || saddr) {
+		t->dst = dst;
+		memcpy(fl, &_fl, sizeof(_fl));
 		goto out;
+	}
 
 	bp = &asoc->base.bind_addr;
 	scope = sctp_scope(daddr);
···
 			if ((laddr->a.sa.sa_family == AF_INET6) &&
 			    (sctp_v6_cmp_addr(&dst_saddr, &laddr->a))) {
 				rcu_read_unlock();
+				t->dst = dst;
+				memcpy(fl, &_fl, sizeof(_fl));
 				goto out;
 			}
 		}
···
 			if (!IS_ERR_OR_NULL(dst))
 				dst_release(dst);
 			dst = bdst;
+			t->dst = dst;
+			memcpy(fl, &_fl, sizeof(_fl));
 			break;
 		}
 
···
 			dst_release(dst);
 		dst = bdst;
 		matchlen = bmatchlen;
+		t->dst = dst;
+		memcpy(fl, &_fl, sizeof(_fl));
 	}
 	rcu_read_unlock();
 
···
 		struct rt6_info *rt;
 
 		rt = (struct rt6_info *)dst;
-		t->dst = dst;
 		t->dst_cookie = rt6_get_cookie(rt);
 		pr_debug("rt6_dst:%pI6/%d rt6_src:%pI6\n",
 			 &rt->rt6i_dst.addr, rt->rt6i_dst.plen,
-			 &fl6->saddr);
+			 &fl->u.ip6.saddr);
 	} else {
 		t->dst = NULL;
-
 		pr_debug("no route\n");
 	}
 }

net/sctp/protocol.c

···
 {
 	struct sctp_association *asoc = t->asoc;
 	struct rtable *rt;
-	struct flowi4 *fl4 = &fl->u.ip4;
+	struct flowi _fl;
+	struct flowi4 *fl4 = &_fl.u.ip4;
 	struct sctp_bind_addr *bp;
 	struct sctp_sockaddr_entry *laddr;
 	struct dst_entry *dst = NULL;
···
 
 	if (t->dscp & SCTP_DSCP_SET_MASK)
 		tos = t->dscp & SCTP_DSCP_VAL_MASK;
-	memset(fl4, 0x0, sizeof(struct flowi4));
+	memset(&_fl, 0x0, sizeof(_fl));
 	fl4->daddr  = daddr->v4.sin_addr.s_addr;
 	fl4->fl4_dport = daddr->v4.sin_port;
 	fl4->flowi4_proto = IPPROTO_SCTP;
···
 		 &fl4->saddr);
 
 	rt = ip_route_output_key(sock_net(sk), fl4);
-	if (!IS_ERR(rt))
+	if (!IS_ERR(rt)) {
 		dst = &rt->dst;
+		t->dst = dst;
+		memcpy(fl, &_fl, sizeof(_fl));
+	}
 
 	/* If there is no association or if a source address is passed, no
 	 * more validation is required.
···
 		odev = __ip_dev_find(sock_net(sk), laddr->a.v4.sin_addr.s_addr,
 				     false);
 		if (!odev || odev->ifindex != fl4->flowi4_oif) {
-			if (!dst)
+			if (!dst) {
 				dst = &rt->dst;
-			else
+				t->dst = dst;
+				memcpy(fl, &_fl, sizeof(_fl));
+			} else {
 				dst_release(&rt->dst);
+			}
 			continue;
 		}
 
 		dst_release(dst);
 		dst = &rt->dst;
+		t->dst = dst;
+		memcpy(fl, &_fl, sizeof(_fl));
 		break;
 	}
 
 out_unlock:
 	rcu_read_unlock();
 out:
-	t->dst = dst;
-	if (dst)
+	if (dst) {
 		pr_debug("rt_dst:%pI4, rt_src:%pI4\n",
-			 &fl4->daddr, &fl4->saddr);
-	else
+			 &fl->u.ip4.daddr, &fl->u.ip4.saddr);
+	} else {
+		t->dst = NULL;
 		pr_debug("no route\n");
+	}
 }
 
 /* For v4, the source address is cached in the route entry(dst). So no need