[PATCH 3.16 119/134] ipv4: restore rt->fi for reference counting

[Ben Hutchings]

3.16.47-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: WANG Cong <xiyou.wangcong@xxxxxxxxx>

commit 82486aa6f1b9bc8145e6d0fa2bc0b44307f3b875 upstream.

IPv4 dst could use fi->fib_metrics to store metrics but fib_info
itself is refcnt'ed, so without taking a refcnt fi and
fi->fib_metrics could be freed while dst metrics still points to
it. This triggers use-after-free as reported by Andrey twice.

This patch reverts commit 2860583fe840 ("ipv4: Kill rt->fi") to
restore this reference counting. It is a quick fix for -net and
-stable, for -net-next, as Eric suggested, we can consider doing
reference counting for metrics itself instead of relying on fib_info.

IPv6 is very different, it copies or steals the metrics from mx6_config
in fib6_commit_metrics() so probably doesn't need a refcnt.

Decnet has already done the refcnt'ing, see dn_fib_semantic_match().

Fixes: 2860583fe840 ("ipv4: Kill rt->fi")
Reported-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Tested-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
Signed-off-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[bwh: Backported to 3.16:
 - Update all 5 places where rtable is initialised
 - Open-code fib_info_hold()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/include/net/route.h
+++ b/include/net/route.h
@@ -64,6 +64,7 @@ struct rtable {
 	u32			rt_pmtu;
 
 	struct list_head	rt_uncached;
+	struct fib_info		*fi; /* for refcnt to shared metrics */
 };
 
 static inline bool rt_is_input_route(const struct rtable *rt)
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1346,6 +1346,11 @@ static void ipv4_dst_destroy(struct dst_
 {
 	struct rtable *rt = (struct rtable *) dst;
 
+	if (rt->fi) {
+		fib_info_put(rt->fi);
+		rt->fi = NULL;
+	}
+
 	if (!list_empty(&rt->rt_uncached)) {
 		spin_lock_bh(&rt_uncached_lock);
 		list_del(&rt->rt_uncached);
@@ -1378,6 +1383,16 @@ static bool rt_cache_valid(const struct
 		!rt_is_expired(rt);
 }
 
+static void rt_init_metrics(struct rtable *rt, struct fib_info *fi)
+{
+	if (fi->fib_metrics != (u32 *)dst_default_metrics) {
+		atomic_inc(&fi->fib_clntref);
+		rt->fi = fi;
+	}
+
+	dst_init_metrics(&rt->dst, fi->fib_metrics, true);
+}
+
 static void rt_set_nexthop(struct rtable *rt, __be32 daddr,
 			   const struct fib_result *res,
 			   struct fib_nh_exception *fnhe,
@@ -1392,7 +1407,7 @@ static void rt_set_nexthop(struct rtable
 			rt->rt_gateway = nh->nh_gw;
 			rt->rt_uses_gateway = 1;
 		}
-		dst_init_metrics(&rt->dst, fi->fib_metrics, true);
+		rt_init_metrics(rt, fi);
 #ifdef CONFIG_IP_ROUTE_CLASSID
 		rt->dst.tclassid = nh->nh_tclassid;
 #endif
@@ -1480,6 +1495,7 @@ static int ip_route_input_mc(struct sk_b
 	rth->rt_pmtu	= 0;
 	rth->rt_gateway	= 0;
 	rth->rt_uses_gateway = 0;
+	rth->fi = NULL;
 	INIT_LIST_HEAD(&rth->rt_uncached);
 	if (our) {
 		rth->dst.input= ip_local_deliver;
@@ -1649,6 +1665,7 @@ rt_cache:
 	rth->rt_pmtu	= 0;
 	rth->rt_gateway	= 0;
 	rth->rt_uses_gateway = 0;
+	rth->fi = NULL;
 	INIT_LIST_HEAD(&rth->rt_uncached);
 	RT_CACHE_STAT_INC(in_slow_tot);
 
@@ -1823,6 +1840,7 @@ local_input:
 	rth->rt_pmtu	= 0;
 	rth->rt_gateway	= 0;
 	rth->rt_uses_gateway = 0;
+	rth->fi = NULL;
 	INIT_LIST_HEAD(&rth->rt_uncached);
 	RT_CACHE_STAT_INC(in_slow_tot);
 	if (res.type == RTN_UNREACHABLE) {
@@ -2037,6 +2055,7 @@ add:
 	rth->rt_pmtu	= 0;
 	rth->rt_gateway = 0;
 	rth->rt_uses_gateway = 0;
+	rth->fi = NULL;
 	INIT_LIST_HEAD(&rth->rt_uncached);
 
 	RT_CACHE_STAT_INC(out_slow_tot);
@@ -2316,6 +2335,9 @@ struct dst_entry *ipv4_blackhole_route(s
 		rt->rt_type = ort->rt_type;
 		rt->rt_gateway = ort->rt_gateway;
 		rt->rt_uses_gateway = ort->rt_uses_gateway;
+		rt->fi = ort->fi;
+		if (rt->fi)
+			atomic_inc(&rt->fi->fib_clntref);
 
 		INIT_LIST_HEAD(&rt->rt_uncached);