net: ipv4: don't let PMTU updates increase route MTU

When an MTU update with PMTU smaller than net.ipv4.route.min_pmtu is
received, we must clamp its value. However, we can receive a PMTU
exception with PMTU < old_mtu < ip_rt_min_pmtu, which would lead to an
increase in PMTU.

To fix this, take the smallest of the old MTU and ip_rt_min_pmtu.

Before this patch, in case of an update, the exception's MTU would
always change. Now, an exception can have only its lock flag updated,
but not the MTU, so we need to add a check on locking to the following
"is this exception getting updated, or close to expiring?" test.

Fixes: d52e5a7e ("ipv4: lock mtu in fnhe when received PMTU < net.ipv4.route.min_pmtu")
Signed-off-by: NSabrina Dubroca <sd@queasysnail.net>
Reviewed-by: NStefano Brivio <sbrivio@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1001,21 +1001,22 @@ out:	kfree_skb(skb);
 static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
 {
 	struct dst_entry *dst = &rt->dst;
+	u32 old_mtu = ipv4_mtu(dst);
 	struct fib_result res;
 	bool lock = false;
 
 	if (ip_mtu_locked(dst))
 		return;
 
-	if (ipv4_mtu(dst) < mtu)
+	if (old_mtu < mtu)
 		return;
 
 	if (mtu < ip_rt_min_pmtu) {
 		lock = true;
-		mtu = ip_rt_min_pmtu;
+		mtu = min(old_mtu, ip_rt_min_pmtu);
 	}
 
-	if (rt->rt_pmtu == mtu &&
+	if (rt->rt_pmtu == mtu && !lock &&
 	    time_before(jiffies, dst->expires - ip_rt_mtu_expires / 2))
 		return;