ipv6: remove from fib tree aged out RTF_CACHE dst

The commit 2b760fcf ("ipv6: hook up exception table to store
dst cache") partially reverted the commit 1e2ea8ad ("ipv6: set
dst.obsolete when a cached route has expired").

As a result, RTF_CACHE dst referenced outside the fib tree will
not be removed until the next sernum change; dst_check() does not
fail on aged-out dst, and dst->__refcnt can't decrease: the aged
out dst will stay valid for a potentially unlimited time after the
timeout expiration.

This change explicitly removes RTF_CACHE dst from the fib tree when
aged out. The rt6_remove_exception() logic will then obsolete the
dst and other entities will drop the related reference on next
dst_check().

pMTU exceptions are not aged-out, and are removed from the exception
table only when the - usually considerably longer - ip6_rt_mtu_expires
timeout expires.

v1 -> v2:
  - do not touch dst.obsolete in rt6_remove_exception(), not needed
v2 -> v3:
  - take care of pMTU exceptions, too

Fixes: 2b760fcf ("ipv6: hook up exception table to store dst cache")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1575,7 +1575,13 @@ static void rt6_age_examine_exception(struct rt6_exception_bucket *bucket,
 {
 	struct rt6_info *rt = rt6_ex->rt6i;
 
-	if (atomic_read(&rt->dst.__refcnt) == 1 &&
+	/* we are pruning and obsoleting aged-out and non gateway exceptions
+	 * even if others have still references to them, so that on next
+	 * dst_check() such references can be dropped.
+	 * EXPIRES exceptions - e.g. pmtu-generated ones are pruned when
+	 * expired, independently from their aging, as per RFC 8201 section 4
+	 */
+	if (!(rt->rt6i_flags & RTF_EXPIRES) &&
 	    time_after_eq(now, rt->dst.lastuse + gc_args->timeout)) {
 		RT6_TRACE("aging clone %p\n", rt);
 		rt6_remove_exception(bucket, rt6_ex);
@@ -1595,6 +1601,10 @@ static void rt6_age_examine_exception(struct rt6_exception_bucket *bucket,
 			rt6_remove_exception(bucket, rt6_ex);
 			return;
 		}
+	} else if (__rt6_check_expired(rt)) {
+		RT6_TRACE("purging expired route %p\n", rt);
+		rt6_remove_exception(bucket, rt6_ex);
+		return;
 	}
 	gc_args->more++;
 }