ipv6: route: enforce RCU protection in rt6_update_exception_stamp_rt()

mainline inclusion
from mainline-v5.0-rc7
commit 193f3685
category: bugfix
bugzilla: 10918
CVE: NA

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

We must access rt6_info->from under RCU read lock: move the
dereference under such lock, with proper annotation.

v1 -> v2:
 - avoid using multiple, racy, fetch operations for rt->from

Fixes: a68886a6 ("net/ipv6: Make from in rt6_info rcu protected")
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NLin Miaohe <linmiaohe@huawei.com>
Reviewed-by: NMao Wenan <maowenan@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/ipv6/route.c

@@ -1612,15 +1612,15 @@ static int rt6_remove_exception_rt(struct rt6_info *rt)
 static void rt6_update_exception_stamp_rt(struct rt6_info *rt)
 {
 	struct rt6_exception_bucket *bucket;
-	struct fib6_info *from = rt->from;
 	struct in6_addr *src_key = NULL;
 	struct rt6_exception *rt6_ex;
-
-	if (!from ||
-	    !(rt->rt6i_flags & RTF_CACHE))
-		return;
+	struct fib6_info *from;
 
 	rcu_read_lock();
+	from = rcu_dereference(rt->from);
+	if (!from || !(rt->rt6i_flags & RTF_CACHE))
+		goto unlock;
+
 	bucket = rcu_dereference(from->rt6i_exception_bucket);
 
 #ifdef CONFIG_IPV6_SUBTREES
@@ -1639,6 +1639,7 @@ static void rt6_update_exception_stamp_rt(struct rt6_info *rt)
 	if (rt6_ex)
 		rt6_ex->stamp = jiffies;
 
+unlock:
 	rcu_read_unlock();
 }