ipv6: prevent infinite loop in rt6_nlmsg_size()

While testing prior patch, I was able to trigger
an infinite loop in rt6_nlmsg_size() in the following place:

list_for_each_entry_rcu(sibling, &f6i->fib6_siblings,
			fib6_siblings) {
	rt6_nh_nlmsg_size(sibling->fib6_nh, &nexthop_len);
}

This is because fib6_del_route() and fib6_add_rt2node()
uses list_del_rcu(), which can confuse rcu readers,
because they might no longer see the head of the list.

Restart the loop if f6i->fib6_nsiblings is zero.

Fixes: d9ccb18f83ea ("ipv6: Fix soft lockups in fib6_select_path under high next hop churn")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250725140725.3626540-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 93578b2ec35fb55ca1f56c3e558661989568639e..af7db69d9eac97033fde46c5ee57e0cfb53a36db 100644 (file)
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -1265,7 +1265,7 @@ add:
                                                         &rt->fib6_siblings,
                                                         fib6_siblings)
                                        sibling->fib6_nsiblings--;
-                               rt->fib6_nsiblings = 0;
+                               WRITE_ONCE(rt->fib6_nsiblings, 0);
                                list_del_rcu(&rt->fib6_siblings);
                                rcu_read_lock();
                                rt6_multipath_rebalance(next_sibling);
@@ -2015,7 +2015,7 @@ static void fib6_del_route(struct fib6_table *table, struct fib6_node *fn,
                list_for_each_entry_safe(sibling, next_sibling,
                                         &rt->fib6_siblings, fib6_siblings)
                        sibling->fib6_nsiblings--;
-               rt->fib6_nsiblings = 0;
+               WRITE_ONCE(rt->fib6_nsiblings, 0);
                list_del_rcu(&rt->fib6_siblings);
                rt6_multipath_rebalance(next_sibling);
        }

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 9f92129efa05087d435575aa84c81b30430db249..6d4e147ae46bc245ccd195e5fb0e254f34ec65a4 100644 (file)
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -5670,32 +5670,34 @@ static int rt6_nh_nlmsg_size(struct fib6_nh *nh, void *arg)
 
 static size_t rt6_nlmsg_size(struct fib6_info *f6i)
 {
+       struct fib6_info *sibling;
+       struct fib6_nh *nh;
        int nexthop_len;
 
        if (f6i->nh) {
                nexthop_len = nla_total_size(4); /* RTA_NH_ID */
                nexthop_for_each_fib6_nh(f6i->nh, rt6_nh_nlmsg_size,
                                         &nexthop_len);
-       } else {
-               struct fib6_nh *nh = f6i->fib6_nh;
-               struct fib6_info *sibling;
-
-               nexthop_len = 0;
-               if (f6i->fib6_nsiblings) {
-                       rt6_nh_nlmsg_size(nh, &nexthop_len);
-
-                       rcu_read_lock();
+               goto common;
+       }
 
-                       list_for_each_entry_rcu(sibling, &f6i->fib6_siblings,
-                                               fib6_siblings) {
-                               rt6_nh_nlmsg_size(sibling->fib6_nh, &nexthop_len);
-                       }
+       rcu_read_lock();
+retry:
+       nh = f6i->fib6_nh;
+       nexthop_len = 0;
+       if (READ_ONCE(f6i->fib6_nsiblings)) {
+               rt6_nh_nlmsg_size(nh, &nexthop_len);
 
-                       rcu_read_unlock();
+               list_for_each_entry_rcu(sibling, &f6i->fib6_siblings,
+                                       fib6_siblings) {
+                       rt6_nh_nlmsg_size(sibling->fib6_nh, &nexthop_len);
+                       if (!READ_ONCE(f6i->fib6_nsiblings))
+                               goto retry;
                }
-               nexthop_len += lwtunnel_get_encap_size(nh->fib_nh_lws);
        }
-
+       rcu_read_unlock();
+       nexthop_len += lwtunnel_get_encap_size(nh->fib_nh_lws);
+common:
        return NLMSG_ALIGN(sizeof(struct rtmsg))
               + nla_total_size(16) /* RTA_SRC */
               + nla_total_size(16) /* RTA_DST */