[PATCH v2 net] ipv6: Fix dst leak for uncached routes.

[Kuniyuki Iwashima <kuniyu@google.com>]

ip6_route_output_flags(), ip6_rt_put_flags(), and ip6_dst_check()
detect an uncached route by list_empty(&rt->dst.rt_uncached),
which replaced the static DST_NOCACHE flag check in commit
a4c2fd7f7891 ("net: remove DST_NOCACHE flag").

When a device is unregistered, rt6_uncached_list_flush_dev()
unlinks uncached routes tied to the device from rt6_uncached_list.

Previously, they were moved to another list with list_move()
(__list_del_entry() + list_add()), and since commit 98aa546af5e4
("inet: remove (struct uncached_list)->quarantine"), the routes
are just unlinked with list_del_init().

If list_del_init() runs concurrently, list_empty() evaluates to
true; ip6_route_output_flags() calls dst_hold_safe() incorrectly
and ip6_rt_put_flags() skips ip6_rt_put(), leaking dst, and thus
dev tied via rt->from as well.

The same race is partially fixed by commit 9a6f0c4d5796 ("dst:
fix races in rt6_uncached_list_del() and rt_del_uncached_list()").

Let's check rt6->dst.rt_uncached_list instead.

Note that IPv4 does not have the same issue.

Fixes: 98aa546af5e4 ("inet: remove (struct uncached_list)->quarantine")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Reviewed-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
---
v2: Add a comment, update stale comments, correct Fixes: tag
v1: https://lore.kernel.org/netdev/20260918041439.2575935-1-kuniyu@google.com/
---
 include/net/ip6_route.h | 4 ++--
 net/ipv6/route.c        | 7 ++++---
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index b9e8d2b759e9..0f9b7a260d25 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -101,12 +101,12 @@ static inline struct dst_entry *ip6_route_output(struct net *net,
 }
 
 /* Only conditionally release dst if flags indicates
- * !RT6_LOOKUP_F_DST_NOREF or dst is in uncached_list.
+ * !RT6_LOOKUP_F_DST_NOREF or dst is uncached.
  */
 static inline void ip6_rt_put_flags(struct rt6_info *rt, int flags)
 {
 	if (!(flags & RT6_LOOKUP_F_DST_NOREF) ||
-	    !list_empty(&rt->dst.rt_uncached))
+	    rt->dst.rt_uncached_list)
 		ip6_rt_put(rt);
 }
 
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 08bd68f1b5bb..70d643336610 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -139,6 +139,7 @@ void rt6_uncached_list_add(struct rt6_info *rt)
 {
 	struct uncached_list *ul = raw_cpu_ptr(&rt6_uncached_list);
 
+	/* Set once and never cleared: non-NULL marks an uncached route. */
 	rt->dst.rt_uncached_list = ul;
 
 	spin_lock_bh(&ul->lock);
@@ -2721,8 +2722,8 @@ struct dst_entry *ip6_route_output_flags(struct net *net,
 	rcu_read_lock();
 	dst = ip6_route_output_flags_noref(net, sk, fl6, flags);
 	rt6 = dst_rt6_info(dst);
-	/* For dst cached in uncached_list, refcnt is already taken. */
-	if (list_empty(&rt6->dst.rt_uncached) && !dst_hold_safe(dst)) {
+	/* For an uncached dst, refcnt is already taken. */
+	if (!rt6->dst.rt_uncached_list && !dst_hold_safe(dst)) {
 		dst = &net->ipv6.ip6_null_entry->dst;
 		dst_hold(dst);
 	}
@@ -2831,7 +2832,7 @@ INDIRECT_CALLABLE_SCOPE struct dst_entry *ip6_dst_check(struct dst_entry *dst,
 	from = rcu_dereference(rt->from);
 
 	if (from && (rt->rt6i_flags & RTF_PCPU ||
-	    unlikely(!list_empty(&rt->dst.rt_uncached))))
+	    unlikely(rt->dst.rt_uncached_list)))
 		dst_ret = rt6_dst_from_check(rt, from, cookie);
 	else
 		dst_ret = rt6_check(rt, from, cookie);
-- 
2.55.0.1082.g2b9226bbc0-goog