Re: [PATCH v1 net] ipv6: Fix dst leak for uncached routes.

[Hangbin Liu <hangbin.liu@linux.dev>]

On Fri, Sep 18, 2026 at 04:14:37AM +0000, Kuniyuki Iwashima wrote:
> 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: 7d9e5f422150 ("ipv6: convert major tx path to use RT6_LOOKUP_F_DST_NOREF")
> Fixes: d64a1f574a29 ("ipv6: honor RT6_LOOKUP_F_DST_NOREF in rule lookup logic")
> Fixes: a4c2fd7f7891 ("net: remove DST_NOCACHE flag")
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> ---
>  include/net/ip6_route.h | 2 +-
>  net/ipv6/route.c        | 4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
> index b9e8d2b759e9..2c5ded121f54 100644
> --- a/include/net/ip6_route.h
> +++ b/include/net/ip6_route.h
> @@ -106,7 +106,7 @@ static inline struct dst_entry *ip6_route_output(struct net *net,
>  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..b18cd0d9148c 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -2722,7 +2722,7 @@ struct dst_entry *ip6_route_output_flags(struct net *net,
>  	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)) {
> +	if (!rt6->dst.rt_uncached_list && !dst_hold_safe(dst)) {
>  		dst = &net->ipv6.ip6_null_entry->dst;
>  		dst_hold(dst);
>  	}
> @@ -2831,7 +2831,7 @@ 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
> 

Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>