Re: [PATCH net v2] net: bridge: mcast: don't truncate the port group walk on teardown

On 26/08/2026 04:41, Jun Yang wrote:
> From: Jun Yang <[email protected]>
> 
> __br_multicast_disable_port_ctx() and br_multicast_del_port() walk
> port->mglist with hlist_for_each_entry_safe(). However,
> br_multicast_find_del_pg() can also delete other entries from the same
> list through br_multicast_fwd_src_remove() or __fwd_del_star_excl().
> 
> If such an entry is the iterator's saved next node, hlist_del_init()
> clears its ->next and terminates the walk early. The reproducer triggers
> this in both teardown walks, leaving port groups in the bridge mdb with
> a dangling ->key.port after del_nbp() frees the port:
> 
>    BUG: KASAN: slab-use-after-free in __mdb_fill_info+0x1191/0x1320
>     __mdb_fill_info+0x1191/0x1320
>     br_mdb_dump+0x594/0xe40
>     rtnl_mdb_dump+0x1cf/0x5d0
> 
> Use hlist_del_init_rcu() to unlink the group while preserving ->next.
> br_multicast_del_pg() and the teardown walks run under
> br->multicast_lock. The GC worker must acquire the same lock before
> detaching the group for destruction, so the node remains alive while
> the walk uses the preserved pointer.
> 
> Fixes: b08123684bd5 ("net: bridge: mcast: install S,G entries automatically based on reports")
> Cc: [email protected]
> Suggested-by: Nikolay Aleksandrov <[email protected]>
> Reported-by: TencentOS Corvus AI <[email protected]>
> Assisted-by: tencentos-corvus-ai:kimi-k3
> Signed-off-by: Jun Yang <[email protected]>
> ---
> v2: delete with hlist_del_init_rcu() to preserve ->next for an in-progress
>      teardown walk, per Nikolay's suggestion, instead of restarting the walks.
>      Tested on 7.2-rc7 with KASAN: the original use-after-free is no longer
>      reported.
> 
> v1: https://lore.kernel.org/all/[email protected]/
> 
>   net/bridge/br_multicast.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
> index 00aa9b2879d6..8022c7f12169 100644
> --- a/net/bridge/br_multicast.c
> +++ b/net/bridge/br_multicast.c
> @@ -808,7 +808,11 @@ void br_multicast_del_pg(struct net_bridge_mdb_entry *mp,
>   	struct hlist_node *tmp;
>   
>   	rcu_assign_pointer(*pp, pg->next);
> -	hlist_del_init(&pg->mglist);
> +	/* Keep ->next (held under multicast_lock, freed later by the GC work):
> +	 * a port->mglist teardown walk may have latched this node as its next,
> +	 * and deleting other groups of the same port must not truncate it.
> +	 */
> +	hlist_del_init_rcu(&pg->mglist);
>   	br_multicast_eht_clean_sets(pg);
>   	hlist_for_each_entry_safe(ent, tmp, &pg->src_list, node)
>   		br_multicast_del_group_src(ent, false);

Acked-by: Nikolay Aleksandrov <[email protected]>