Re: [PATCH] net: sched: fix crash when deleting secondary chains

Re: [PATCH] net: sched: fix crash when deleting secondary chains

[Jiri Pirko]

Sun, Nov 19, 2017 at 12:44:05PM CET, code@rkapl.cz wrote:
>If you flush (delete) a filter chain other than chain 0 (such as when
>deleting the device), the kernel may run into a use-after-free. The
>chain refcount must not be decremented unless we are sure we are done
>with the chain.
>
>To reproduce the bug, run:
>    ip link add dtest type dummy
>    tc qdisc add dev dtest ingress
>    tc filter add dev dtest chain 1  parent ffff: flower
>    ip link del dtest
>
>Introduced in: commit f93e1cdcf42c ("net/sched: fix filter flushing"),
>but unless you have KAsan or luck, you won't notice it until
>commit 0dadc117ac8b ("cls_flower: use tcf_exts_get_net() before call_rcu()")


Patch looks fine. Please repost with proper "Fixes: " tag.
Feel free to add my:
Acked-by: Jiri Pirko <jiri@mellanox.com>
tag.

Thanks!



>
>Signed-off-by: Roman Kapl <code@rkapl.cz>
>---
> net/sched/cls_api.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
>diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
>index ab255b421781..7d97f612c9b9 100644
>--- a/net/sched/cls_api.c
>+++ b/net/sched/cls_api.c
>@@ -205,13 +205,14 @@ static void tcf_chain_head_change(struct tcf_chain *chain,
> 
> static void tcf_chain_flush(struct tcf_chain *chain)
> {
>-	struct tcf_proto *tp;
>+	struct tcf_proto *tp = rtnl_dereference(chain->filter_chain);
> 
> 	tcf_chain_head_change(chain, NULL);
>-	while ((tp = rtnl_dereference(chain->filter_chain)) != NULL) {
>+	while (tp) {
> 		RCU_INIT_POINTER(chain->filter_chain, tp->next);
>-		tcf_chain_put(chain);
> 		tcf_proto_destroy(tp);
>+		tp = rtnl_dereference(chain->filter_chain);
>+		tcf_chain_put(chain);
> 	}
> }
> 
>-- 
>2.15.0
>