Re: [PATCH v2] net: sched: fix memory leak in tcindex_set_parms

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 14 Nov 2022 01:05:08 +0800 Hawkins Jiawei wrote:
> diff --git a/net/sched/cls_tcindex.c b/net/sched/cls_tcindex.c
> index 1c9eeb98d826..d2fac9559d3e 100644
> --- a/net/sched/cls_tcindex.c
> +++ b/net/sched/cls_tcindex.c
> @@ -338,6 +338,7 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
>  	struct tcf_result cr = {};
>  	int err, balloc = 0;
>  	struct tcf_exts e;
> +	struct tcf_exts old_e = {};

This is not a valid way of initializing a structure.
tcf_exts_init() is supposed to be called.
If we add a list member to that structure this code will break, again.

>  	err = tcf_exts_init(&e, net, TCA_TCINDEX_ACT, TCA_TCINDEX_POLICE);
>  	if (err < 0)
> @@ -479,6 +480,7 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
>  	}
>  
>  	if (old_r && old_r != r) {
> +		old_e = old_r->exts;
>  		err = tcindex_filter_result_init(old_r, cp, net);
>  		if (err < 0) {
>  			kfree(f);
> @@ -510,6 +512,12 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
>  		tcf_exts_destroy(&new_filter_result.exts);
>  	}
>  
> +	/* Note: old_e should be destroyed after the RCU grace period,
> +	 * to avoid possible use-after-free by concurrent readers.
> +	 */
> +	synchronize_rcu();
> +	tcf_exts_destroy(&old_e);

I don't think this dance is required, @cp is a copy of the original
data, and the original (@p) is destroyed in a safe manner below.

>  	if (oldp)
>  		tcf_queue_work(&oldp->rwork, tcindex_partial_destroy_work);
>  	return 0;