Re: [PATCH net] net/sched: cls_fw: Fix improper refcount update leads to use-after-free

[Pedro Tammela <pctammela@mojatatu.com>]

On 05/07/2023 13:15, jhs@mojatatu.com wrote:
> From: M A Ramdhan <ramdhan@starlabs.sg>
> 
> In the event of a failure in tcf_change_indev(), fw_set_parms() will
> immediately return an error after incrementing or decrementing
> reference counter in tcf_bind_filter().  If attacker can control
> reference counter to zero and make reference freed, leading to
> use after free.
> 
> In order to prevent this, move the point of possible failure above the
> point where the TC_FW_CLASSID is handled.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: M A Ramdhan <ramdhan@starlabs.sg>
> Signed-off-by: M A Ramdhan <ramdhan@starlabs.sg>
> Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>

LGTM,

Reviewed-by: Pedro Tammela <pctammela@mojatatu.com>

> ---
>   net/sched/cls_fw.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/net/sched/cls_fw.c b/net/sched/cls_fw.c
> index ae9439a6c56c..8641f8059317 100644
> --- a/net/sched/cls_fw.c
> +++ b/net/sched/cls_fw.c
> @@ -212,11 +212,6 @@ static int fw_set_parms(struct net *net, struct tcf_proto *tp,
>   	if (err < 0)
>   		return err;
>   
> -	if (tb[TCA_FW_CLASSID]) {
> -		f->res.classid = nla_get_u32(tb[TCA_FW_CLASSID]);
> -		tcf_bind_filter(tp, &f->res, base);
> -	}
> -
>   	if (tb[TCA_FW_INDEV]) {
>   		int ret;
>   		ret = tcf_change_indev(net, tb[TCA_FW_INDEV], extack);
> @@ -233,6 +228,11 @@ static int fw_set_parms(struct net *net, struct tcf_proto *tp,
>   	} else if (head->mask != 0xFFFFFFFF)
>   		return err;
>   
> +	if (tb[TCA_FW_CLASSID]) {
> +		f->res.classid = nla_get_u32(tb[TCA_FW_CLASSID]);
> +		tcf_bind_filter(tp, &f->res, base);
> +	}
> +
>   	return 0;
>   }
>