Re: [PATCH net] net/sched: taprio: always validate TCA_TAPRIO_ATTR_PRIOMAP

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Tue, Jun 04, 2024 at 06:15:11PM +0000, Eric Dumazet wrote:
> If one TCA_TAPRIO_ATTR_PRIOMAP attribute has been provided,
> taprio_parse_mqprio_opt() must validate it, or userspace
> can inject arbitrary data to the kernel, the second time
> taprio_change() is called.
> 
> First call (with valid attributes) sets dev->num_tc
> to a non zero value.
> 
> Second call (with arbitrary mqprio attributes)
> returns early from taprio_parse_mqprio_opt()
> and bad things can happen.
> 
> Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")
> Reported-by: Noam Rathaus <noamr@ssd-disclosure.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Vinicius Costa Gomes <vinicius.gomes@intel.com>
> Cc: Vladimir Oltean <vladimir.oltean@nxp.com>
> ---
>  net/sched/sch_taprio.c | 15 ++++++---------
>  1 file changed, 6 insertions(+), 9 deletions(-)
> 
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index 937a0c513c17..b284a06b5a75 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -1176,16 +1176,13 @@ static int taprio_parse_mqprio_opt(struct net_device *dev,
>  {
>  	bool allow_overlapping_txqs = TXTIME_ASSIST_IS_ENABLED(taprio_flags);
>  
> -	if (!qopt && !dev->num_tc) {
> -		NL_SET_ERR_MSG(extack, "'mqprio' configuration is necessary");
> -		return -EINVAL;
> -	}
> -
> -	/* If num_tc is already set, it means that the user already
> -	 * configured the mqprio part
> -	 */
> -	if (dev->num_tc)
> +	if (!qopt) {
> +		if (!dev->num_tc) {
> +			NL_SET_ERR_MSG(extack, "'mqprio' configuration is necessary");
> +			return -EINVAL;
> +		}
>  		return 0;
> +	}
>  
>  	/* taprio imposes that traffic classes map 1:n to tx queues */
>  	if (qopt->num_tc > dev->num_tx_queues) {
> -- 
> 2.45.2.505.gda0bf45e8d-goog
>

Correct, the mqprio qopt structure should be validated whenever passed,
not just if dev->num_tc == 0.

But... what bad things can happen?

Whenever I try to trick the kernel into doing something with the second
mqprio, I get:

"Changing the traffic mapping of a running schedule is not supported."