Re: [PATCH v2 2/3] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 10, 2021 at 03:13:05PM +0000, Quentin Perret wrote:
> SCHED_FLAG_KEEP_PARAMS can be passed to sched_setattr to specify that
> the call must not touch scheduling parameters (nice or priority). This
> is particularly handy for uclamp when used in conjunction with
> SCHED_FLAG_KEEP_POLICY as that allows to issue a syscall that only
> impacts uclamp values.
> 
> However, sched_setattr always checks whether the priorities and nice
> values passed in sched_attr are valid first, even if those never get
> used down the line. This is useless at best since userspace can
> trivially bypass this check to set the uclamp values by specifying low
> priorities. However, it is cumbersome to do so as there is no single
> expression of this that skips both RT and CFS checks at once. As such,
> userspace needs to query the task policy first with e.g. sched_getattr
> and then set sched_attr.sched_priority accordingly. This is racy and
> slower than a single call.
> 
> As the priority and nice checks are useless when SCHED_FLAG_KEEP_PARAMS
> is specified, simply inherit them in this case to match the policy
> inheritance of SCHED_FLAG_KEEP_POLICY.
> 
> Reported-by: Wei Wang <wvw@google.com>
> Signed-off-by: Quentin Perret <qperret@google.com>
> ---
>  kernel/sched/core.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3b213402798e..1d4aedbbcf96 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6585,6 +6585,10 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
>  	rcu_read_unlock();
>  
>  	if (likely(p)) {
> +		if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS) {
> +			attr.sched_priority = p->rt_priority;
> +			attr.sched_nice = task_nice(p);
> +		}
>  		retval = sched_setattr(p, &attr);
>  		put_task_struct(p);
>  	}

I don't like this much... afaict the KEEP_PARAMS clause in
__setscheduler() also covers the DL params, and you 'forgot' to copy
those.

Can't we short circuit the validation logic?