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 <[email protected]>
Signed-off-by: Quentin Perret <[email protected]>
---
kernel/sched/core.c | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e514a093a0ba..ad055fb9ed2d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6523,6 +6523,16 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
return -E2BIG;
}
+static void get_params(struct task_struct *p, struct sched_attr *attr)
+{
+ if (task_has_dl_policy(p))
+ __getparam_dl(p, attr);
+ else if (task_has_rt_policy(p))
+ attr->sched_priority = p->rt_priority;
+ else
+ attr->sched_nice = task_nice(p);
+}
+
/**
* sys_sched_setscheduler - set/change the scheduler policy and RT priority
* @pid: the pid in question.
@@ -6584,6 +6594,8 @@ 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)
+ get_params(p, &attr);
retval = sched_setattr(p, &attr);
put_task_struct(p);
}
@@ -6732,12 +6744,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
kattr.sched_policy = p->policy;
if (p->sched_reset_on_fork)
kattr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
- if (task_has_dl_policy(p))
- __getparam_dl(p, &kattr);
- else if (task_has_rt_policy(p))
- kattr.sched_priority = p->rt_priority;
- else
- kattr.sched_nice = task_nice(p);
+ get_params(p, &kattr);
#ifdef CONFIG_UCLAMP_TASK
/*
--
2.32.0.288.g62a8d224e6-goog
On 06/23/21 12:34, 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 <[email protected]>
> Signed-off-by: Quentin Perret <[email protected]>
> ---
LGTM.
Reviewed-by: Qais Yousef <[email protected]>
Cheers
--
Qais Yousef