Messages in this thread Patch in this message |  | | Date | Mon, 19 Jul 2021 17:16:56 +0100 | Subject | [PATCH v4 2/2] sched: Skip priority checks with SCHED_FLAG_KEEP_PARAMS | From | Quentin Perret <> |
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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> Reviewed-by: Qais Yousef <qais.yousef@arm.com> Signed-off-by: Quentin Perret <qperret@google.com> --- 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 e801d2c3077b..914076eab242 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7332,6 +7332,16 @@ 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. @@ -7393,6 +7403,8 @@ 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); } @@ -7541,12 +7553,7 @@ 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 /* -- 2.32.0.402.g57bb445576-goog
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