[PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default

[Patrick Bellasi]

The number of clamp groups supported is limited and defined at compile
time. However, a malicious user can currently ask for many different
clamp values thus consuming all the available clamp groups.

Since on properly configured systems we expect only a limited set of
different clamp values, the previous problem can be mitigated by
allowing access to clamp groups configuration only to privileged tasks.
This should still allow a System Management Software to properly
pre-configure the system.

Let's restrict the tuning of utilization clamp values, by default, to
tasks with CAP_SYS_ADMIN capabilities.

Whenever this should be considered too restrictive and/or not required
for a specific platforms, a kernel boot option is provided to change
this default behavior thus allowing non privileged tasks to change their
utilization clamp values.

Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Cc: Paul Turner <pjt@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Todd Kjos <tkjos@xxxxxxxxxx>
Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Steve Muckle <smuckle@xxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Quentin Perret <quentin.perret@xxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-pm@xxxxxxxxxxxxxxx

---
Changes in v4:
 Others:
 - new patch added in this version
 - rebased on v4.19-rc1
---
 .../admin-guide/kernel-parameters.txt         |  3 +++
 kernel/sched/core.c                           | 22 ++++++++++++++++---
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9871e649ffef..481f8214ea9a 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4561,6 +4561,9 @@
 			<port#>,<js1>,<js2>,<js3>,<js4>,<js5>,<js6>,<js7>
 			See also Documentation/input/devices/joystick-parport.rst
 
+	uclamp_user	[KNL] Enable task-specific utilization clamping tuning
+			also from tasks without CAP_SYS_ADMIN capability.
+
 	udbg-immortal	[PPC] When debugging early kernel crashes that
 			happen after console_init() and before a proper
 			console driver takes over, this boot options might
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 222397edb8a7..8341ce580a9a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1510,14 +1510,29 @@ static inline int alloc_uclamp_sched_group(struct task_group *tg,
 static inline void free_uclamp_sched_group(struct task_group *tg) { }
 #endif /* CONFIG_UCLAMP_TASK_GROUP */
 
+static bool uclamp_user_allowed __read_mostly;
+static int __init uclamp_user_allow(char *str)
+{
+	uclamp_user_allowed = true;
+
+	return 0;
+}
+early_param("uclamp_user", uclamp_user_allow);
+
 static inline int __setscheduler_uclamp(struct task_struct *p,
-					const struct sched_attr *attr)
+					const struct sched_attr *attr,
+					bool user)
 {
 	int group_id[UCLAMP_CNT] = { UCLAMP_NOT_VALID };
 	int lower_bound, upper_bound;
 	struct uclamp_se *uc_se;
 	int result = 0;
 
+	if (!capable(CAP_SYS_ADMIN) &&
+	    user && !uclamp_user_allowed) {
+		return -EPERM;
+	}
+
 	mutex_lock(&uclamp_mutex);
 
 	/* Find a valid group_id for each required clamp value */
@@ -1702,7 +1717,8 @@ static inline int alloc_uclamp_sched_group(struct task_group *tg,
 	return 1;
 }
 static inline int __setscheduler_uclamp(struct task_struct *p,
-					const struct sched_attr *attr)
+					const struct sched_attr *attr,
+					bool user)
 {
 	return -EINVAL;
 }
@@ -5217,7 +5233,7 @@ static int __sched_setscheduler(struct task_struct *p,
 
 	/* Configure utilization clamps for the task */
 	if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) {
-		retval = __setscheduler_uclamp(p, attr);
+		retval = __setscheduler_uclamp(p, attr, user);
 		if (retval)
 			return retval;
 	}
-- 
2.18.0