[PATCH] sched: Defend cfs and rt bandwidth quota against overflow

[Huaixin Chang]

When users write some huge number into cpu.cfs_quota_us or
cpu.rt_runtime_us, overflow might happen during to_ratio() shifts of
schedulable checks.

to_ratio() could be altered to avoid unnecessary internal overflow, but
min_cfs_quota_period is less than 1 << BW_SHIFT, so a cutoff would still
be needed. Set a cap MAX_BW for cfs_quota_us and rt_runtime_us to
prevent overflow.

Signed-off-by: Huaixin Chang <changhuaixin@xxxxxxxxxxxxxxxxx>
---
 kernel/sched/core.c  | 8 ++++++++
 kernel/sched/rt.c    | 9 +++++++++
 kernel/sched/sched.h | 2 ++
 3 files changed, 19 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3a61a3b8eaa9..0be1782e15c9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7390,6 +7390,8 @@ static DEFINE_MUTEX(cfs_constraints_mutex);
 
 const u64 max_cfs_quota_period = 1 * NSEC_PER_SEC; /* 1s */
 static const u64 min_cfs_quota_period = 1 * NSEC_PER_MSEC; /* 1ms */
+/* More than 203 days if BW_SHIFT equals 20. */
+static const u64 max_cfs_runtime = MAX_BW * NSEC_PER_USEC;
 
 static int __cfs_schedulable(struct task_group *tg, u64 period, u64 runtime);
 
@@ -7417,6 +7419,12 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 	if (period > max_cfs_quota_period)
 		return -EINVAL;
 
+	/*
+	 * Bound quota to defend quota against overflow during bandwidth shift.
+	 */
+	if (quota != RUNTIME_INF && quota > max_cfs_runtime)
+		return -EINVAL;
+
 	/*
 	 * Prevent race between setting of cfs_rq->runtime_enabled and
 	 * unthrottle_offline_cfs_rqs().
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index df11d88c9895..7ba49625cdbd 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2569,6 +2569,9 @@ static int __rt_schedulable(struct task_group *tg, u64 period, u64 runtime)
 	return ret;
 }
 
+/* More than 4 hours if BW_SHIFT equals 20. */
+static const u64 max_rt_runtime = MAX_BW;
+
 static int tg_set_rt_bandwidth(struct task_group *tg,
 		u64 rt_period, u64 rt_runtime)
 {
@@ -2585,6 +2588,12 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
 	if (rt_period == 0)
 		return -EINVAL;
 
+	/*
+	 * Bound quota to defend quota against overflow during bandwidth shift.
+	 */
+	if (rt_runtime != RUNTIME_INF && rt_runtime > max_rt_runtime)
+		return -EINVAL;
+
 	mutex_lock(&rt_constraints_mutex);
 	err = __rt_schedulable(tg, rt_period, rt_runtime);
 	if (err)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index db3a57675ccf..1f58677a8f23 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1918,6 +1918,8 @@ extern void init_dl_inactive_task_timer(struct sched_dl_entity *dl_se);
 #define BW_SHIFT		20
 #define BW_UNIT			(1 << BW_SHIFT)
 #define RATIO_SHIFT		8
+#define MAX_BW_BITS		(64 - BW_SHIFT)
+#define MAX_BW			((1ULL << MAX_BW_BITS) - 1)
 unsigned long to_ratio(u64 period, u64 runtime);
 
 extern void init_entity_runnable_average(struct sched_entity *se);
-- 
2.14.4.44.g2045bb6