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From:   bsegall@google.com
To:     Huaixin Chang <changhuaixin@linux.alibaba.com>
Cc:     bsegall@google.com, chiluk+linux@indeed.com,
        linux-kernel@vger.kernel.org, mingo@redhat.com, pauld@redhead.com,
        peterz@infradead.org, vincent.guittot@linaro.org
Subject: Re: [PATCH] sched: Defend cfs and rt bandwidth quota against overflow
References: <xm261roim4hi.fsf@google.com>
        <20200423133752.12787-1-changhuaixin@linux.alibaba.com>
Date:   Thu, 23 Apr 2020 13:33:33 -0700
In-Reply-To: <20200423133752.12787-1-changhuaixin@linux.alibaba.com> (Huaixin
        Chang's message of "Thu, 23 Apr 2020 21:37:52 +0800")
Message-ID: <xm26k126kkma.fsf@google.com>
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Huaixin Chang <changhuaixin@linux.alibaba.com> writes:

> 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@linux.alibaba.com>
> ---
>  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;
> +

We probably _do_ also want this in sched_rt_global_validate now that I
think of it. Other than missing that, it looks good.

>  	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);