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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 v2] sched: Defend cfs and rt bandwidth quota against overflow
References: <xm26k126kkma.fsf@google.com>
        <20200425105248.60093-1-changhuaixin@linux.alibaba.com>
Date:   Mon, 27 Apr 2020 11:29:30 -0700
In-Reply-To: <20200425105248.60093-1-changhuaixin@linux.alibaba.com> (Huaixin
        Chang's message of "Sat, 25 Apr 2020 18:52:48 +0800")
Message-ID: <xm26ftcolr3p.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.

Reviewed-by: Ben Segall <bsegall@google.com>

>
> Signed-off-by: Huaixin Chang <changhuaixin@linux.alibaba.com>
> ---
>  kernel/sched/core.c  |  8 ++++++++
>  kernel/sched/rt.c    | 12 +++++++++++-
>  kernel/sched/sched.h |  2 ++
>  3 files changed, 21 insertions(+), 1 deletion(-)
>
> 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..6d60ba21ed29 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -9,6 +9,8 @@
>  
>  int sched_rr_timeslice = RR_TIMESLICE;
>  int sysctl_sched_rr_timeslice = (MSEC_PER_SEC / HZ) * RR_TIMESLICE;
> +/* More than 4 hours if BW_SHIFT equals 20. */
> +static const u64 max_rt_runtime = MAX_BW;
>  
>  static int do_sched_rt_period_timer(struct rt_bandwidth *rt_b, int overrun);
>  
> @@ -2585,6 +2587,12 @@ 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)
> @@ -2702,7 +2710,9 @@ static int sched_rt_global_validate(void)
>  		return -EINVAL;
>  
>  	if ((sysctl_sched_rt_runtime != RUNTIME_INF) &&
> -		(sysctl_sched_rt_runtime > sysctl_sched_rt_period))
> +		((sysctl_sched_rt_runtime > sysctl_sched_rt_period) ||
> +		 ((u64)sysctl_sched_rt_runtime *
> +			NSEC_PER_USEC > max_rt_runtime)))
>  		return -EINVAL;
>  
>  	return 0;
> 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);