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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 27 Sep 2021 18:50:23 +0200
Message-ID: <CAKfTPtDvxZt8U=xc104VgSgvyqfrPztSMkbApJvBowr6T7i1Bw@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Null terminate buffer when updating tunable_scaling
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Mike Galbraith <efault@gmx.de>, Ingo Molnar <mingo@kernel.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, 27 Sept 2021 at 13:46, Mel Gorman <mgorman@techsingularity.net> wrote:
>
> This patch null-terminates the temporary buffer in sched_scaling_write()
> so kstrtouint() does not return failure and checks the value is valid.
>
> Before
> $ cat /sys/kernel/debug/sched/tunable_scaling
> 1
> $ echo 0 > /sys/kernel/debug/sched/tunable_scaling
> -bash: echo: write error: Invalid argument
> $ cat /sys/kernel/debug/sched/tunable_scaling
> 1
>
> After
> $ cat /sys/kernel/debug/sched/tunable_scaling
> 1
> $ echo 0 > /sys/kernel/debug/sched/tunable_scaling
> $ cat /sys/kernel/debug/sched/tunable_scaling
> 0
> $ echo 3 > /sys/kernel/debug/sched/tunable_scaling
> -bash: echo: write error: Invalid argument
>
> Fixes: 8a99b6833c88 ("sched: Move SCHED_DEBUG sysctl to debugfs")
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

Acked-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>  kernel/sched/debug.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index 49716228efb4..17a653b67006 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -173,16 +173,22 @@ static ssize_t sched_scaling_write(struct file *filp, const char __user *ubuf,
>                                    size_t cnt, loff_t *ppos)
>  {
>         char buf[16];
> +       unsigned int scaling;
>
>         if (cnt > 15)
>                 cnt = 15;
>
>         if (copy_from_user(&buf, ubuf, cnt))
>                 return -EFAULT;
> +       buf[cnt] = '\0';
>
> -       if (kstrtouint(buf, 10, &sysctl_sched_tunable_scaling))
> +       if (kstrtouint(buf, 10, &scaling))
>                 return -EINVAL;
>
> +       if (scaling >= SCHED_TUNABLESCALING_END)
> +               return -EINVAL;
> +
> +       sysctl_sched_tunable_scaling = scaling;
>         if (sched_update_scaling())
>                 return -EINVAL;
>