Re: [LTP] [PATCH] sched/rt: Disallow writing invalid values to sched_rt_period_us

[Petr Vorel <pvorel@suse.cz>]

> The validation of the value written to sched_rt_period_us was broken
> because:

> - the sysclt_sched_rt_period is declared as unsigned int
> - parsed by proc_do_intvec()
> - the range is asserted after the value parsed by proc_do_intvec()

> Because of this negative values written to the file were written into a
> unsigned integer that were later on interpreted as large positive
> integers which did passed the check:

> if (sysclt_sched_rt_period <= 0)
> 	return EINVAL;

> This commit fixes the parsing by setting explicit range for both
> perid_us and runtime_us into the sched_rt_sysctls table and processes
> the values with proc_dointvec_minmax() instead.

> Alternatively if we wanted to use full range of unsigned int for the
> period value we would have to split the proc_handler and use
> proc_douintvec() for it however even the
> Documentation/scheduller/sched-rt-group.rst describes the range as 1 to
> INT_MAX.

> As far as I can tell the only problem this causes is that the sysctl
> file allows writing negative values which when read back may confuse
> userspace.

> There is also a LTP test being submitted for these sysctl files at:

> http://patchwork.ozlabs.org/project/ltp/patch/20230901144433.2526-1-chrubis@suse.cz/

> Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
> ---
>  kernel/sched/rt.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)

> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 0597ba0f85ff..aed3d55de2dd 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -37,6 +37,8 @@ static struct ctl_table sched_rt_sysctls[] = {
>  		.maxlen         = sizeof(unsigned int),
>  		.mode           = 0644,
>  		.proc_handler   = sched_rt_handler,
> +		.extra1         = SYSCTL_ONE,
> +		.extra2         = SYSCTL_INT_MAX,
>  	},
>  	{
>  		.procname       = "sched_rt_runtime_us",
> @@ -44,6 +46,8 @@ static struct ctl_table sched_rt_sysctls[] = {
>  		.maxlen         = sizeof(int),
>  		.mode           = 0644,
>  		.proc_handler   = sched_rt_handler,
> +		.extra1         = SYSCTL_NEG_ONE,
> +		.extra2         = SYSCTL_INT_MAX,
Documentation/scheduller/sched-rt-group.rst [1] specifies this as values from -1 to
(INT_MAX - 1), I guess due int range. Looking into proc_dointvec_minmax() [2]
even INT_MAX would pass the check. I suppose we can do nothing about that,
because there is no value in sysctl_vals[] which would represent INT_MAX - 1.

And you specify in LTP test range: from -1 to INT_MAX.

But even much shorter value than INT_MAX fails:

$ echo 1234567 > /proc/sys/kernel/sched_rt_runtime_us
sh: echo: write error: Invalid argument

Ranges in /proc/sys/kernel/sched_rt_period_us works as expected.

[1] https://www.kernel.org/doc/html/latest/scheduler/sched-rt-group.html#system-wide-settings
[2] https://elixir.bootlin.com/linux/latest/source/kernel/sysctl.c#L843

>  	},
>  	{
>  		.procname       = "sched_rr_timeslice_ms",
> @@ -2985,9 +2989,6 @@ static int sched_rt_global_constraints(void)
>  #ifdef CONFIG_SYSCTL
>  static int sched_rt_global_validate(void)
>  {
> -	if (sysctl_sched_rt_period <= 0)
> -		return -EINVAL;
> -
>  	if ((sysctl_sched_rt_runtime != RUNTIME_INF) &&
>  		((sysctl_sched_rt_runtime > sysctl_sched_rt_period) ||
>  		 ((u64)sysctl_sched_rt_runtime *
> @@ -3018,7 +3019,7 @@ static int sched_rt_handler(struct ctl_table *table, int write, void *buffer,
>  	old_period = sysctl_sched_rt_period;
>  	old_runtime = sysctl_sched_rt_runtime;

> -	ret = proc_dointvec(table, write, buffer, lenp, ppos);
> +	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);

>  	if (!ret && write) {
>  		ret = sched_rt_global_validate();

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