linux-kernel - Re: [PATCH v2] x86/resctrl: Avoid overflow in MB settings in bw

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Message-ID: <ZumkrWn7FtiMxESR@carol.k8r.cz>
Date: Tue, 17 Sep 2024 17:47:57 +0200
From: Martin Kletzander <nert.pinx@...il.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: Fenghua Yu <fenghua.yu@...el.com>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/resctrl: Avoid overflow in MB settings in
 bw_validate()

On Tue, Sep 17, 2024 at 08:08:50AM -0700, Reinette Chatre wrote:
>Hi Martin,
>
>On 9/17/24 1:12 AM, Martin Kletzander wrote:
>> On Mon, Sep 16, 2024 at 09:56:01AM -0700, Reinette Chatre wrote:
>>> On 9/16/24 6:07 AM, Martin Kletzander wrote:
>>>> When resctrl is mounted with the "mba_MBps" option the default (maximum)
>>>> bandwidth is the maximum unsigned value for the type.  However when
>>>> using the same value that already exists in the schemata file it is then
>>>> rounded up to the bandwidth granularity and overflows to a small number
>>>> instead, making it difficult to reset memory bandwidth allocation value
>>>> back to its default.
>>>>
>>>> Since the granularity and minimum bandwidth are not used when the
>>>> software controller is used (resctrl is mounted with the "mba_MBps"),
>>>> skip the rounding up as well and return early from bw_validate().
>>>
>>> Thank you very much for finding the issue and proposing a fix.
>>>
>>>>
>>>> Signed-off-by: Martin Kletzander <nert.pinx@...il.com>
>>>> ---
>>>> Changes in v2:
>>>>   - actually save the value in the output parameter @data
>>>>
>>>>   arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 9 +++++++--
>>>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>>>> index 50fa1fe9a073..702b1a372e9c 100644
>>>> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>>>> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>>>> @@ -48,8 +48,13 @@ static bool bw_validate(char *buf, unsigned long *data, struct rdt_resource *r)
>>>>           return false;
>>>>       }
>>>>
>>>> -    if ((bw < r->membw.min_bw || bw > r->default_ctrl) &&
>>>> -        !is_mba_sc(r)) {
>>>> +    /* Nothing else to do if software controller is enabled */
>>>> +    if (is_mba_sc(r)) {
>>>> +        *data = bw;
>>>> +        return true;
>>>
>>> While this would fix the scenario tested I do not believe this fully addresses the
>>> overflow issue. As I understand the test wrote U32_MAX to the schemata file,
>>> which triggered the overflow because of the rounding and is fixed by this patch. Looks like,
>>> after this patch, writing "U32_MAX + 1" will still trigger the overflow.
>>>
>>> The overflow appears to result from some inconsistent type use and not using
>>> appropriate parsing API that is able to detect overflow.
>>>
>>> How about something like below:
>>>
>>
>> That makes much more sense, I have not considered changing the data type
>> as I wanted to keep the changes at minimum, but your solution is even
>> better.  Should I leave the fix up to you or do you want me to send a v3?
>
>Could you please try it out to ensure it works for you and then send a v3?
>

I wanted but the diff has some weird line numbering and could not be
applied.  I'll write it manually later, test it out, and send a v3.
Thanks!

>Thank you very much.
>
>Reinette