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

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Message-ID: <Zuk5_3qzSfld5dvD@wheatley.k8r.cz>
Date: Tue, 17 Sep 2024 10:12:47 +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 Mon, Sep 16, 2024 at 09:56:01AM -0700, Reinette Chatre wrote:
>Hi Martin,
>
>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?

>diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>index 50fa1fe9a073..53defc5a6784 100644
>--- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>+++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>@@ -29,10 +29,10 @@
>   * hardware. The allocated bandwidth percentage is rounded to the next
>   * control step available on the hardware.
>   */
>-static bool bw_validate(char *buf, unsigned long *data, struct rdt_resource *r)
>+static bool bw_validate(char *buf, u32 *data, struct rdt_resource *r)
>  {
>-	unsigned long bw;
>  	int ret;
>+	u32 bw;
>
>  	/*
>  	 * Only linear delay values is supported for current Intel SKUs.
>@@ -42,14 +42,19 @@ static bool bw_validate(char *buf, unsigned long *data, struct rdt_resource *r)
>  		return false;
>  	}
>
>-	ret = kstrtoul(buf, 10, &bw);
>+	ret = kstrtou32(buf, 10, &bw);
>  	if (ret) {
>-		rdt_last_cmd_printf("Non-decimal digit in MB value %s\n", buf);
>+		rdt_last_cmd_printf("Invalid MB value %s\n", buf);
>  		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;
>+	}
>+
>+	if (bw < r->membw.min_bw || bw > r->default_ctrl) {
>  		rdt_last_cmd_printf("MB value %ld out of range [%d,%d]\n", bw,
>  				    r->membw.min_bw, r->default_ctrl);
>  		return false;
>@@ -65,7 +70,7 @@ int parse_bw(struct rdt_parse_data *data, struct resctrl_schema *s,
>  	struct resctrl_staged_config *cfg;
>  	u32 closid = data->rdtgrp->closid;
>  	struct rdt_resource *r = s->res;
>-	unsigned long bw_val;
>+	u32 bw_val;
>
>  	cfg = &d->staged_config[s->conf_type];
>  	if (cfg->have_new_ctrl) {
>
>
>
>> +	}
>> +
>> +	if (bw < r->membw.min_bw || bw > r->default_ctrl) {
>>   		rdt_last_cmd_printf("MB value %ld out of range [%d,%d]\n", bw,
>>   				    r->membw.min_bw, r->default_ctrl);
>>   		return false;
>
>Reinette