Re: [PATCH v2] x86/resctrl: Avoid overflow in MB settings in bw_validate()

[Reinette Chatre <reinette.chatre@intel.com>]

Hi Martin,

On 9/17/24 8:47 AM, Martin Kletzander wrote:
> On Tue, Sep 17, 2024 at 08:08:50AM -0700, Reinette Chatre wrote:
>> 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@gmail.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!

Apologies. Please try:

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) {