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

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Message-ID: <ZugtbMDP0AiPVrmt@wheatley.k8r.cz>
Date: Mon, 16 Sep 2024 15:06:52 +0200
From: Martin Kletzander <nert.pinx@...il.com>
To: Fenghua Yu <fenghua.yu@...el.com>,
	Reinette Chatre <reinette.chatre@...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] x86/resctrl: Avoid overflow in MB settings in
 bw_validate()

On Mon, Sep 16, 2024 at 12:37:13PM +0200, 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().
>
>Signed-off-by: Martin Kletzander <nert.pinx@...il.com>
>---
> arch/x86/kernel/cpu/resctrl/ctrlmondata.c | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
>diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>index 50fa1fe9a073..7e6014176a29 100644
>--- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>+++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>@@ -48,8 +48,11 @@ 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))
>+		return true;

I'm so sorry, I sent an incomplete patch.  This still needs to modify
the *data argument.  I'll send a v2 shortly.

>+
>+	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;
>-- 
>2.46.0
>