linux-kernel - Re: WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46 kfence

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Date:   Fri, 18 Nov 2022 10:19:24 +0100
From:   Marco Elver <elver@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Naresh Kamboju <naresh.kamboju@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        kasan-dev <kasan-dev@...glegroups.com>, X86 ML <x86@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, regressions@...ts.linux.dev,
        lkft-triage@...ts.linaro.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: WARNING: CPU: 0 PID: 0 at arch/x86/include/asm/kfence.h:46
 kfence_protect

On Thu, Nov 17, 2022 at 03:54PM -0800, Dave Hansen wrote:
> On 11/17/22 15:23, Marco Elver wrote:
> > Yes - it's the 'level != PG_LEVEL_4K'.
> 
> That plus the bisect made it pretty easy to find, thanks for the effort!
> 
> Could you double-check that the attached patch fixes it?  It seemed to
> for me.

Yes, that works - thanks!

> The issue was that the new "No changes, easy!" check in the suspect
> commit didn't check the cpa->force_split option.  It didn't split down
> to 4k and then all hell broke loose.
> 
> Oh, and I totally misread the kfence ability to tolerate partial TLB
> flushes.  Sorry for the noise there!

> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
> index 220361ceb997..9b4e2ad957f6 100644
> --- a/arch/x86/mm/pat/set_memory.c
> +++ b/arch/x86/mm/pat/set_memory.c
> @@ -1727,7 +1727,8 @@ static int __change_page_attr_set_clr(struct cpa_data *cpa, int primary)
>  	/*
>  	 * No changes, easy!
>  	 */
> -	if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr)))
> +	if (!(pgprot_val(cpa->mask_set) | pgprot_val(cpa->mask_clr))
> +	    && !cpa->force_split)
>  		return ret;
>  
>  	while (rempages) {

Tested-by: Marco Elver <elver@...gle.com>