linux-kernel - Re: [PATCH v2 5/5] x86/kasan: Populate shadow for shared chunk of the CPU entry area

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Message-ID: <Y3KAp+yNQ54IKvTn@google.com>
Date:   Mon, 14 Nov 2022 17:53:43 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        syzbot+ffb4f000dc2872c93f62@...kaller.appspotmail.com,
        syzbot+8cdd16fd5a6c0565e227@...kaller.appspotmail.com
Subject: Re: [PATCH v2 5/5] x86/kasan: Populate shadow for shared chunk of
 the CPU entry area

On Mon, Nov 14, 2022, Peter Zijlstra wrote:
> On Mon, Nov 14, 2022 at 05:44:00PM +0300, Andrey Ryabinin wrote:
> > Going back kasan_populate_shadow() seems like safer and easier choice.
> > The only disadvantage of it that we might waste 1 page, which is not
> > much compared to the KASAN memory overhead.
> 
> So the below delta?
> 
> ---
> --- a/arch/x86/mm/kasan_init_64.c
> +++ b/arch/x86/mm/kasan_init_64.c
> @@ -388,7 +388,7 @@ void __init kasan_init(void)
>  	shadow_cea_end = kasan_mem_to_shadow_align_up(CPU_ENTRY_AREA_BASE +
>  						      CPU_ENTRY_AREA_MAP_SIZE);
>  
> -	kasan_populate_early_shadow(
> +	kasan_populate_shadow(
>  		kasan_mem_to_shadow((void *)PAGE_OFFSET + MAXMEM),
>  		kasan_mem_to_shadow((void *)VMALLOC_START));

Wrong one, that's the existing mapping.  To get back to v1:

diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index af82046348a0..0302491d799d 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -416,8 +416,8 @@ void __init kasan_init(void)
         * area is randomly placed somewhere in the 512GiB range and mapping
         * the entire 512GiB range is prohibitively expensive.
         */
-       kasan_populate_early_shadow((void *)shadow_cea_begin,
-                                   (void *)shadow_cea_per_cpu_begin);
+       kasan_populate_shadow(shadow_cea_begin,
+                             shadow_cea_per_cpu_begin, 0);
 
        kasan_populate_early_shadow((void *)shadow_cea_end,
                        kasan_mem_to_shadow((void *)__START_KERNEL_map));