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

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Date:   Mon, 14 Nov 2022 17:44:00 +0300
From:   Andrey Ryabinin <ryabinin.a.a@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "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 11/10/22 23:35, Sean Christopherson wrote:

>  
> +	/*
> +	 * Populate the shadow for the shared portion of the CPU entry area.
> +	 * Shadows for the per-CPU areas are mapped on-demand, as each CPU's
> +	 * 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);
> +

I know I suggested to use "early" here, but I just realized that this might be a problem.
This will actually map shadow page for the 8 pages (KASAN_SHADOW_SCALE_SHIFT) of the original memory.
In case there is some per-cpu entry area starting right at CPU_ENTRY_AREA_PER_CPU the shadow for it will
be covered with kasan_early_shadow_page instead of the usual one.

So we need to go back to your v1 PATCH, or alternatively we can round up CPU_ENTRY_AREA_PER_CPU
#define CPU_ENTRY_AREA_PER_CPU		(CPU_ENTRY_AREA_RO_IDT + PAGE_SIZE << KASAN_SHADOW_SCALE_SHIFT)

Such change will also require fixing up max_cea calculation in init_cea_offsets()


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.



>  	kasan_populate_early_shadow((void *)shadow_cea_end,
>  			kasan_mem_to_shadow((void *)__START_KERNEL_map));
>