linux-kernel - Re: [PATCH 3/3] x86/kasan: Populate shadow for shared chunk of the CPU entry area

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Message-ID: <Y2q2GFWjLKMp5eUr@google.com>
Date:   Tue, 8 Nov 2022 20:03:36 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc:     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, Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        "H. Peter Anvin" <hpa@...or.com>, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org,
        syzbot+8cdd16fd5a6c0565e227@...kaller.appspotmail.com
Subject: Re: [PATCH 3/3] x86/kasan: Populate shadow for shared chunk of the
 CPU entry area

On Tue, Nov 08, 2022, Andrey Ryabinin wrote:
> 
> On 11/4/22 21:32, Sean Christopherson wrote:
> > @@ -409,6 +410,15 @@ void __init kasan_init(void)
> >  		kasan_mem_to_shadow((void *)VMALLOC_END + 1),
> >  		(void *)shadow_cea_begin);
> >  
> > +	/*
> > +	 * 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_shadow(shadow_cea_begin,
> > +			      shadow_cea_per_cpu_begin, 0);
> > +
> 
> I think we can extend the kasan_populate_early_shadow() call above up to
> shadow_cea_per_cpu_begin point, instead of this.
> populate_early_shadow() maps single RO zeroed page. No one should write to the shadow for IDT.
> KASAN only needs writable shadow for linear mapping/stacks/vmalloc/global variables.

Is that the only difference between the "early" and "normal" variants?  If so,
renaming them to kasan_populate_ro_shadow() vs. kasan_populate_rw_shadow() would
make this code much more intuitive for non-KASAN folks.

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