linux-kernel - Re: [PATCH v3 00/12] mm/kasan: make kasan=on|off work for all three modes

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Message-ID: <CA+fCnZd9unSjY-UnEwc4rGkSRgZX3nrs=WgBb2eDQNEpZX10cA@mail.gmail.com>
Date: Fri, 5 Sep 2025 21:44:00 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>, Baoquan He <bhe@...hat.com>, snovitoll@...il.com, 
	glider@...gle.com, dvyukov@...gle.com, elver@...gle.com, linux-mm@...ck.org, 
	vincenzo.frascino@....com, akpm@...ux-foundation.org, 
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, 
	kexec@...ts.infradead.org, sj@...nel.org, lorenzo.stoakes@...cle.com
Subject: Re: [PATCH v3 00/12] mm/kasan: make kasan=on|off work for all three modes

On Fri, Sep 5, 2025 at 9:13 PM Christophe Leroy
<christophe.leroy@...roup.eu> wrote:
>
> > Hm, I thought it worked like that, but then what threw me off just now
> > was seeing that zero_pte_populate()->pte_wrprotect() (on arm64) resets
> > the PTE_WRITE bit and sets the PTE_RDONLY bit. So I thought the
> > kasan_early_shadow_page is marked as read-only and then the
> > instrumentation is disabled for all early code that might write into
> > the page before the proper shadow is set up. Or am I reading this
> > bit-setting code wrong?
>
> But that zero_pte_populate() is called by kasan_init() when everything
> is ready.
>
> kasan_init()->kasan_init_shadow()->kasan_populate_early_shadow()->zero_p4d_populate()->zero_pud_populate()->zero_pmd_populate()->zero_pte_populate()
>
> Here we are talking about the shadow set at startup kasan_early_init(),
> aren't we ?

Ah, you're right, thanks!

I was confused by the name of kasan_populate_early_shadow(). I think
we should rename it to kasan_populate_shadow_read_only() or something
like that and also update the comment. As this function is not
intended for populating early shadow (that is done via
kasan_early_init() in the arch code instead), we're populating normal
shadow for pages that can be accessed but whose shadow won't be
written to. Perhaps it makes sense to come up with a better name for
the kasan_early_shadow_page variable too to point out its dual
purpose.