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+fCnZdWxWD99t9yhmB90VPefi3Gohn8Peo6=cxrvw8Zdz+3qQ@mail.gmail.com>
Date: Fri, 5 Sep 2025 20:08:36 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc: 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, 
	christophe.leroy@...roup.eu
Subject: Re: [PATCH v3 00/12] mm/kasan: make kasan=on|off work for all three modes

On Fri, Sep 5, 2025 at 7:12 PM Andrey Ryabinin <ryabinin.a.a@...il.com> wrote:
>
> > But have you tried running kasan=off + CONFIG_KASAN_STACK=y +
> > CONFIG_VMAP_STACK=y (+ CONFIG_KASAN_VMALLOC=y)? I would expect this
> > should causes crashes, as the early shadow is mapped as read-only and
> > the inline stack instrumentation will try writing into it (or do the
> > writes into the early shadow somehow get ignored?..).
> >
>
> It's not read-only, otherwise we would crash very early before full shadow
> setup and won't be able to boot at all. So writes still happen, and shadow
> checked, but reports are disabled.

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?