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

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Message-ID: <c0bd173c-c84f-41d5-8532-2afb8eca9313@csgroup.eu>
Date: Fri, 5 Sep 2025 21:13:28 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Andrey Konovalov <andreyknvl@...il.com>,
 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
Subject: Re: [PATCH v3 00/12] mm/kasan: make kasan=on|off work for all three
 modes



Le 05/09/2025 à 20:08, Andrey Konovalov a écrit :
> 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?

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 ?

Christophe