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

Andrey Konovalov
2025-09-05T18:08:54Z

On Fri, Sep 5, 2025 at 7:12 PM Andrey Ryabinin <ryabinin.a.a@gmail.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?