linux-kernel - Re: KASAN vs realloc

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Message-ID: <CANP3RGerB8-soNb1pBx7TqmOivR7Vp4Q-iqjE5QH0fzHrxEseQ@mail.gmail.com>
Date: Fri, 6 Feb 2026 13:26:37 -0800
From: Maciej Żenczykowski <maze@...gle.com>
To: Maciej Wieczor-Retman <m.wieczorretman@...me>
Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>, Kees Cook <kees@...nel.org>, 
	joonki.min@...sung-slsi.corp-partner.google.com, 
	Andrew Morton <akpm@...gle.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Marco Elver <elver@...gle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Uladzislau Rezki <urezki@...il.com>, 
	Danilo Krummrich <dakr@...nel.org>, jiayuan.chen@...ux.dev, 
	syzbot+997752115a851cb0cf36@...kaller.appspotmail.com, 
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, kasan-dev@...glegroups.com, 
	Kernel hackers <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: KASAN vs realloc

On Fri, Feb 6, 2026 at 12:14 PM Maciej Wieczor-Retman
<m.wieczorretman@...me> wrote:
>
> From what I see kasan_poison_last_granule() is called through:
>
> __kasan_vrealloc()
> --> __kasan_unpoison_vmalloc()
> ----> kasan_unpoison()
> ------> kasan_poison_last_granule()
>
> and the arguments are "addr + old_size" and "new_size - old_size" so it looks
> okay I think.

Cool, thanks for checking.

> On 2026-02-06 at 11:07:12 -0800, Maciej Żenczykowski wrote:
> >While looking at:
> >  https://android-review.git.corp.google.com/c/kernel/common/+/3939998
> >  UPSTREAM: mm/kasan: fix KASAN poisoning in vrealloc()
> >
> >I noticed a lack of symmetry - I'm not sure if it's a problem or not...
> >but I'd have expected kasan_poison_last_granule() to be called
> >regardless of whether the size shrunk or increased.
> >
> >It is of course possible this is handled automatically by
> >__kasan_unpoison_vmalloc() - I haven't traced that deep,
> >in general these functions seem to have a terrible api surface full of
> >razors... with hidden assumptions about what is and is not granule
> >aligned.
>
> --
> Kind regards
> Maciej Wieczór-Retman
>

--
Maciej Żenczykowski, Kernel Networking Developer @ Google