linux-kernel - Re: [PATCH v1] mm/kasan: Fix incorrect unpoisoning in vrealloc for KASAN

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Message-ID: <wkcpaobuckopokvqfyb37ugox24bpcyzl6pztaz42s6byzf4pg@z4ndbjoxbjq7>
Date: Wed, 03 Dec 2025 07:30:10 +0000
From: Maciej Wieczór-Retman <m.wieczorretman@...me>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, linux-mm@...ck.org, syzbot+997752115a851cb0cf36@...kaller.appspotmail.com, Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>, Andrew Morton <akpm@...ux-foundation.org>, Uladzislau Rezki <urezki@...il.com>, Danilo Krummrich <dakr@...nel.org>, Kees Cook <kees@...nel.org>, kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm/kasan: Fix incorrect unpoisoning in vrealloc for KASAN

On 2025-12-03 at 02:05:11 +0000, Jiayuan Chen wrote:
>December 3, 2025 at 04:48, "Maciej Wieczor-Retman"
><maciej.wieczor-retman@...el.com
>mailto:maciej.wieczor-retman@...el.com?to=%22Maciej%20Wieczor-Retman%22%20%3Cmaciej.wieczor-retman%40intel.com%3E
>> wrote:
>> 
>> Hi, I'm working on [1]. As Andrew pointed out to me the patches are quite
>> similar. I was wondering if you mind if the reuse_tag was an actual tag value?
>> Instead of just bool toggling the usage of kasan_random_tag()?
>> 
>> I tested the problem I'm seeing, with your patch and the tags end up being reset.
>> That's because the vms[area] pointers that I want to unpoison don't have a tag
>> set, but generating a different random tag for each vms[] pointer crashes the
>> kernel down the line. So __kasan_unpoison_vmalloc() needs to be called on each
>> one but with the same tag.
>> 
>> Arguably I noticed my series also just resets the tags right now, but I'm
>> working to correct it at the moment. I can send a fixed version tomorrow. Just
>> wanted to ask if having __kasan_unpoison_vmalloc() set an actual predefined tag
>> is a problem from your point of view?
>> 
>> [1] https://lore.kernel.org/all/cover.1764685296.git.m.wieczorretman@pm.me/
>> 
>
>
>Hi Maciej,
>
>It seems we're focusing on different issues, but feel free to reuse or modify the 'reuse_tag'.
>It's intended to preserve the tag in one 'vma'.
>
>I'd also be happy to help reproduce and test your changes to ensure the issue I encountered
>isn't regressed once you send a patch based on mine. 
>
>Thanks.

Yes, the final issues are different, just we both want to use
__kasan_unpoison_vmalloc slightly differently.

Okay, then I'll rebase my patches onto your patch, restest on my end and
resubmit my series. I'll add you to CC and reply in this thread too.
Thanks :)

kind regards
Maciej Wieczór-Retman