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

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Message-ID: <fe3436bcc785ae432edb9a24aa8993e8d25dad9f@linux.dev>
Date: Wed, 03 Dec 2025 02:05:11 +0000
From: "Jiayuan Chen" <jiayuan.chen@...ux.dev>
To: "Maciej Wieczor-Retman" <maciej.wieczor-retman@...el.com>
Cc: 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

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.