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Date: Sun, 4 Jan 2026 10:02:30 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Ryan Roberts <ryan.roberts@arm.com>
Cc: Alexander Potapenko <glider@google.com>, Marco Elver <elver@google.com>,
 Dmitry Vyukov <dvyukov@google.com>, kasan-dev@googlegroups.com,
 linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v1] mm: kmsan: Fix poisoning of high-order non-compound
 pages
Message-Id: <20260104100230.09abd1beaca2123d174022b2@linux-foundation.org>
In-Reply-To: <20260104134348.3544298-1-ryan.roberts@arm.com>
References: <20260104134348.3544298-1-ryan.roberts@arm.com>
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On Sun,  4 Jan 2026 13:43:47 +0000 Ryan Roberts <ryan.roberts@arm.com> wrote:

> kmsan_free_page() is called by the page allocator's free_pages_prepare()
> during page freeing. It's job is to poison all the memory covered by the
> page. It can be called with an order-0 page, a compound high-order page
> or a non-compound high-order page. But page_size() only works for
> order-0 and compound pages. For a non-compound high-order page it will
> incorrectly return PAGE_SIZE.
> 
> The implication is that the tail pages of a high-order non-compound page
> do not get poisoned at free, so any invalid access while they are free
> could go unnoticed. It looks like the pages will be poisoned again at
> allocaiton time, so that would bookend the window.
> 
> Fix this by using the order parameter to calculate the size.
> 
> Fixes: b073d7f8aee4 ("mm: kmsan: maintain KMSAN metadata for page operations")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> 
> Hi,
> 
> I noticed this during code review, so perhaps I've just misunderstood the intent
> of the code.
>
> I don't have the means to compile and run on x86 with KMSAN enabled though, so
> punting this out hoping someone might be able to validate/test. I guess there is
> a small chance this could lead to KMSAN finding some new issues?

We'll see, I'll park this in mm-new to get it a little testing, see if
anything is shaken out.  If all looks good and if the KMSAN maintainers
are OK with it I'll later move the patch into mm-hotfixes for more
expedited upstreaming.