linux-kernel - Re: [PATCH v2 1/7] mm: fix off-by-one error in VMA count limit checks

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Message-Id: <20250917074413.58886-1-sj@kernel.org>
Date: Wed, 17 Sep 2025 00:44:13 -0700
From: SeongJae Park <sj@...nel.org>
To: Kalesh Singh <kaleshsingh@...gle.com>
Cc: SeongJae Park <sj@...nel.org>,
	akpm@...ux-foundation.org,
	minchan@...nel.org,
	lorenzo.stoakes@...cle.com,
	david@...hat.com,
	Liam.Howlett@...cle.com,
	rppt@...nel.org,
	pfalcato@...e.de,
	kernel-team@...roid.com,
	android-mm@...gle.com,
	stable@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>,
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	Vlastimil Babka <vbabka@...e.cz>,
	Suren Baghdasaryan <surenb@...gle.com>,
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	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
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	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
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	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ben Segall <bsegall@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Jann Horn <jannh@...gle.com>,
	Shuah Khan <shuah@...nel.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-trace-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 1/7] mm: fix off-by-one error in VMA count limit checks

On Mon, 15 Sep 2025 09:36:32 -0700 Kalesh Singh <kaleshsingh@...gle.com> wrote:

> The VMA count limit check in do_mmap() and do_brk_flags() uses a
> strict inequality (>), which allows a process's VMA count to exceed
> the configured sysctl_max_map_count limit by one.
> 
> A process with mm->map_count == sysctl_max_map_count will incorrectly
> pass this check and then exceed the limit upon allocation of a new VMA
> when its map_count is incremented.
> 
> Other VMA allocation paths, such as split_vma(), already use the
> correct, inclusive (>=) comparison.
> 
> Fix this bug by changing the comparison to be inclusive in do_mmap()
> and do_brk_flags(), bringing them in line with the correct behavior
> of other allocation paths.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: <stable@...r.kernel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Cc: Mike Rapoport <rppt@...nel.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Pedro Falcato <pfalcato@...e.de>
> Signed-off-by: Kalesh Singh <kaleshsingh@...gle.com>

Acked-by: SeongJae Park <sj@...nel.org>


Thanks,
SJ

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