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

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Message-ID: <ad019c14-5211-4261-bfb8-c4e0dd3b2535@redhat.com>
Date: Wed, 17 Sep 2025 12:52:54 +0200
From: David Hildenbrand <david@...hat.com>
To: Kalesh Singh <kaleshsingh@...gle.com>, akpm@...ux-foundation.org,
 minchan@...nel.org, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 rppt@...nel.org, pfalcato@...e.de
Cc: kernel-team@...roid.com, android-mm@...gle.com, stable@...r.kernel.org,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 Kees Cook <kees@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 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 15.09.25 18:36, Kalesh Singh 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>
> ---

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb