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

[SeongJae Park <sj@kernel.org>]

On Mon, 15 Sep 2025 09:36:32 -0700 Kalesh Singh <kaleshsingh@google.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@vger.kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Pedro Falcato <pfalcato@suse.de>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

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


Thanks,
SJ

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