linux-kernel - Re: [PATCH v2] mm: fix copy_vma() error handling for hugetlb mappings

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Message-ID: <aDK5mGfiRxBREBc0@localhost.localdomain>
Date: Sun, 25 May 2025 08:32:56 +0200
From: Oscar Salvador <osalvador@...e.de>
To: Ricardo Cañuelo Navarro <rcn@...lia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Pedro Falcato <pfalcato@...e.de>, revest@...gle.com,
	kernel-dev@...lia.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: fix copy_vma() error handling for hugetlb mappings

On Fri, May 23, 2025 at 02:19:10PM +0200, Ricardo Cañuelo Navarro wrote:
> If, during a mremap() operation for a hugetlb-backed memory mapping,
> copy_vma() fails after the source vma has been duplicated and
> opened (ie. vma_link() fails), the error is handled by closing the new
> vma. This updates the hugetlbfs reservation counter of the reservation
> map which at this point is referenced by both the source vma and the new
> copy. As a result, once the new vma has been freed and copy_vma()
> returns, the reservation counter for the source vma will be incorrect.
> 
> This patch addresses this corner case by clearing the hugetlb private
> page reservation reference for the new vma and decrementing the
> reference before closing the vma, so that vma_close() won't update the
> reservation counter. This is also what copy_vma_and_data() does with the
> source vma if copy_vma() succeeds, so a helper function has been added
> to do the fixup in both functions.
> 
> The issue was reported by a private syzbot instance and can be
> reproduced using the C reproducer in [1]. It's also a possible duplicate
> of public syzbot report [2]. The WARNING report is:
> 
... 
> Signed-off-by: Ricardo Cañuelo Navarro <rcn@...lia.com>
> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Link: https://people.igalia.com/rcn/kernel_logs/20250422__WARNING_in_page_counter_cancel__repro.c [1]
> Link: https://lore.kernel.org/all/67000a50.050a0220.49194.048d.GAE@google.com/ [2]

Reviewed-by: Oscar Salvador <osalvador@...e.de>


-- 
Oscar Salvador
SUSE Labs