linux-kernel - Re: [PATCH v1 1/2] mm: Clear uffd-wp PTE/PMD state on mremap()

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Message-ID: <Z4gaUAt9w8s1rLPK@x1n>
Date: Wed, 15 Jan 2025 15:28:00 -0500
From: Peter Xu <peterx@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Shuah Khan <shuah@...nel.org>, David Hildenbrand <david@...hat.com>,
	Mikołaj Lenczewski <miko.lenczewski@....com>,
	Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH v1 1/2] mm: Clear uffd-wp PTE/PMD state on mremap()

On Tue, Jan 07, 2025 at 02:47:52PM +0000, Ryan Roberts wrote:
> When mremap()ing a memory region previously registered with userfaultfd
> as write-protected but without UFFD_FEATURE_EVENT_REMAP, an
> inconsistency in flag clearing leads to a mismatch between the vma flags
> (which have uffd-wp cleared) and the pte/pmd flags (which do not have
> uffd-wp cleared). This mismatch causes a subsequent mprotect(PROT_WRITE)
> to trigger a warning in page_table_check_pte_flags() due to setting the
> pte to writable while uffd-wp is still set.
> 
> Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any
> such mremap() so that the values are consistent with the existing
> clearing of VM_UFFD_WP. Be careful to clear the logical flag regardless
> of its physical form; a PTE bit, a swap PTE bit, or a PTE marker. Cover
> PTE, huge PMD and hugetlb paths.
> 
> Co-developed-by: Mikołaj Lenczewski <miko.lenczewski@....com>
> Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@....com>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> Closes: https://lore.kernel.org/linux-mm/810b44a8-d2ae-4107-b665-5a42eae2d948@arm.com/
> Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
> Cc: stable@...r.kernel.org

Nothing I see wrong:

Reviewed-by: Peter Xu <peterx@...hat.com>

One trivial thing: some multiple-line comments is following the net/ coding
style rather than mm/, but well.. I don't think it's a huge deal.

https://www.kernel.org/doc/html/v4.10/process/coding-style.html#commenting

Thanks again.

-- 
Peter Xu