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

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Message-ID: <873aede9-bfcd-4c95-a93d-ec1881554f39@arm.com>
Date: Thu, 16 Jan 2025 09:04:37 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Peter Xu <peterx@...hat.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 15/01/2025 20:28, Peter Xu wrote:
> 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>

Great thanks!

> 
> 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

Noted, I'll aim to get it right in future.

> 
> Thanks again.
>