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

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Message-ID: <3964f3ff-ce24-4656-ae4b-bc9ffdf6532a@redhat.com>
Date: Mon, 20 Jan 2025 15:01:07 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, 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>, 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 16.01.25 10:04, Ryan Roberts wrote:
> 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!

Thanks Peter, for your feedback while I was out.

I remember that I skimmed over it without anything obvious jumping at 
me, but decided to set it aside for later to take a closer look .... 
which never happened.

Took another look, and it looks good to me! (we really must clear the 
uffd-wp flags when losing the VMA flag)

-- 
Cheers,

David / dhildenb