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

[Ryan Roberts <ryan.roberts@arm.com>]

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@arm.com>
>> Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@arm.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.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@vger.kernel.org
> 
> Nothing I see wrong:
> 
> Reviewed-by: Peter Xu <peterx@redhat.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.
>