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

[Peter Xu <peterx@redhat.com>]

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>

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