Re: [PATCH 1/2] mm/userfaultfd: recheck WP registration in the hugetlb UFFDIO_COPY retry

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On 10/31/26 10:24 AM, Mike Rapoport wrote:
> Hi Jose,
> 
> On Thu, Oct 29, 2026 at 11:11:01PM +0100, Jose A. Perez de Azpillaga wrote:
>> mfill_atomic_hugetlb() drops its locks to copy the source page with page
>> faults enabled and then looks the VMA up again. The retry doesn't check
>> that the VMA is still registered for WP, so if the range is re-registered
>> without WP in that window, a UFFDIO_COPY_MODE_WP copy installs a uffd-wp
>> PTE in a VMA without VM_UFFD_WP. That is one of the leftover uffd bits
>> hugetlb_fault() has to tolerate.
> 
> The "leftover" sentence will be confusing for future readers

sure, will drop it.

>  
>> The non-hugetlb retry goes through mfill_get_vma() again and fails with
>> -EINVAL. Do the same check in the hugetlb retry.
> 
> No need to reference non-hugetlb path, just describe the added check.

okay, will do in v2.

>  
>> Fixes: 6041c6917903 ("mm/hugetlb: take care of UFFDIO_COPY_MODE_WP")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
> 
> With those fixed:
> 
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

thanks for the review :)

> 
>> ---
>>  mm/userfaultfd.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
>> index 3b71712edab4..a70ecfc8a998 100644
>> --- a/mm/userfaultfd.c
>> +++ b/mm/userfaultfd.c
>> @@ -810,6 +810,10 @@ static __always_inline ssize_t mfill_atomic_hugetlb(
>>  		if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
>>  			goto out_unlock_vma;
>>  
>> +		/* The range may have been re-registered without WP meanwhile. */
>> +		if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP))
>> +			goto out_unlock_vma;
>> +
>>  		/*
>>  		 * If memory mappings are changing because of non-cooperative
>>  		 * operation (e.g. mremap) running in parallel, bail out and
>> -- 
>> 2.55.0
>>
> 


-- 
cheers,
jose a. p-a