Re: [PATCH v3] mm/hugetlb: Fix uffd wr-protection for CoW optimization path

[David Hildenbrand <david@redhat.com>]

On 24.03.23 23:27, Mike Kravetz wrote:
> On 03/24/23 10:26, Peter Xu wrote:
>> This patch fixes an issue that a hugetlb uffd-wr-protected mapping can be
>> writable even with uffd-wp bit set.  It only happens with hugetlb private
>> mappings, when someone firstly wr-protects a missing pte (which will
>> install a pte marker), then a write to the same page without any prior
>> access to the page.
>>
>> Userfaultfd-wp trap for hugetlb was implemented in hugetlb_fault() before
>> reaching hugetlb_wp() to avoid taking more locks that userfault won't need.
>> However there's one CoW optimization path that can trigger hugetlb_wp()
>> inside hugetlb_no_page(), which will bypass the trap.
>>
>> This patch skips hugetlb_wp() for CoW and retries the fault if uffd-wp bit
>> is detected.  The new path will only trigger in the CoW optimization path
>> because generic hugetlb_fault() (e.g. when a present pte was wr-protected)
>> will resolve the uffd-wp bit already.  Also make sure anonymous UNSHARE
>> won't be affected and can still be resolved, IOW only skip CoW not CoR.
>>
>> This patch will be needed for v5.19+ hence copy stable.
>>
>> Reported-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
>> Cc: linux-stable <stable@vger.kernel.org>
>> Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection")
>> Signed-off-by: Peter Xu <peterx@redhat.com>
>> ---
>>
>> Notes:
>>
>> v2 is not on the list but in an attachment in the reply; this v3 is mostly
>> to make sure it's not the same as the patch used to be attached.  Sorry
>> Andrew, we need to drop the queued one as I rewrote the commit message.
> 
> My appologies!  I saw the code path missed in v2 and assumed you did not
> think it applied.  So, I said nothing.  My bad!
> 
>> Muhammad, I didn't attach your T-b because of the slight functional change.
>> Please feel free to re-attach if it still works for you (which I believe
>> should).
>>
>> thanks,
>> ---
>>   mm/hugetlb.c | 14 ++++++++++++--
>>   1 file changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 8bfd07f4c143..a58b3739ed4b 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -5478,7 +5478,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>>   		       struct folio *pagecache_folio, spinlock_t *ptl)
>>   {
>>   	const bool unshare = flags & FAULT_FLAG_UNSHARE;
>> -	pte_t pte;
>> +	pte_t pte = huge_ptep_get(ptep);
>>   	struct hstate *h = hstate_vma(vma);
>>   	struct page *old_page;
>>   	struct folio *new_folio;
>> @@ -5487,6 +5487,17 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>>   	unsigned long haddr = address & huge_page_mask(h);
>>   	struct mmu_notifier_range range;
>>   
>> +	/*
>> +	 * Never handle CoW for uffd-wp protected pages.  It should be only
>> +	 * handled when the uffd-wp protection is removed.
>> +	 *
>> +	 * Note that only the CoW optimization path (in hugetlb_no_page())
>> +	 * can trigger this, because hugetlb_fault() will always resolve
>> +	 * uffd-wp bit first.
>> +	 */
>> +	if (!unshare && huge_pte_uffd_wp(pte))
>> +		return 0;
> 
> This looks correct.  However, since the previous version looked correct I must
> ask.  Can we have unshare set and huge_pte_uffd_wp true?  If so, then it seems
> we would need to possibly propogate that uffd_wp to the new pte as in v2

We can. A reproducer would share an anon hugetlb page because parent and 
child. In the parent, we would uffd-wp that page. We could trigger 
unsharing by R/O-pinning that page.

-- 
Thanks,

David / dhildenb