lkml.org 
[lkml]   [2023]   [Mar]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v3] mm/hugetlb: Fix uffd wr-protection for CoW optimization path
From
On 27.03.23 20:34, Mike Kravetz wrote:
> On 03/26/23 10:46, Peter Xu wrote:
>> On Fri, Mar 24, 2023 at 11:36:53PM +0100, David Hildenbrand wrote:
>>>>> @@ -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
>>
>> Good point, thanks for spotting!
>>
>>>
>>> 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.
>>
>> Right. This seems to be a separate bug.. It should be triggered in
>> totally different context and much harder due to rare use of RO pins,
>> meanwhile used with userfault-wp.
>>
>> If both of you agree, I can prepare a separate patch for this bug, and I'll
>> better prepare a reproducer/selftest with it.
>>
>
> I am OK with separate patches, and agree that the R/O pinning case is less
> likely to happen.

Yes, the combination should be rather rare and we can fix that
separately. Ideally, we'd try to mimic the same uffd code flow in
hugetlb cow/unshare handling that we use in memory.c

>
> Since this patch addresses the issue found by Muhammad,
>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>

Hopefully we didn't forget about yet another case :D

Acked-by: David Hildenbrand <david@redhat.com>

--
Thanks,

David / dhildenb

\
 
 \ /
  Last update: 2023-03-27 22:58    [W:0.054 / U:1.716 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site