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

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Message-ID: <c762297d-5c65-f20a-4c11-90d2f966c675@redhat.com>
Date:   Mon, 27 Mar 2023 22:57:46 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        linux-stable <stable@...r.kernel.org>
Subject: Re: [PATCH v3] mm/hugetlb: Fix uffd wr-protection for CoW
 optimization path

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@...cle.com>

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

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

-- 
Thanks,

David / dhildenb