Re: [RFC PATCH v2 0/2] mm: fix races due to deferred TLB flushes

Nadav Amit
2021-03-02T22:56:47Z

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> On Mar 2, 2021, at 2:13 PM, Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Dec 25, 2020 at 01:25:27AM -0800, Nadav Amit wrote:
>> From: Nadav Amit <namit@vmware.com>
>>
>> This patch-set went from v1 to RFCv2, as there is still an ongoing
>> discussion regarding the way of solving the recently found races due to
>> deferred TLB flushes. These patches are only sent for reference for now,
>> and can be applied later if no better solution is taken.
>>
>> In a nutshell, write-protecting PTEs with deferred TLB flushes was mostly
>> performed while holding mmap_lock for write. This prevented concurrent
>> page-fault handler invocations from mistakenly assuming that a page is
>> write-protected when in fact, due to the deferred TLB flush, other CPU
>> could still write to the page. Such a write can cause a memory
>> corruption if it takes place after the page was copied (in
>> cow_user_page()), and before the PTE was flushed (by wp_page_copy()).
>>
>> However, the userfaultfd and soft-dirty mechanisms did not take
>> mmap_lock for write, but only for read, which made such races possible.
>> Since commit 09854ba94c6a ("mm: do_wp_page() simplification") these
>> races became more likely to take place as non-COW'd pages are more
>> likely to be COW'd instead of being reused. Both of the races that
>> these patches are intended to resolve were produced on v5.10.
>>
>> To avoid the performance overhead some alternative solutions that do not
>> require to acquire mmap_lock for write were proposed, specifically for
>> userfaultfd. So far no better solution that can be backported was
>> proposed for the soft-dirty case.
>>
>> v1->RFCv2:
>> - Better (i.e., correct) description of the userfaultfd buggy case [Yu]
>> - Patch for the soft-dirty case
>
> Nadav,
>
> Do you plan to post a new version to fix the tlb corrupt issue that this series
> wanted to solve?

Yes, yes. Sorry for that. Will do so later today.

Regards,
Nadav

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