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Date:   Tue, 12 Jan 2021 20:02:24 +0100
From:   Laurent Dufour <>
To:     Peter Zijlstra <>
Cc:     Vinayak Menon <>,
        Linus Torvalds <>,
        Andy Lutomirski <>,
        Peter Xu <>,
        Nadav Amit <>, Yu Zhao <>,
        Andrea Arcangeli <>,
        linux-mm <>,
        lkml <>,
        Pavel Emelyanov <>,
        Mike Kravetz <>,
        Mike Rapoport <>,
        stable <>,
        Minchan Kim <>,
        Will Deacon <>,
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

Le 12/01/2021 à 17:57, Peter Zijlstra a écrit :
> On Tue, Jan 12, 2021 at 04:47:17PM +0100, Laurent Dufour wrote:
>> Le 12/01/2021 à 12:43, Vinayak Menon a écrit :
>>> Possibility of race against other PTE modifiers
>>> 1) Fork - We have seen a case of SPF racing with fork marking PTEs RO and that
>>> is described and fixed here
> Right, that's exactly the kind of thing I was worried about.
>>> 2) mprotect - change_protection in mprotect which does the deferred flush is
>>> marked under vm_write_begin/vm_write_end, thus SPF bails out on faults
>>> on those VMAs.
> Sure, mprotect also changes vm_flags, so it really needs that anyway.
>>> 3) userfaultfd - mwriteprotect_range is not protected unlike in (2) above.
>>> But SPF does not take UFFD faults.
>>> 4) hugetlb - hugetlb_change_protection - called from mprotect and covered by
>>> (2) above.
>>> 5) Concurrent faults - SPF does not handle all faults. Only anon page faults.
> What happened to shared/file-backed stuff? ISTR I had that working.

File-backed mappings are not processed in a speculative way, there were options 
to manage some of them depending on the underlying file system but that's still 
not done.

Shared anonymous mapping, are also not yet handled in a speculative way (vm_ops 
is not null).

>>> Of which do_anonymous_page and do_swap_page are NONE/NON-PRESENT->PRESENT
>>> transitions without tlb flush. And I hope do_wp_page with RO->RW is fine as well.
> The tricky one is demotion, specifically write to non-write.
>>> I could not see a case where speculative path cannot see a PTE update done via
>>> a fault on another CPU.
> One you didn't mention is the NUMA balancing scanning crud; although I
> think that's fine, loosing a PTE update there is harmless. But I've not
> thought overly hard on it.

That's a good point, I need to double check on that side.

>> You explained it fine. Indeed SPF is handling deferred TLB invalidation by
>> marking the VMA through vm_write_begin/end(), as for the fork case you
>> mentioned. Once the PTL is held, and the VMA's seqcount is checked, the PTE
>> values read are valid.
> That should indeed work, but are we really sure we covered them all?
> Should we invest in better TLBI APIs to make sure we can't get this
> wrong?

That may be a good option to identify deferred TLB invalidation but I've no clue 
on what this API would look like.

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