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    SubjectRe: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
    On Tue, Jan 12, 2021 at 11:15:43AM -0800, Nadav Amit wrote:
    > > On Jan 12, 2021, at 11:02 AM, Laurent Dufour <ldufour@linux.vnet.ibm.com> wrote:
    > >
    > > 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 https://lore.kernel.org/patchwork/patch/1062672/
    > >> 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.
    >
    > I will send an RFC soon for per-table deferred TLB flushes tracking.
    > The basic idea is to save a generation in the page-struct that tracks
    > when deferred PTE change took place, and track whenever a TLB flush
    > completed. In addition, other users - such as mprotect - would use
    > the tlb_gather interface.
    >
    > Unfortunately, due to limited space in page-struct this would only
    > be possible for 64-bit (and my implementation is only for x86-64).

    I don't want to discourage you but I don't think this would end up
    well. PPC doesn't necessarily follow one-page-struct-per-table rule,
    and I've run into problems with this before while trying to do
    something similar.

    I'd recommend per-vma and per-category (unmapping, clearing writable
    and clearing dirty) tracking, which only rely on arch-independent data
    structures, i.e., vm_area_struct and mm_struct.

    > It would still require to do the copying while holding the PTL though.

    IMO, this is unacceptable. Most archs don't support per-table PTL, and
    even x86_64 can be configured to use per-mm PTL. What if we want to
    support a larger page size in the feature?

    It seems to me the only way to solve the problem with self-explanatory
    code and without performance impact is to check mm_tlb_flush_pending
    and the writable bit (and two other cases I mentioned above) at the
    same time. Of course, this requires a lot of effort to audit the
    existing uses, clean them up and properly wrap them up with new
    primitives, BUG_ON all invalid cases and document the exact workflow
    to prevent misuses.

    I've mentioned the following before -- it only demonstrates the rough
    idea.

    diff --git a/mm/memory.c b/mm/memory.c
    index 5e9ca612d7d7..af38c5ee327e 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -4403,8 +4403,11 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
    goto unlock;
    }
    if (vmf->flags & FAULT_FLAG_WRITE) {
    - if (!pte_write(entry))
    + if (!pte_write(entry)) {
    + if (mm_tlb_flush_pending(vmf->vma->vm_mm))
    + flush_tlb_page(vmf->vma, vmf->address);
    return do_wp_page(vmf);
    + }
    entry = pte_mkdirty(entry);
    }
    entry = pte_mkyoung(entry);
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