Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect

[Laurent Dufour]

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.