Re: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte

[Peter Xu]

 Hi, Andrew,

On Wed, Nov 30, 2022 at 02:24:25PM -0800, Andrew Morton wrote:
> On Tue, 15 Nov 2022 19:17:43 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote:
> 
> > On 14.11.22 01:04, Peter Xu wrote:
> > > Ives van Hoorne from codesandbox.io reported an issue regarding possible
> > > data loss of uffd-wp when applied to memfds on heavily loaded systems.  The
> > > symptom is some read page got data mismatch from the snapshot child VMs.
> > > 
> > > Here I can also reproduce with a Rust reproducer that was provided by Ives
> > > that keeps taking snapshot of a 256MB VM, on a 32G system when I initiate
> > > 80 instances I can trigger the issues in ten minutes.
> > > 
> > > It turns out that we got some pages write-through even if uffd-wp is
> > > applied to the pte.
> > > 
> > > The problem is, when removing migration entries, we didn't really worry
> > > about write bit as long as we know it's not a write migration entry.  That
> > > may not be true, for some memory types (e.g. writable shmem) mk_pte can
> > > return a pte with write bit set, then to recover the migration entry to its
> > > original state we need to explicit wr-protect the pte or it'll has the
> > > write bit set if it's a read migration entry.  For uffd it can cause
> > > write-through.
> > > 
> > > The relevant code on uffd was introduced in the anon support, which is
> > > commit f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration",
> > > 2020-04-07).  However anon shouldn't suffer from this problem because anon
> > > should already have the write bit cleared always, so that may not be a
> > > proper Fixes target, while I'm adding the Fixes to be uffd shmem support.
> > > 
> >
> > ...
> >
> > > --- a/mm/migrate.c
> > > +++ b/mm/migrate.c
> > > @@ -213,8 +213,14 @@ static bool remove_migration_pte(struct folio *folio,
> > >   			pte = pte_mkdirty(pte);
> > >   		if (is_writable_migration_entry(entry))
> > >   			pte = maybe_mkwrite(pte, vma);
> > > -		else if (pte_swp_uffd_wp(*pvmw.pte))
> > > +		else
> > > +			/* NOTE: mk_pte can have write bit set */
> > > +			pte = pte_wrprotect(pte);
> > > +
> > > +		if (pte_swp_uffd_wp(*pvmw.pte)) {
> > > +			WARN_ON_ONCE(pte_write(pte));
> 
> Will this warnnig trigger in the scenario you and Ives have discovered?

If without the above newly added wr-protect, yes.  This is the case where
we found we got write bit set even if uffd-wp bit is also set, hence allows
the write to go through even if marked protected.

> 
> > >   			pte = pte_mkuffd_wp(pte);
> > > +		}
> > >   
> > >   		if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
> > >   			rmap_flags |= RMAP_EXCLUSIVE;
> > 
> > As raised, I don't agree to this generic non-uffd-wp change without 
> > further, clear justification.
> 
> Pater, can you please work this further?

I didn't reply here because I have already replied with the question in
previous version with a few attempts.  Quotting myself:

https://lore.kernel.org/all/Y3KgYeMTdTM0FN5W@x1n/

        The thing is recovering the pte into its original form is the
        safest approach to me, so I think we need justification on why it's
        always safe to set the write bit.

I've also got another longer email trying to explain why I think it's the
other way round to be justfied, rather than justifying removal of the write
bit for a read migration entry, here:

https://lore.kernel.org/all/Y3O5bCXSbvKJrjRL@x1n/

> 
> > I won't nack it, but I won't ack it either.
> 
> I wouldn't mind seeing a little code comment which explains why we're
> doing this.

I've got one more fixup to the same patch attached, with enriched comments
on why we need wr-protect for read migration entries.

Please have a look to see whether that helps, thanks.

-- 
Peter Xu