Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte

[Peter Xu <peterx@redhat.com>]

On Mon, Nov 14, 2022 at 05:09:32PM +0100, David Hildenbrand wrote:
> On 10.11.22 21:31, 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
> > sympton 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.  I didn't verify, but I think it'll be
> > the same for mprotect()ed pages and after migration we can miss the sigbus
> > instead.
> 
> I don't think so. mprotect() handling relies on vma->vm_page_prot, which is
> supposed to do the right thing. E.g., map the pte protnone without
> VM_READ/VM_WRITE/....

I've removed that example when I posted v3, feel free to have a look.

> 
> > 
> > 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.  To satisfy the need on the backport, I'm attaching
> > the Fixes tag to the uffd-wp shmem support.  Since no one had issue with
> > mprotect, so I assume that's also the kernel version we should start to
> > backport for stable, and we shouldn't need to worry before that.
> > 
> > Cc: Andrea Arcangeli <aarcange@redhat.com>
> > Cc: stable@vger.kernel.org
> > Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
> > Reported-by: Ives van Hoorne <ives@codesandbox.io>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >   mm/migrate.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index dff333593a8a..8b6351c08c78 100644
> > --- 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);
> 
> 
> Any particular reason why not to simply glue this to pte_swp_uffd_wp(),
> because only that needs special care:
> 
> if (pte_swp_uffd_wp(*pvmw.pte)) {
> 	pte = pte_wrprotect(pte);
> 	pte = pte_mkuffd_wp(pte);
> }
> 
> 
> And that would match what actually should have been done in commit
> f45ec5ff16a7 -- only special-case uffd-wp.
> 
> Note that I think there are cases where we have a PTE that was !writable,
> but after migration we can map it writable.

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.

Or do you perhaps have solid clue and think it's always safe?

> 
> BTW, does unuse_pte() need similar care?
> 
> new_pte = pte_mkold(mk_pte(page, vma->vm_page_prot));
> if (pte_swp_uffd_wp(*pte))
> 	new_pte = pte_mkuffd_wp(new_pte);
> set_pte_at(vma->vm_mm, addr, pte, new_pte);

I think unuse path is fine because unuse only applies to private mappings,
so we should always have the W bit removed there within mk_pte().

Thanks,

-- 
Peter Xu