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

[Peter Xu <peterx@redhat.com>]

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.

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>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Tested-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);
+
+		if (pte_swp_uffd_wp(*pvmw.pte)) {
+			WARN_ON_ONCE(pte_write(pte));
 			pte = pte_mkuffd_wp(pte);
+		}
 
 		if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
 			rmap_flags |= RMAP_EXCLUSIVE;
-- 
2.37.3