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Message-Id: <20221114000447.1681003-2-peterx@redhat.com>
Date: Sun, 13 Nov 2022 19:04:46 -0500
From: Peter Xu <peterx@...hat.com>
To: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: Mike Rapoport <rppt@...ux.vnet.ibm.com>,
Nadav Amit <nadav.amit@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>, peterx@...hat.com,
Andrea Arcangeli <aarcange@...hat.com>,
Ives van Hoorne <ives@...esandbox.io>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Alistair Popple <apopple@...dia.com>, stable@...r.kernel.org
Subject: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte
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@...hat.com>
Cc: stable@...r.kernel.org
Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
Reported-by: Ives van Hoorne <ives@...esandbox.io>
Reviewed-by: Alistair Popple <apopple@...dia.com>
Tested-by: Ives van Hoorne <ives@...esandbox.io>
Signed-off-by: Peter Xu <peterx@...hat.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
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