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.
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. > Signed-off-by: Peter Xu <peterx@io 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
I verified these latest changes with my initial test case, and can confirm that I haven't been able to reproduce the problem anymore! Usually, I would be able to reproduce the error after ~1-5 runs, given that I would fill the page cache before the run, now I have been running the same test case for 50+ times, and I haven't seen the error anymore.
Thanks again Peter for taking the time to find the issue and coming up with a fix!
On Thu, Nov 10, 2022 at 21:31:31, Peter Xu <peterx@xxxxxxxxxx> wrote:
