Messages in this thread Patch in this message | | | Date | Mon, 14 Nov 2022 17:09:32 +0100 | From | David Hildenbrand <> | Subject | Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte |
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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/....
> > 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.
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);
-- Thanks,
David / dhildenb
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