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

[David Hildenbrand <david@redhat.com>]

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