From: Peter Xu <peterx@redhat.com>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrea Arcangeli <aarcange@redhat.com>,
Axel Rasmussen <axelrasmussen@google.com>,
Ives van Hoorne <ives@codesandbox.io>,
Nadav Amit <nadav.amit@gmail.com>,
Andrew Morton <akpm@linux-foundation.org>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
stable@vger.kernel.org
Subject: Re: [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte
Date: Mon, 14 Nov 2022 15:09:05 -0500 [thread overview]
Message-ID: <Y3KgYeMTdTM0FN5W@x1n> (raw)
In-Reply-To: <9af36be3-313b-e39c-85bb-bf30011bccb8@redhat.com>
On Mon, Nov 14, 2022 at 05:09:32PM +0100, David Hildenbrand wrote:
> 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/....
I've removed that example when I posted v3, feel free to have a look.
>
> >
> > 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.
The thing is recovering the pte into its original form is the safest
approach to me, so I think we need justification on why it's always safe to
set the write bit.
Or do you perhaps have solid clue and think it's always safe?
>
> 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);
I think unuse path is fine because unuse only applies to private mappings,
so we should always have the W bit removed there within mk_pte().
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2022-11-14 20:10 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-10 20:31 [PATCH v2 0/2] mm/migrate: Fix writable pte for read migration entry Peter Xu
2022-11-10 20:31 ` [PATCH v2 1/2] mm/migrate: Fix read-only page got writable when recover pte Peter Xu
2022-11-10 21:28 ` Nadav Amit
2022-11-10 22:09 ` Peter Xu
[not found] ` <CADnc5G2-7B7qyPitDY33Wb0D5a=pq-1PC=gp0f9peTtVnOvzjw@mail.gmail.com>
2022-11-10 22:08 ` Peter Xu
2022-11-10 23:42 ` Alistair Popple
2022-11-13 23:56 ` Peter Xu
2022-11-14 6:22 ` Alistair Popple
2022-11-14 16:09 ` David Hildenbrand
2022-11-14 20:09 ` Peter Xu [this message]
2022-11-15 9:13 ` David Hildenbrand
2022-11-15 16:08 ` Peter Xu
2022-11-15 17:22 ` David Hildenbrand
2022-11-15 17:54 ` David Hildenbrand
2022-11-15 18:11 ` Peter Xu
2022-11-15 18:16 ` David Hildenbrand
2022-11-15 18:03 ` Peter Xu
2022-11-15 18:08 ` David Hildenbrand
2022-11-10 20:31 ` [PATCH v2 2/2] mm/uffd: Sanity check write bit for uffd-wp protected ptes Peter Xu
2022-11-11 22:06 ` kernel test robot
2022-11-13 22:33 ` Peter Xu
2022-11-12 2:59 ` kernel test robot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=Y3KgYeMTdTM0FN5W@x1n \
--to=peterx@redhat.com \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=axelrasmussen@google.com \
--cc=david@redhat.com \
--cc=ives@codesandbox.io \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=nadav.amit@gmail.com \
--cc=rppt@linux.vnet.ibm.com \
--cc=stable@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).