linux-kernel - Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in hugetlb_mcopy_atomic

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Message-ID: <CAJHvVcji14hUsqg=yrtevEqwMtZi7Xwyj4yUnLMaZi0DBqPTHA@mail.gmail.com>
Date:   Fri, 15 Jul 2022 10:28:44 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/hugetlb: avoid corrupting page->mapping in hugetlb_mcopy_atomic_pte

On Fri, Jul 15, 2022 at 10:07 AM Peter Xu <peterx@...hat.com> wrote:
>
> On Fri, Jul 15, 2022 at 09:45:37AM -0700, Axel Rasmussen wrote:
> > I agree we should either:
> > - Update the UFFD selftest to exercise this case
> > - Or, don't allow it, update vma_can_userfault() to also require VM_SHARED
> > for VM_UFFD_MINOR registration.
> >
> > The first one is unfortunately not completely straightforward as Peter
> > described. I would say it's probably not worth holding up this fix while we
> > wait for it to happen?
>
> Agreed, Andrew has already queued it.  It actually is a real fix since we
> never forbid the user running private mappings upon minor faults, so
> it's literally a bug in kernel irrelevant of the kselftest.
>
> >
> > I don't really have a strong preference between the two. The second option
> > is what I originally proposed in the first version of the minor fault
> > series, so going back to that isn't a problem at least from my perspective.
> > If in the future we find a real use case for this, we could always easily
> > re-enable it and add selftests for it at that point.
>
> I'd go for fixing the test case if possible.  Mike, would it be fine if we
> go back to /dev/hugepages path based approach in the test case?

One possible alternative, can we use memfd_create() with MFD_HUGE_*?
This afaict lets us have an fd so we can create two mappings,
without having to mount hugetlbfs, pass in a path to the test, ...

>
>
> Thanks,

>
> --
> Peter Xu
>