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

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Message-ID: <YtGe2qIO038e627p@xz-m1.local>
Date:   Fri, 15 Jul 2022 13:07:38 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.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 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?

Thanks,

-- 
Peter Xu