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SubjectRe: [PATCH 1/1] userfaultfd: avoid huge_zero_page in UFFDIO_MOVE
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On 12.01.24 02:39, Suren Baghdasaryan wrote:
> While testing UFFDIO_MOVE ioctl, syzbot triggered VM_BUG_ON_PAGE caused
> by a call to PageAnonExclusive() with a huge_zero_page as a parameter.
> UFFDIO_MOVE does not yet handle zeropages and returns EBUSY when one is
> encountered. Add an early huge_zero_page check in the PMD move path
> to avoid this situation.
>
> Reported-by: syzbot+705209281e36404998f6@syzkaller.appspotmail.com
> Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> Applies cleanly over linux-next, mm-stable and mm-unstable branches
>
> mm/userfaultfd.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 216ab4c8621f..20e3b0d9cf7e 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -1393,6 +1393,12 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
> err = -ENOENT;
> break;
> }
> + /* Avoid moving zeropages for now */
> + if (is_huge_zero_pmd(*src_pmd)) {
> + spin_unlock(ptl);
> + err = -EBUSY;
> + break;
> + }

As an alternative (and more future proof?) just reject anything that is
not an anon folio as well?

Anyhow, this should handle the issue at hand.

Acked-by: David Hildenbrand <david@redhat.com>

--
Cheers,

David / dhildenb


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