Re: [PATCH 1/1] userfaultfd: avoid huge_zero_page in UFFDIO_MOVE

[David Hildenbrand <david@redhat.com>]

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