Re: [PATCH v2 1/5] KVM: do not allow mapping valid but non-refcounted pages

[Christian Borntraeger]

On 25.06.21 09:36, David Stevens wrote:
> From: Nicholas Piggin <npiggin@xxxxxxxxx>
>
> It's possible to create a region which maps valid but non-refcounted
> pages (e.g., tail pages of non-compound higher order allocations). These
> host pages can then be returned by gfn_to_page, gfn_to_pfn, etc., family
> of APIs, which take a reference to the page, which takes it from 0 to 1.
> When the reference is dropped, this will free the page incorrectly.
>
> Fix this by only taking a reference on the page if it was non-zero,
> which indicates it is participating in normal refcounting (and can be
> released with put_page).
>
> Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>

I guess this would be the small fix for stable? Do we want to add that cc?

Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> ---
>   virt/kvm/kvm_main.c | 19 +++++++++++++++++--
>   1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 3dcc2abbfc60..f7445c3bcd90 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2175,6 +2175,13 @@ static bool vma_is_valid(struct vm_area_struct *vma, bool write_fault)
>   	return true;
>   }
>
> +static int kvm_try_get_pfn(kvm_pfn_t pfn)
> +{
> +	if (kvm_is_reserved_pfn(pfn))
> +		return 1;
> +	return get_page_unless_zero(pfn_to_page(pfn));
> +}
> +
>   static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>   			       unsigned long addr, bool *async,
>   			       bool write_fault, bool *writable,
> @@ -2224,13 +2231,21 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
>   	 * Whoever called remap_pfn_range is also going to call e.g.
>   	 * unmap_mapping_range before the underlying pages are freed,
>   	 * causing a call to our MMU notifier.
> +	 *
> +	 * Certain IO or PFNMAP mappings can be backed with valid
> +	 * struct pages, but be allocated without refcounting e.g.,
> +	 * tail pages of non-compound higher order allocations, which
> +	 * would then underflow the refcount when the caller does the
> +	 * required put_page. Don't allow those pages here.
>   	 */
> -	kvm_get_pfn(pfn);
> +	if (!kvm_try_get_pfn(pfn))
> +		r = -EFAULT;
>
>   out:
>   	pte_unmap_unlock(ptep, ptl);
>   	*p_pfn = pfn;
> -	return 0;
> +
> +	return r;
>   }
>
>   /*