[PATCH 0/6] KVM: Remove uses of struct page from x86 and arm64 MMU

[Christian Borntraeger]

On 24.06.21 14:57, Nicholas Piggin wrote:
> Excerpts from Paolo Bonzini's message of June 24, 2021 10:41 pm:
>> On 24/06/21 13:42, Nicholas Piggin wrote:
>>> Excerpts from Nicholas Piggin's message of June 24, 2021 8:34 pm:
>>>> Excerpts from David Stevens's message of June 24, 2021 1:57 pm:
>>>>> KVM supports mapping VM_IO and VM_PFNMAP memory into the guest by using
>>>>> follow_pte in gfn_to_pfn. However, the resolved pfns may not have
>>>>> assoicated struct pages, so they should not be passed to pfn_to_page.
>>>>> This series removes such calls from the x86 and arm64 secondary MMU. To
>>>>> do this, this series modifies gfn_to_pfn to return a struct page in
>>>>> addition to a pfn, if the hva was resolved by gup. This allows the
>>>>> caller to call put_page only when necessated by gup.
>>>>>
>>>>> This series provides a helper function that unwraps the new return type
>>>>> of gfn_to_pfn to provide behavior identical to the old behavior. As I
>>>>> have no hardware to test powerpc/mips changes, the function is used
>>>>> there for minimally invasive changes. Additionally, as gfn_to_page and
>>>>> gfn_to_pfn_cache are not integrated with mmu notifier, they cannot be
>>>>> easily changed over to only use pfns.
>>>>>
>>>>> This addresses CVE-2021-22543 on x86 and arm64.
>>>>
>>>> Does this fix the problem? (untested I don't have a POC setup at hand,
>>>> but at least in concept)
>>>
>>> This one actually compiles at least. Unfortunately I don't have much
>>> time in the near future to test, and I only just found out about this
>>> CVE a few hours ago.
>>
>> And it also works (the reproducer gets an infinite stream of userspace
>> exits and especially does not crash).  We can still go for David's
>> solution later since MMU notifiers are able to deal with this pages, but
>> it's a very nice patch for stable kernels.
> 
> Oh nice, thanks for testing. How's this?
> 
> Thanks,
> Nick
> 
> ---
> 
> KVM: Fix page ref underflow for regions with valid but non-refcounted pages
> 
> 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 at gmail.com>
> ---
>   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 6a6bc7af0e28..46fb042837d2 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2055,6 +2055,13 @@ 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,
> @@ -2104,13 +2111,21 @@ 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;
>   

Right. That should also take care of s390 (pin_guest_page in vsie.c
which calls gfn_to_page).
FWIW, the current API is really hard to follow as it does not tell
which functions take a reference and which dont.

Anyway, this patch (with cc stable?)

Reviewed-by: Christian Borntraeger <borntraeger at de.ibm.com>

>   out:
>   	pte_unmap_unlock(ptep, ptl);
>   	*p_pfn = pfn;
> -	return 0;
> +
> +	return r;
>   }
>   
>   /*
>