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

Paolo Bonzini
2021-06-25T08:07:32Z

On 25/06/21 09:58, Christian Borntraeger wrote:
>
>
> On 25.06.21 09:36, David Stevens wrote:
>> From: Nicholas Piggin <npiggin@gmail.com>
>>
>> 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@gmail.com>
>
> I guess this would be the small fix for stable? Do we want to add that cc?
>
> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>

Yes, this one is going to Linus today. The rest is for 5.15.

Paolo

>> ---
>>   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;
>>   }
>>
>>   /*
>>
>