Re: [Qemu-devel] [PATCH v3] vfio/common: Check iova with limit not with size

[Pierre Morel <pmorel@linux.vnet.ibm.com>]

On 01/12/2016 07:16 PM, Alex Williamson wrote:
> On Tue, 2016-01-12 at 16:11 +0100, Pierre Morel wrote:
>> In vfio_listener_region_add(), we try to validate that the region is
>> not
>> zero sized and hasn't overflowed the addresses space.
>>
>> But the calculation uses the size of the region instead of
>> using the region's limit (size - 1).
>>
>> This leads to Int128 overflow when the region has
>> been initialized to UINT64_MAX because in this case
>> memory_region_init() transform the size from UINT64_MAX
>> to int128_2_64().
>>
>> Let's really use the limit by sustracting one to the size
>> and take care to use the limit for functions using limit
>> and size to call functions which need size.
>>
>> Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
>> ---
>>
>> Changes from v2:
>>      - all, just ignore v2, sorry about this,
>>        this is build after v1
>>
>> Changes from v1:
>>      - adjust the tests by knowing we already substracted one to end.
>>
>>   hw/vfio/common.c |   14 +++++++-------
>>   1 files changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
>> index 6797208..a5f6643 100644
>> --- a/hw/vfio/common.c
>> +++ b/hw/vfio/common.c
>> @@ -348,12 +348,12 @@ static void
>> vfio_listener_region_add(MemoryListener *listener,
>>       if (int128_ge(int128_make64(iova), llend)) {
>>           return;
>>       }
>> -    end = int128_get64(llend);
>> +    end = int128_get64(int128_sub(llend, int128_one()));
>>   
>> -    if ((iova < container->min_iova) || ((end - 1) > container-
>>> max_iova)) {
>> +    if ((iova < container->min_iova) || (end  > container-
>>> max_iova)) {
>>           error_report("vfio: IOMMU container %p can't map guest IOVA
>> region"
>>                        " 0x%"HWADDR_PRIx"..0x%"HWADDR_PRIx,
>> -                     container, iova, end - 1);
>> +                     container, iova, end);
>>           ret = -EFAULT;
>>           goto fail;
>>       }
>> @@ -363,7 +363,7 @@ static void
>> vfio_listener_region_add(MemoryListener *listener,
>>       if (memory_region_is_iommu(section->mr)) {
>>           VFIOGuestIOMMU *giommu;
>>   
>> -        trace_vfio_listener_region_add_iommu(iova, end - 1);
>> +        trace_vfio_listener_region_add_iommu(iova, end);
>>           /*
>>            * FIXME: We should do some checking to see if the
>>            * capabilities of the host VFIO IOMMU are adequate to model
>> @@ -394,13 +394,13 @@ static void
>> vfio_listener_region_add(MemoryListener *listener,
>>               section->offset_within_region +
>>               (iova - section->offset_within_address_space);
>>   
>> -    trace_vfio_listener_region_add_ram(iova, end - 1, vaddr);
>> +    trace_vfio_listener_region_add_ram(iova, end, vaddr);
>>   
>> -    ret = vfio_dma_map(container, iova, end - iova, vaddr, section-
>>> readonly);
>> +    ret = vfio_dma_map(container, iova, end - iova + 1, vaddr,
>> section->readonly);
>>       if (ret) {
>>           error_report("vfio_dma_map(%p, 0x%"HWADDR_PRIx", "
>>                        "0x%"HWADDR_PRIx", %p) = %d (%m)",
>> -                     container, iova, end - iova, vaddr, ret);
>> +                     container, iova, end - iova + 1, vaddr, ret);
>>           goto fail;
>>       }
>>   
> Hmm, did we just push the overflow from one place to another?  If we're
> mapping a full region of size int128_2_64() starting at iova zero, then
> this becomes (0xffff_ffff_ffff_ffff - 0 + 1) = 0.  So I think we need
> to calculate size with 128bit arithmetic too and let it assert if we
> overflow, ie:
>
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index a5f6643..13ad90b 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -321,7 +321,7 @@ static void vfio_listener_region_add(MemoryListener *listener,
>                                        MemoryRegionSection *section)
>   {
>       VFIOContainer *container = container_of(listener, VFIOContainer, listener);
> -    hwaddr iova, end;
> +    hwaddr iova, end, size;
>       Int128 llend;
>       void *vaddr;
>       int ret;
> @@ -348,7 +348,9 @@ static void vfio_listener_region_add(MemoryListener *listener,
>       if (int128_ge(int128_make64(iova), llend)) {
>           return;
>       }
> +
>       end = int128_get64(int128_sub(llend, int128_one()));
> +    size = int128_get64(int128_sub(llend, int128_make64(iova)));

here again, if iova is null, since llend is section->size (2^64) ...

>   
>       if ((iova < container->min_iova) || (end  > container->max_iova)) {
>           error_report("vfio: IOMMU container %p can't map guest IOVA region"
> @@ -396,11 +398,11 @@ static void vfio_listener_region_add(MemoryListener *listener,
>   
>       trace_vfio_listener_region_add_ram(iova, end, vaddr);
>   
> -    ret = vfio_dma_map(container, iova, end - iova + 1, vaddr, section->readonly);
> +    ret = vfio_dma_map(container, iova, size, vaddr, section->readonly);
>       if (ret) {
>           error_report("vfio_dma_map(%p, 0x%"HWADDR_PRIx", "
>                        "0x%"HWADDR_PRIx", %p) = %d (%m)",
> -                     container, iova, end - iova + 1, vaddr, ret);
> +                     container, iova, size, vaddr, ret);
>           goto fail;
>       }
>   
>
> Does that still solve your scenario?  Perhaps vfio-iommu-type1 should
> have used first/last rather than start/size for mapping since we seem
> to have an off-by-one for mapping a full 64bit space.  Seems like we
> could do it with two calls to vfio_dma_map if we really wanted to.
> Thanks,
>
> Alex
>

You are right, every try to solve this will push the overflow somewhere 
else.

There is just no way to express 2^64 with 64 bits, we have the int128() 
solution,
but if we solve it here, we fall in the linux ioctl call anyway.

Intuitively, making two calls do not seem right to me.

But, what do you think of something like:

- creating a new VFIO extention

- and in ioctl(), since we have a flag entry in the 
vfio_iommu_type1_dma_map,
may be adding a new flag meaning "map all virtual memory" ?
or meaning "use first/last" ?
I think this would break existing code unless we add a new VFIO extension.