Re: [Qemu-devel] [PATCH] vfio: add check for memory region overflow condition

[Bandan Das <bsd@redhat.com>]

Alex Williamson <alex.williamson@redhat.com> writes:

> On Mon, 21 Mar 2016 18:00:50 -0400
> Bandan Das <bsd@redhat.com> wrote:
>
>> vfio_listener_region_add for a iommu mr results in
>> an overflow assert since emulated iommu memory region is initialized
>> with UINT64_MAX. Add a check just like memory_region_size()
>> does.
>> 
>> Signed-off-by: Bandan Das <bsd@redhat.com>
>> ---
>>  hw/vfio/common.c | 7 ++++++-
>>  1 file changed, 6 insertions(+), 1 deletion(-)
>> 
>> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
>> index fb588d8..269244b 100644
>> --- a/hw/vfio/common.c
>> +++ b/hw/vfio/common.c
>> @@ -349,7 +349,12 @@ static void vfio_listener_region_add(MemoryListener *listener,
>>      if (int128_ge(int128_make64(iova), llend)) {
>>          return;
>>      }
>> -    end = int128_get64(llend);
>> +
>> +    if (int128_eq(llend, int128_2_64())) {
>> +            end = UINT64_MAX;
>> +    } else {
>> +            end = int128_get64(llend);
>> +    }
>>  
>>      if ((iova < container->min_iova) || ((end - 1) > container->max_iova)) {
>>          error_report("vfio: IOMMU container %p can't map guest IOVA region"
>
> But now all the calculations where we use end-1 are wrong.  See the
> discussion with Pierre Morel in the January qemu-devel archives.
> There's a solution in there, but I never saw a follow-up from Pierre
> with a revised patch.  Thanks,

I am missing something. When end < UIN64_MAX, end - 1 calculations are valid because
the patch doesn't change that behavior. When end is UINT64_MAX, int128_get64() doesn't know how
to calculate this value and we are just feeding it manually. The patch is just the opposite
of what memory_region_init() did to init the mem region in the first place:
   mr->size = int128_make64(size);
   if (size == UINT64_MAX) {
      mr->size = int128_2_64();
   }
So, end - 1 is still valid for end = UINT64_MAX, no ?

> Alex