linux-kernel - Re: [PATCH v2 1/2] vfio iommu type1: Fix size argument to vfio_find

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Date:   Tue, 6 Dec 2016 23:50:59 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     <pbonzini@...hat.com>, <kraxel@...hat.com>, <cjia@...dia.com>,
        <qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] vfio iommu type1: Fix size argument to
 vfio_find_dma() during DMA UNMAP.



On 12/6/2016 11:08 PM, Alex Williamson wrote:
> On Tue, 6 Dec 2016 22:43:30 +0530
> Kirti Wankhede <kwankhede@...dia.com> wrote:
> 
>> vfio_dma keeps track of address range from (dma->iova + 0) to
>> (dma->iova + dma->size - 1), while vfio_find_dma() search logic looks for
>> range from (dma->iova + 1) to (dma->iova + dma->size).
> 
> This is not true.  The issue is with the non-inclusive address at the
> end of a range being incompatible with passing zero for the range
> size.  Passing zero for the range size is a bit of a hack and doing
> such triggers a corner case in the end of range detection where we test
> (start + size <= dma->iova).  Using <= here covers the non-inclusive
> range end, ie. if the range was start=0x0, size=0x1000, the range is
> actually 0x0-0xfff.  Thus if start+size is 0x1000, it should not be
> considered part of a range beginning at 0x1000.  So there's no
> incompatibility as implied in the statement above, it's more that using
> zero for the size isn't compatible with matching the start address of
> an existing vfio_dma.  Thanks,
> 

Makes sense. Updating the description of both patch.

Thanks,
Kirti

> Alex
> 
>> In vfio_find_dma(), when the start address is equal to dma->iova and size
>> is 0, check for the end of search range makes it to take wrong side of
>> RB-tree. That fails the search even though the address is present in
>> mapped dma ranges. Due to this, in vfio_dma_do_unmap(), while checking
>> boundary conditions, size should be set to 1 for verifying start address
>> of unmap range.
>> vfio_find_dma() is also used to verify last address in unmap range with
>> size = 0, but in that case address to be searched is calculated with
>> size - 1 and so it works correctly.
>>
>> Signed-off-by: Kirti Wankhede <kwankhede@...dia.com>
>> Signed-off-by: Neo Jia <cjia@...dia.com>
>> ---
>>  drivers/vfio/vfio_iommu_type1.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index a28fbddb505c..8e9e94ccb2ff 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -826,7 +826,7 @@ again:
>>  	 * mappings within the range.
>>  	 */
>>  	if (iommu->v2) {
>> -		dma = vfio_find_dma(iommu, unmap->iova, 0);
>> +		dma = vfio_find_dma(iommu, unmap->iova, 1);
>>  		if (dma && dma->iova != unmap->iova) {
>>  			ret = -EINVAL;
>>  			goto unlock;
>