Re: [PATCH v4 3/9] swiotlb: Zero out bounce buffer for untrusted device

[Lu Baolu <baolu.lu@linux.intel.com>]

Hi,

On 6/12/19 9:05 AM, Konrad Rzeszutek Wilk wrote:
> On Wed, Jun 12, 2019 at 08:43:40AM +0800, Lu Baolu wrote:
>> Hi Konrad,
>>
>> Thanks a lot for your reviewing.
>>
>> On 6/10/19 11:45 PM, Konrad Rzeszutek Wilk wrote:
>>> On Mon, Jun 03, 2019 at 09:16:14AM +0800, Lu Baolu wrote:
>>>> This is necessary to avoid exposing valid kernel data to any
>>>> milicious device.
>>>
>>> malicious
>>
>> Yes, thanks.
>>
>>>
>>>>
>>>> Suggested-by: Christoph Hellwig <hch@lst.de>
>>>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>>>> ---
>>>>    kernel/dma/swiotlb.c | 6 ++++++
>>>>    1 file changed, 6 insertions(+)
>>>>
>>>> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
>>>> index f956f785645a..ed41eb7f6131 100644
>>>> --- a/kernel/dma/swiotlb.c
>>>> +++ b/kernel/dma/swiotlb.c
>>>> @@ -35,6 +35,7 @@
>>>>    #include <linux/scatterlist.h>
>>>>    #include <linux/mem_encrypt.h>
>>>>    #include <linux/set_memory.h>
>>>> +#include <linux/pci.h>
>>>>    #ifdef CONFIG_DEBUG_FS
>>>>    #include <linux/debugfs.h>
>>>>    #endif
>>>> @@ -560,6 +561,11 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
>>>>    	 */
>>>>    	for (i = 0; i < nslots; i++)
>>>>    		io_tlb_orig_addr[index+i] = orig_addr + (i << IO_TLB_SHIFT);
>>>> +
>>>> +	/* Zero out the bounce buffer if the consumer is untrusted. */
>>>> +	if (dev_is_untrusted(hwdev))
>>>> +		memset(phys_to_virt(tlb_addr), 0, alloc_size);
>>>
>>> What if the alloc_size is less than a PAGE? Should this at least have ALIGN or such?
>>
>> It's the consumer (iommu subsystem) who requires this to be page
>> aligned. For swiotlb, it just clears out all data in the allocated
>> bounce buffer.
> 
> I am thinking that the if you don't memset the full page the malicious hardware could read stale date from the rest of the page
> that hasn't been cleared?

Yes. My point is that this should be guaranteed by the bounce page
implementation in iommu.

Best regards,
Baolu

> 
>>
>> Best regards,
>> Baolu
>>
>>>
>>>> +
>>>>    	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
>>>>    	    (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL))
>>>>    		swiotlb_bounce(orig_addr, tlb_addr, mapping_size, DMA_TO_DEVICE);
>>>> -- 
>>>> 2.17.1
>>>>
>>>
>