Re: [PATCH 1/1] iommu/arm-smmu-v3: eliminate a potential memory corruption on Hi16xx soc

[Leizhen (ThunderTown)]

On 2018/10/15 20:46, Andrew Murray wrote:
> Hi Zhen,
> 
> On Mon, Oct 15, 2018 at 04:36:16PM +0800, Zhen Lei wrote:
>> ITS translation register map:
>> 0x0000-0x003C	Reserved
>> 0x0040		GITS_TRANSLATER
>> 0x0044-0xFFFC	Reserved
>>
>> The standard GITS_TRANSLATER register in ITS is only 4 bytes, but Hisilicon
>> expands the next 4 bytes to carry some IMPDEF information. That means, 8 bytes
>> data will be written to MSIAddress each time.
>>
>> MSIAddr: |----4bytes----|----4bytes----|
>> 	 |    MSIData   |    IMPDEF    |
>>
>> There is no problem for ITS, because the next 4 bytes space is reserved in ITS.
>> But it will overwrite the 4 bytes memory following "sync_count". It's very
>> luckly that the previous and the next neighbour of "sync_count" are both aligned
>> by 8 bytes, so no problem is met now.
> 
> My understanding is that MSI's are 32bit memory writes and as such the SMMU
> performs a 32bit write in response to the MSI. If so then what is different
> with the Hi16xx that causes a problem? Have you been able to able to adjust
> the layout of the arm_smmu_device struct to demonstrate this?

In normal, only 32bits MSIdata will be written into sync_count:
|----4bytes----|----4bytes----|
|  sync_count  |     xxxx     |

But for Hi16xx, the ITS hardware will write extra 32bits IMDDEF data into "xxxx". If
"xxxx" is the space of the next struct member, its value will be overwritten.

> 
> Thanks,
> 
> Andrew Murray
> 
>>
>> It's good to explicitly add a workaround:
>> 1. Add gcc __attribute__((aligned(8))) to make sure that "sync_count" is always
>>    aligned by 8 bytes.
>> 2. Add a "u64" union member to make sure the 4 bytes padding is always exist.
>>
>> There is no functional change.
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>> ---
>>  drivers/iommu/arm-smmu-v3.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>> index 5059d09..a07bc0d 100644
>> --- a/drivers/iommu/arm-smmu-v3.c
>> +++ b/drivers/iommu/arm-smmu-v3.c
>> @@ -586,7 +586,10 @@ struct arm_smmu_device {
>>  
>>  	struct arm_smmu_strtab_cfg	strtab_cfg;
>>  
>> +	union {
>> +	u64				padding; /* workaround for Hisilicon */
>>  	u32				sync_count;
>> +	} __attribute__((aligned(8)));
>>  
>>  	/* IOMMU core code handle */
>>  	struct iommu_device		iommu;
>> -- 
>> 1.8.3
>>
>>
>> _______________________________________________
>> iommu mailing list
>> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
>> https://lists.linuxfoundation.org/mailman/listinfo/iommu
> 
> .
> 

-- 
Thanks!
BestRegards