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

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2018/10/16 1:21, Will Deacon wrote:
> 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.
>>
>> 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@huawei.com>
>> ---
>>  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)));
> 
> Won't this already be aligned by the ABI?
> 
> Anyway, you'll need to swizzle things for big-endian, I suspect. Maybe you
> can do something clever like making sync_count an array of two elements
> and determining the offset based on the endianness. Or just keep it simple
> like we do for things like struct qrwlock and struct qspinlock and use
> #ifdefs.

This workaround is a special case, the sync_count is only written by ITS hardware,
and is only read by software. Although Hisilicon ITS will write 8 bytes at
MSIAddress(required it aligned by 8 bytes), but it can sure that the value of
MSIdata will be written at the lower 4 bytes(the start address of sync_count).
Because the type of sync_count is u32, so that CPU is also read the 4 bytes at
the lower address.

> 
> Also -- you need a comment to explain this insanity :)
> 
> Will
> 
> .
> 

-- 
Thanks!
BestRegards