linux-kernel - Re: [PATCH v2 1/1] iommu/arm-smmu-v3: eliminate a potential memory corruption on Hi16xx soc

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Date:   Tue, 30 Oct 2018 09:26:35 +0000
From:   John Garry <john.garry@...wei.com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
        Will Deacon <will.deacon@....com>
CC:     Joerg Roedel <joro@...tes.org>, LinuxArm <linuxarm@...wei.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 1/1] iommu/arm-smmu-v3: eliminate a potential memory
 corruption on Hi16xx soc

On 30/10/2018 01:52, Leizhen (ThunderTown) wrote:
>
>
> On 2018/10/30 1:59, Will Deacon wrote:
>> On Sat, Oct 20, 2018 at 03:36:54PM +0800, Zhen Lei wrote:
>>> 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, total 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 fortunately that the previous and the next neighbour of the
>>> "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 "int" struct member to make sure the 4 bytes padding is always
>>>    exist.
>>>
>>> There is no functional change.
>>>
>>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>>> ---
>>>  drivers/iommu/arm-smmu-v3.c | 15 ++++++++++++++-
>>>  1 file changed, 14 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>>> index 5059d09..624fdd0 100644
>>> --- a/drivers/iommu/arm-smmu-v3.c
>>> +++ b/drivers/iommu/arm-smmu-v3.c
>>> @@ -586,7 +586,20 @@ struct arm_smmu_device {
>>>
>>>  	struct arm_smmu_strtab_cfg	strtab_cfg;
>>>
>>> -	u32				sync_count;
>>> +	/*
>>> +	 * The alignment and padding is required by Hi16xx of Hisilicon.

Nit: I know that there is no functional change related to this bug which 
depends on the chip version, but how about "hi1620 and earlier"? hi16xx 
is very broad.

Thanks

>>> +	 * Because the ITS hardware on Hi16xx will truncate the MSIAddress(Here
>>> +	 * it's the address of "sync_count") to 8 bytes boundary first, then
>>> +	 * write 32 bits MSIdata at offset 0, and 32 bits IMPDEF data at offset
>>> +	 * 4. Without this workaround, the adjacent member maybe overwritten.
>>> +	 *
>>> +	 *                    |---4bytes---|---4bytes---|
>>> +	 * MSIAddress & (~0x7):   MSIdata  | IMPDEF data|
>>> +	 */
>>> +	struct {
>>> +		u32			sync_count;
>>> +		int			padding;
>>> +	} __attribute__((aligned(8)));
>>
>> I thought the conclusion after reviewing your original patch was to maintain
>> the union and drop the alignment directive? e.g.
>>
>> 	union {
>> 		u32	sync_count;
>> 		u64	padding; /* Hi16xx writes an extra 32 bits of goodness */
>> 	};
> OK, I will sent v3.
>
>>
>> Will
>>
>> .
>>
>