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Date:   Mon, 29 Oct 2018 17:59:05 +0000
From:   Will Deacon <will.deacon@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        LinuxArm <linuxarm@...wei.com>
Subject: Re: [PATCH v2 1/1] iommu/arm-smmu-v3: eliminate a potential memory
 corruption on Hi16xx soc

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.
> +	 * 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 */
	};

Will

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