Subject: [PATCH] arm: dma-mapping: fix off-by-one check in arm_setup_iommu_dma_ops

From: [email-masked] (Robin Murphy)
Date: Tue, 28 Apr 2015 16:10:27 +0100

On 28/04/15 15:52, Robin Murphy wrote:
> Hi Marek,
>
> On 28/04/15 10:08, Marek Szyprowski wrote:
>> Patch 22b3c181c6c324a46f71aae806d8ddbe61d25761 ("arm: dma-mapping: limit
>> IOMMU mapping size") added a check for IO address space size. However
>> this patch broke IOMMU initialization for typical platforms initialized
>> from device tree, which get the default IO address space size of 4GiB.
>> This value doesn't fit into size_t and fails a check introduced by that
>> commit resulting in failed dma-mapping/iommu initialization. This patch
>> fixes this issue by adding proper support for full 4GiB address space
>> size.
>>
>> Signed-off-by: Marek Szyprowski <[email-masked]>
>
> Much nicer than my hack of just passing in size-1, thanks! I'd offer a
> tested-by for the default 32-bit case, however...
>
>> ---
>>    arch/arm/include/asm/dma-iommu.h | 2 +-
>>    arch/arm/mm/dma-mapping.c        | 9 +++------
>>    2 files changed, 4 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/dma-iommu.h b/arch/arm/include/asm/dma-iommu.h
>> index 8e3fcb924db6..2ef282f96651 100644
>> --- a/arch/arm/include/asm/dma-iommu.h
>> +++ b/arch/arm/include/asm/dma-iommu.h
>> @@ -25,7 +25,7 @@ struct dma_iommu_mapping {
>>    };
>>
>>    struct dma_iommu_mapping *
>> -arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, size_t size);
>> +arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, u64 size);
>>
>>    void arm_iommu_release_mapping(struct dma_iommu_mapping *mapping);
>>
>> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
>> index 09c5fe3d30c2..b43b762eebc1 100644
>> --- a/arch/arm/mm/dma-mapping.c
>> +++ b/arch/arm/mm/dma-mapping.c
>> @@ -1878,7 +1878,7 @@ struct dma_map_ops iommu_coherent_ops = {
>>     * arm_iommu_attach_device function.
>>     */
>>    struct dma_iommu_mapping *
>> -arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, size_t size)
>> +arm_iommu_create_mapping(struct bus_type *bus, dma_addr_t base, u64 size)
>>    {
>>    	unsigned int bits = size >> PAGE_SHIFT;
>
> ...doesn't this u64->int conversion now have the potential to truncate
> (if the device has a large enough DMA mask specified) and end up
> generating a weirdly small mapping rather than failing outright?
> Admittedly you'd have to have a 44-bit or larger DMA mask, and I'm not
> sure what the practical likelihood of seeing that even on LPAE systems
> is, but still I'm a little uneasy about it.

...by which I mean we should move the current size check into 
arm_iommu_create_mapping itself, rather than relying on the caller, 
because looking at dependent bits of code in isolation can be confusing ;)

Robin.

>>    	unsigned int bitmap_size = BITS_TO_LONGS(bits) * sizeof(long);
>> @@ -2057,11 +2057,8 @@ static bool arm_setup_iommu_dma_ops(struct device *dev, u64 dma_base, u64 size,
>>    	if (!iommu)
>>    		return false;
>>
>> -	/*
>> -	 * currently arm_iommu_create_mapping() takes a max of size_t
>> -	 * for size param. So check this limit for now.
>> -	 */
>> -	if (size > SIZE_MAX)
>> +	/* Only 32-bit DMA address space is supported for now */
>> +	if (size > DMA_BIT_MASK(32) + 1)
>>    		return false;
>>
>>    	mapping = arm_iommu_create_mapping(dev->bus, dma_base, size);
>>
>
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