Re: [PATCH v3 1/2] dma-mapping: Add vmap checks to dma_map_single()

[Robin Murphy <robin.murphy@arm.com>]

On 2019-10-11 6:02 am, Greg Kroah-Hartman wrote:
> On Thu, Oct 10, 2019 at 03:28:28PM -0700, Kees Cook wrote:
>> As we've seen from USB and other areas[1], we need to always do runtime
>> checks for DMA operating on memory regions that might be remapped. This
>> adds vmap checks (similar to those already in USB but missing in other
>> places) into dma_map_single() so all callers benefit from the checking.
>>
>> [1] https://git.kernel.org/linus/3840c5b78803b2b6cc1ff820100a74a092c40cbb
>>
>> Suggested-by: Laura Abbott <labbott@redhat.com>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>>   include/linux/dma-mapping.h | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
>> index 4a1c4fca475a..ff4e91c66f44 100644
>> --- a/include/linux/dma-mapping.h
>> +++ b/include/linux/dma-mapping.h
>> @@ -583,6 +583,12 @@ static inline unsigned long dma_get_merge_boundary(struct device *dev)
>>   static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
>>   		size_t size, enum dma_data_direction dir, unsigned long attrs)
>>   {
>> +	/* DMA must never operate on areas that might be remapped. */
>> +	if (unlikely(is_vmalloc_addr(ptr))) {
>> +		dev_warn_once(dev, "bad map: %zu bytes in vmalloc\n", size);
> 
> Can we get a bit better error text here?  In USB we were at least giving
> people a hint as to what went wrong, "bad map" might not really make
> that much sense to a USB developer as to what they needed to do to fix
> this issue.

Agreed, something along the lines of "dma_map_single of vmalloc'ed 
buffer..." might be clearer. Also I'd be inclined to use dev_WARN_ONCE() 
to include a diagnostically-useful backtrace, as the existing USB code 
would.

Thanks,
Robin.
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