Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory

[Liam Mark <lmark@codeaurora.org>]

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+ Sumit

Hi Sumit,

Do you have any thoughts on this patch? 

It fixes a potential crash in on older kernel and I think limiting 
begin/end_cpu_access to only apply cache maintenance when the buffer is 
dma mapped makes sense from a logical perspective and performance 
perspective.



On Wed, 6 Feb 2019, Ørjan Eide wrote:

> 
> I've run some testing, and this patch does indeed fix the crash in
> dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg
> list.
> 
> Tested-by: Ørjan Eide <orjan.eide@arm.com>
> 
> I tested this on an older kernel, v4.14, since the dma-mapping code
> moved, in v4.19, to ignore the dma_address and instead use sg_phys() to
> get a valid address from the page, which is always valid in the ion sg
> lists. While this wouldn't crash on newer kernels, it's still good to
> avoid the unnecessary work when no CMO is needed.
> 

Isn't a fix like this also required from a stability perspective for 
future kernels? I understand from your analysis below that the crash has 
been fixed after 4.19 by using sg_phys to get the address but aren't we 
breaking the DMA API contract by calling dma_sync_* without first dma 
mapping the memory, if so then we have no guarantee that future 
implementations of functions like dma_direct_sync_sg_for_cpu will properly 
handle calls to dma_sync_* if the memory is not dma mapped.

> Is this patch a candidate for the relevant stable kernels, those that 
> have this bug exposed to user space via Ion and DMA_BUF_IOCTL_SYNC?
> 

My belief is that is relevant for older kernels otherwise an unprivileged 
malicious userspace application may be able to crash the system if they 
can call DMA_BUF_IOCTL_SYNC at the right time.

BTW thanks Ørjan testing and anaalsyis you have carried out on this 
change.

Liam

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