Messages in this thread | | | Date | Thu, 28 Feb 2019 15:49:26 -0800 (PST) | From | Liam Mark <> | Subject | Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory |
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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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