Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()"

Sibi Sankar
2022-11-21T10:13:05Z

On 11/21/22 12:12, Manivannan Sadhasivam wrote:
> On Fri, Nov 18, 2022 at 12:33:49PM +0000, Will Deacon wrote:
>> On Fri, Nov 18, 2022 at 04:24:02PM +0530, Manivannan Sadhasivam wrote:
>>> On Mon, Nov 14, 2022 at 05:38:00PM +0000, Catalin Marinas wrote:
>>>> On Mon, Nov 14, 2022 at 03:14:21PM +0000, Robin Murphy wrote:
>>>>> On 2022-11-14 14:11, Will Deacon wrote:
>>>>>> On Mon, Nov 14, 2022 at 04:33:29PM +0530, Manivannan Sadhasivam wrote:
>>>>>>> This reverts commit c44094eee32f32f175aadc0efcac449d99b1bbf7.
>>>>>>>
>>>>>>> As reported by Amit [1], dropping cache invalidation from
>>>>>>> arch_dma_prep_coherent() triggers a crash on the Qualcomm SM8250 platform
>>>>>>> (most probably on other Qcom platforms too). The reason is, Qcom
>>>>>>> qcom_q6v5_mss driver copies the firmware metadata and shares it with modem
>>>>>>> for validation. The modem has a secure block (XPU) that will trigger a
>>>>>>> whole system crash if the shared memory is accessed by the CPU while modem
>>>>>>> is poking at it.
>>>>>>>
>>>>>>> To avoid this issue, the qcom_q6v5_mss driver allocates a chunk of memory
>>>>>>> with no kernel mapping, vmap's it, copies the firmware metadata and
>>>>>>> unvmap's it. Finally the address is then shared with modem for metadata
>>>>>>> validation [2].
>>>>>>>
>>>>>>> Now because of the removal of cache invalidation from
>>>>>>> arch_dma_prep_coherent(), there will be cache lines associated with this
>>>>>>> memory even after sharing with modem. So when the CPU accesses it, the XPU
>>>>>>> violation gets triggered.
>>>>>>
>>>>>> This last past is a non-sequitur: the buffer is no longer mapped on the CPU
>>>>>> side, so how would the CPU access it?
>>>>>
>>>>> Right, for the previous change to have made a difference the offending part
>>>>> of this buffer must be present in some cache somewhere *before* the DMA
>>>>> buffer allocation completes.
>>>>>
>>>>> Clearly that driver is completely broken though. If the DMA allocation came
>>>>> from a no-map carveout vma_dma_alloc_from_dev_coherent() then the vmap()
>>>>> shenanigans wouldn't work, so if it backed by struct pages then the whole
>>>>> dance is still pointless because *a cacheable linear mapping exists*, and
>>>>> it's just relying on the reduced chance that anything's going to re-fetch
>>>>> the linear map address after those pages have been allocated, exactly as I
>>>>> called out previously[1].
>>>>
>>>> So I guess a DMA pool that's not mapped in the linear map, together with
>>>> memremap() instead of vmap(), would work around the issue. But the
>>>> driver needs fixing, not the arch code.
>>>>
>>>
>>> Okay, thanks for the hint. Can you share how to allocate the dma-pool that's
>>> not part of the kernel's linear map? I looked into it but couldn't find a way.
>>
>> The no-map property should take care of this iirc
>>
>
> Yeah, we have been using it in other places of the same driver. But as per
> Sibi, we used dynamic allocation for metadata validation since there was no
> memory reserved statically for that.

Will,

Unlike the other portions in the driver that required statically defined
no-map carveouts, metadata just needed a contiguous memory for
authentication. Re-using existing carveouts for this metadata region
may not work due to modem FW limitations and declaring a new carveout
for metadata will break the device tree bindings. That's the reason for
using DMA_ATTR_NO_KERNEL_MAPPING for dma_alloc_attr and vmpa/vunmap with
VM_FLUSH_RESET_PERMS before passing the memory onto modem. Are there
other suggestions for achieving the same without breaking bindings?

- Sibi

>
> But if we do not have a way to allocate a dynamic memory that is not part of
> kernel's linear map, then we may have to resort to using an existing reserved
> memory.
>
> Thanks,
> Mani
>
>> Will
>