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

Robin Murphy
2022-11-14T15:14:34Z

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].

Robin.

[1]
https://lore.kernel.org/linux-arm-kernel/97fface8-e40e-072c-4335-c94094884e93@arm.com/

> As I just replied to Amit, we need more information about what this
> "access" is and how it is being detected.
>
> Will