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

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:

quoted

On 2022-11-14 14:11, Will Deacon wrote:

quoted

On Mon, Nov 14, 2022 at 04:33:29PM +0530, Manivannan Sadhasivam wrote:

quoted

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.

Thanks,
Mani

-- 
Catalin

-- 
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