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

[Catalin Marinas]

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.

-- 
Catalin