Re: [RFC PATCH v3 0/6] Restricted DMA

[Florian Fainelli]

Hi,

First of all let me say that I am glad that someone is working on a
upstream solution for this issue, would appreciate if you could CC and
Jim Quinlan on subsequent submissions.

On 1/5/21 7:41 PM, Claire Chang wrote:
> This series implements mitigations for lack of DMA access control on
> systems without an IOMMU, which could result in the DMA accessing the
> system memory at unexpected times and/or unexpected addresses, possibly
> leading to data leakage or corruption.
> 
> For example, we plan to use the PCI-e bus for Wi-Fi and that PCI-e bus is
> not behind an IOMMU. As PCI-e, by design, gives the device full access to
> system memory, a vulnerability in the Wi-Fi firmware could easily escalate
> to a full system exploit (remote wifi exploits: [1a], [1b] that shows a
> full chain of exploits; [2], [3]).
> 
> To mitigate the security concerns, we introduce restricted DMA. Restricted
> DMA utilizes the existing swiotlb to bounce streaming DMA in and out of a
> specially allocated region and does memory allocation from the same region.
> The feature on its own provides a basic level of protection against the DMA
> overwriting buffer contents at unexpected times. However, to protect
> against general data leakage and system memory corruption, the system needs
> to provide a way to restrict the DMA to a predefined memory region (this is
> usually done at firmware level, e.g. in ATF on some ARM platforms).

Can you explain how ATF gets involved and to what extent it does help,
besides enforcing a secure region from the ARM CPU's perpsective? Does
the PCIe root complex not have an IOMMU but can somehow be denied access
to a region that is marked NS=0 in the ARM CPU's MMU? If so, that is
still some sort of basic protection that the HW enforces, right?

On Broadcom STB SoCs we have had something similar for a while however
and while we don't have an IOMMU for the PCIe bridge, we do have a a
basic protection mechanism whereby we can configure a region in DRAM to
be PCIe read/write and CPU read/write which then gets used as the PCIe
inbound region for the PCIe EP. By default the PCIe bridge is not
allowed access to DRAM so we must call into a security agent to allow
the PCIe bridge to access the designated DRAM region.

We have done this using a private CMA area region assigned via Device
Tree, assigned with a and requiring the PCIe EP driver to use
dma_alloc_from_contiguous() in order to allocate from this device
private CMA area. The only drawback with that approach is that it
requires knowing how much memory you need up front for buffers and DMA
descriptors that the PCIe EP will need to process. The problem is that
it requires driver modifications and that does not scale over the number
of PCIe EP drivers, some we absolutely do not control, but there is no
need to bounce buffer. Your approach scales better across PCIe EP
drivers however it does require bounce buffering which could be a
performance hit.

Thanks!
-- 
Florian