Re: [PATCH 0/4] Bounced DMA support

[Claire Chang]

On Mon, Jul 13, 2020 at 7:40 PM Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 2020-07-13 10:12, 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 bounced DMA. The bounced
> > DMA ops provide an implementation of DMA ops that bounce streaming DMA
> > in and out of a specially allocated 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).
>
> More to the point, this seems to need some fairly special interconnect
> hardware too. On typical systems that just stick a TZASC directly in
> front of the memory controller it would be hard to block DMA access
> without also blocking CPU access. With something like Arm TZC-400 I
> guess you could set up a "secure" region for most of DRAM that allows NS
> accesses by NSAID from the CPUs, then similar regions for the pools with
> NSAID access for both the respective device and the CPUs, but by that
> point you've probably used up most of the available regions before even
> considering what the firmware and TEE might want for actual Secure memory.
>
> In short, I don't foresee this being used by very many systems.
We're going to use this on MTK SoC with MPU (memory protection unit) to
restrict the DMA access for PCI-e Wi-Fi.
>
> That said,, although the motivation is different, it appears to end up
> being almost exactly the same end result as the POWER secure
> virtualisation thingy (essentially: constrain DMA to a specific portion
> of RAM). The more code can be shared with that, the better.
Could you share a bit more about the POWER secure virtualisation thingy?
>
> > Currently, 32-bit architectures are not supported because of the need to
> > handle HIGHMEM, which increases code complexity and adds more
> > performance penalty for such platforms. Also, bounced DMA can not be
> > enabled on devices behind an IOMMU, as those require an IOMMU-aware
> > implementation of DMA ops and do not require this kind of mitigation
> > anyway.
>
> Note that we do actually have the notion of bounced DMA with IOMMUs
> already (to avoid leakage of unrelated data in the same page). I think
> it's only implemented for intel-iommu so far, but shouldn't take much
> work to generalise to iommu-dma if anyone wanted to. That's already done
> a bunch of work to generalise the SWIOTLB routines to be more reusable,
> so building on top of that would be highly preferable.
Yes, I'm aware of that and I'll try to put this on top of SWIOTLB.
>
> Thirdly, the concept of device-private bounce buffers does in fact
> already exist to some degree too - there are various USB, crypto and
> other devices that can only DMA to a local SRAM buffer (not to mention
> subsystems doing their own bouncing for the sake of alignment/block
> merging/etc.). Again, a slightly more generalised solution that makes
> this a first-class notion for dma-direct itself and could help supplant
> some of those hacks would be really really good.
>
> Robin.
>
> > [1a] https://googleprojectzero.blogspot.com/2017/04/over-air-exploiting-broadcoms-wi-fi_4.html
> > [1b] https://googleprojectzero.blogspot.com/2017/04/over-air-exploiting-broadcoms-wi-fi_11.html
> > [2] https://blade.tencent.com/en/advisories/qualpwn/
> > [3] https://www.bleepingcomputer.com/news/security/vulnerabilities-found-in-highly-popular-firmware-for-wifi-chips/
> >
> >
> > Claire Chang (4):
> >    dma-mapping: Add bounced DMA ops
> >    dma-mapping: Add bounced DMA pool
> >    dt-bindings: of: Add plumbing for bounced DMA pool
> >    of: Add plumbing for bounced DMA pool
> >
> >   .../reserved-memory/reserved-memory.txt       |  36 +++
> >   drivers/of/address.c                          |  37 +++
> >   drivers/of/device.c                           |   3 +
> >   drivers/of/of_private.h                       |   6 +
> >   include/linux/device.h                        |   3 +
> >   include/linux/dma-mapping.h                   |   1 +
> >   kernel/dma/Kconfig                            |  17 +
> >   kernel/dma/Makefile                           |   1 +
> >   kernel/dma/bounced.c                          | 304 ++++++++++++++++++
> >   9 files changed, 408 insertions(+)
> >   create mode 100644 kernel/dma/bounced.c
> >