Message-ID: <6c36635b-9611-30dd-bb6f-92e7d66bd7e3@arm.com>
Date:   Wed, 15 Jul 2020 18:53:17 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Claire Chang <tientzu@...omium.org>
Cc:     Rob Herring <robh+dt@...nel.org>, frowand.list@...il.com,
        hch@....de, m.szyprowski@...sung.com, treding@...dia.com,
        Greg KH <gregkh@...uxfoundation.org>, saravanak@...gle.com,
        suzuki.poulose@....com, dan.j.williams@...el.com,
        heikki.krogerus@...ux.intel.com, bgolaszewski@...libre.com,
        devicetree@...r.kernel.org, lkml <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux-foundation.org, tfiga@...omium.org,
        Nicolas Boichat <drinkcat@...omium.org>
Subject: Re: [PATCH 0/4] Bounced DMA support

On 2020-07-15 04:43, Claire Chang wrote:
> On Mon, Jul 13, 2020 at 7:40 PM Robin Murphy <robin.murphy@....com> 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.

OK, based on failing to really grasp the M4U and LARB side of MTK's 
interconnect for the media stuff, I'm not even going to ask about that 
MPU ;)

What I had in mind and didn't do a good job of actually expressing there 
was that this is a strong justification for using shared code as much as 
possible, to minimise the amount we have to maintain that's specific to 
this relatively niche use-case (it's good for SoCs like yours that *can* 
support it, but most will either have IOMMUs or simply not have the 
option of doing anything).

>> 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?

There are probably more recent resources, but what I could easily turn 
up from memory (other than a lot of back-and-forth about virtio) is this 
old patchset which seems to form a reasonable overview:

https://lore.kernel.org/linux-iommu/20180824162535.22798-1-bauerman@linux.ibm.com/

The main differences are that they have the luxury of being able to make 
the SWIOTLB buffer accessible in-place, rather than having to control 
its initial allocation, and because it's a system-wide thing they could 
pretty much achieve it all with existing machinery - on second look 
there are actually far fewer changes there than I thought - whereas you 
need the extra work to handle it on a per-device basis.

Robin.

>>> 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
>>>

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