Re: [PATCH 0/4] Bounced DMA support
From: Robin Murphy
Date: Wed Jul 15 2020 - 13:53:26 EST
On 2020-07-15 04:43, Claire Chang wrote:
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.
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@xxxxxxxxxxxxx/
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