Messages in this thread | | | Subject | Re: [PATCH 0/4] Bounced DMA support | From | Robin Murphy <> | Date | Wed, 15 Jul 2020 18:53:17 +0100 |
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On 2020-07-15 04:43, Claire Chang wrote: > On Mon, Jul 13, 2020 at 7:40 PM Robin Murphy <robin.murphy@arm.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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