> On 26 Dec 2015, at 21:24, Corey Richardson <email@example.com> wrote:
> On Fri, Dec 25, 2015 at 11:19:55PM -0800, Raymond Jennings wrote:
>> Speaking of which, how exactly does seL4 enforce no dma on systems without
>> an IOMMU?
> I've been unable to determine this myself. Consider the example of an ATA
> controller: it seems you could put any arbitrary address in the PRDT and have
> it spray disk contents into physical memory. In this case, all one needs is an
> IOPort for that device's range on the IO bus and potentially for its place in
> PCI configuration space (to enable bus mastering).
> The manual seems to be self-contradictory here. In the section about the
> BootInfo it indicates that the physical addresses are given to initiate DMA
> when no IOMMU is present, but the IOSpace section states that to use DMA an
> IOMMU must be used.
> I can't really find any way that this is enforced or could be enforced by the
> kernel. In userland it can be done quite easily by just not giving out
seL4 does not enforce absence of DMA. In general it’s not possible to do that, that’s why it’s an assumption to the proof that you need to validate for your system.
Without an IOMMU, you will need to trust the drivers and the hardware of DMA-capable devices to either not use DMA or to use it safely only.
You can lock down systems to not provide any access at all to DMA-devices, which sometimes is enough for simple separation-style systems. There are more trade-off points in the design space, but the trust story does become massively simpler and better when you have IOMMU support.
With an IOMMU, seL4 is in control and you’re fine.
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