4 Jan
2016
4 Jan
'16
12:43 p.m.
At least the boot process gets all the information it needs (including the
physical addresses associated with untyped memory frames) and also gets the
master I/O capability.
On Sun, Jan 3, 2016 at 3:49 PM, Gerwin Klein <Gerwin.Klein(a)nicta.com.au>
wrote:
have
On 26 Dec 2015, at 21:24, Corey Richardson
<corey(a)octayn.net> 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 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
IOPorts.
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.
Cheers,
Gerwin
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