[U-Boot] [PATCH v2 06/12] virt-dt: Allow reservation of the secure region when it is in a RAM carveout.

[Mark Rutland]

On Mon, Feb 16, 2015 at 02:31:21PM +0000, Jan Kiszka wrote:
> On 2015-02-16 15:25, Mark Rutland wrote:
> > On Mon, Feb 16, 2015 at 01:51:37PM +0000, Jan Kiszka wrote:
> >> On 2015-02-16 14:42, Mark Rutland wrote:
> >>> On Mon, Feb 16, 2015 at 12:54:43PM +0000, Jan Kiszka wrote:
> >>>> From: Ian Campbell <ijc at hellion.org.uk>
> >>>>
> >>>> In this case the secure code lives in RAM, and hence needs to be reserved, but
> >>>> it has been relocated, so the reservation of __secure_start does not apply.
> >>>>
> >>>> Add support for setting CONFIG_ARMV7_SECURE_RESERVE_SIZE to reserve such a
> >>>> region.
> >>>>
> >>>> This will be used in a subsequent patch for Jetson-TK1
> >>>
> >>> Using a memreserve and allowing the OS to map the memory but not poke it
> >>> can be problematic due to the potential of mismatched attributes between
> >>> the monitor and the OS.
> >>
> >> OK, here my knowledge is not yet sufficient to process this remark. What
> >> kind of problems can arise from what kind of attribute mismatch? And why
> >> should the OS be able to cause problems for the monitor?
> > 
> > For example, consider the case of the region being mapped cacheable by
> > the OS but not by the monitor. The monitor communicates between cores
> > expecting to never hit in a cache (because it uses a non-cacheable
> > mapping), but the mapping used by the OS can cause the region to be
> > allocated into caches at any point in time even if it never accesses the
> > region explicitly.
> > 
> > The CPU _may_ hit in a cache even if making a non-cacheable access (this
> > is called an "unexepcted data cache hit"), so the cache allocations
> > caused by the OS can mask data other CPUs wrote straight to memory.
> > 
> > Other than that case, I believe the rules given in the ARM ARM for
> > mismatched memory attributes may apply for similar reasons.  Thus
> > allowing the OS to map this memory can cause a loss of coherency on the
> > monitor side, if the OS and monitor map the region with different
> > attributes.
> > 
> > This is all IMPLEMENTATION DEFINED, so it may be that you're fine on the
> > system you're dealing with. I don't immediately know whether that is the
> > case, however. Never telling the OS about the memory in the first place
> > avoids the possibility in all cases.
> 
> But from a security point of view, it must not matter if the OS maps the
> memory or not - the monitor must be robust against that, no? If the
> architecture cannot provide such guarantees, it has to be worked around
> in software in the monitor (I hope you can do so...).

Well, yes and no.

In this case it sounds like due to the security controller you should
never encounter the mismatched attributes issue in the first place,
though you may encounter issues w.r.t. speculative accesses triggering
violations arbitrarily. Not telling the OS about the secure memory means
that said violations shouldn't occur in normal operation; only when the
non-secure OS is trying to do something bad.

If the OS has access to the memory, then you're already trusting it to
not write to there or you can't trust that memory at all (and hence
cannot use it). Given that means you must already assume that the OS is
cooperative, it's simpler to not tell it about the memory than to add
cache maintenance around every memory access within the monitor. You can
never make things secure in this case, but you can at least offer the
abstraction provided by PSCI.

So as far as I can see in either case it's better to not tell the OS
about the memory you wish to use from the monitor. If you have no HW
protection and can't trust the OS then you've already lost, and if you
do have HW protection you don't want it to trigger
continuously/spuriously as a result of speculation.

Thanks,
Mark.