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

Mark Rutland
2015-02-19T13:42:22Z

On Thu, Feb 19, 2015 at 09:25:56AM +0000, Jan Kiszka wrote:
> On 2015-02-19 10:19, Ian Campbell wrote:
> > On Thu, 2015-02-19 at 09:28 +0100, Thierry Reding wrote:
> >> On Tue, Feb 17, 2015 at 11:55:24AM +0000, Mark Rutland wrote:
> >>> [...]
> >>>
> >>>>>> This is getting invasive:
> >>>>>>
> >>>>>> If I add carveouts via adjusting memory banks, I need to account for the
> >>>>>> case that an existing bank is split into two halves, creating additional
> >>>>>> banks this way. But then current fdt_fixup_memory_banks will no longer
> >>>>>> work due to its limitation to the number of physical banks. I could
> >>>>>> always add one spare bank to that service, ok, but then the next use
> >>>>>> case for carveouts will hit the wall again. So I better double that
> >>>>>> limit, or so.
> >>>>>
> >>>>> Yeah, not fun.
> >>>>>
> >>>>> If the code is position-independent then you might be able to simply
> >>>>> carve out a sufficient proportion from the start of the first entry or
> >>>>> the end of the last one, which would avoid splitting. If either of said
> >>>>> regions are too small for the monitor code then it's questionable as to
> >>>>> whether the OS can make use of it.
> >>>>
> >>>> The code /seems/ to be position-independent, but locations are so far
> >>>> hard-coded in those places that prepare it and move it around. Maybe we
> >>>> can decide about the location at runtime, maybe we can simply demand it
> >>>> to be at the end or the beginning of some bank.
> >>>
> >>> If it's possible to do so, it would seem like the nicest option to me.
> >>
> >> Using the top of memory for this seems like the most natural choice,
> >
> > I think it needs to still be below 4G, doesn't it? So on large mem/LPAE
> > systems some care might be needed.
>
> Argh. That would likely mean we had to split a bank (unless >2G comes in
> multiple banks), something I'd like to avoid having to implement.
>
> >
> > It was suggested by Mark earlier in the thread that this stuff is
> > IMPLEMENTATION DEFINED. Is it possible that we simply don't need to
> > worry about these cross-world cache issues on Tegra?
> >
> > (I must confess that until now I'd assumed that the cache lines were
> > tagged with the world which populated them to stop them interfering with
> > each other in this sort of way...)
>
> I'm pretty sure that is no such thing as a cross-world cache problem.
> Otherwise the architecture or some implementation would have serious
> security issues as discussed earlier. To my understanding, Mark's
> suggestion is now targeting the concern that Linux may accidentally
> trigger accesses and, thus, stumble or create warnings at least.

Yup.

If the memory is protected by some configurable security controller (as
seems to be the case on Tegra), the non-secure side accessing any memory
protected by it will result in a violation (and presumably bring down
the non-secure world). We need to prevent speculative accesses (the
security controller can't tell the difference), and therefore cannot map
the memory at all (so a /memreserve/ is insufficient).

Depending on implementation details there are other potential problems,
and carving out the memory explicitly solves all that I am aware of
without having to rely on implementation-specific details.

Thanks,
Mark.