Re: [3/3] irqchip/gic-v3: Bounds check redistributor accesses

[Nishanth Menon]

Marc,

On Tue, Mar 13, 2018 at 9:21 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
> Hi Lokesh,
>
> On 13/03/18 13:38, Lokesh Vutla wrote:
>> Hi All,
>>
>> On Wednesday 11 October 2017 03:11 PM, Punit Agrawal wrote:
>>> The kernel crashes while iterating over a redistributor that is
>>> in-correctly sized by the platform firmware or doesn't contain the last
>>> record.
>>>
>>> Prevent the crash by checking accesses against the size of the region
>>> provided by the firmware. While we are at it, warn the user about
>>> incorrect region size.
>>>
>>> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
>>> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
>>
>> Sorry to bring up an old thread. Just wanted to check what is the status
>> on this series.
>
> So far, I wasn't inclined to merge it, as it only allowed you to detect
> a broken system, as opposed to help with a working one.

Is'nt that a good reason to have it? Why not help an error in dtb with
a debug helper than an obtuse crash to debug painfully?

>
>> This will also be useful when we try to boot linux + hypervisor with
>> less number of cores than the SoC supports. For example:
>> - SoC has 4 cores and Linux tries to boot with 2 cores.
>> - then a type-2 hypervisor gets installed.
>> - Hypervisor tries to boot a VM with linux on core 1.
>>
>> Now the VM boot will fail while it iterates over all the GICR regions
>> till GICR_TYPER is found. Hypervisor will trap any accesses to GICR
>> regions of any invalid cpus(cpu 2, cpu 3 in this case).
>
> It you're passing the redistributors to a guest, you're doing something
> terribly wrong. You're putting the guest in a position to do a DoS on
> the hypervisor (disabling its timer interrupt, for example). Not the
> greatest move. There is a number of other gotchas with this approach
> (virtual interrupts, distributor virtualization...).
>
>> If the $patch is not the right approach, can you suggest on how to
>> handle the above scenario?
>
> The proper way to handle this is to virtualize the distributor and
> redistributor by trap/emulate. The only thing you can safely pass to a
> guest is the CPU interface, either as system registers or in its MMIO
> form (if you have the GICv2 compatibility interface).
>

Dumb question: Would'nt a trap emulate be real expensive with hyp
context in v8 (all the context save/restore we'd have to do? (in the
context of discussion, GICV2 compat is disabled).


-- 
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Regards,
Nishanth Menon