Re: [PATCH] vm_event: Implement ARM SMC events

[Tamas K Lengyel <tamas.k.lengyel@gmail.com>]

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On Apr 12, 2016 01:51, "Corneliu ZUZU" <czuzu@bitdefender.com> wrote:
>
> On 4/11/2016 10:47 PM, Tamas K Lengyel wrote:
>>
>> From: Tamas K Lengyel <tklengyel@sec.in.tum.de>
>>
>> The ARM SMC instructions are already configured to trap to Xen by
default. In
>> this patch we allow a user-space process in a privileged domain to
receive
>> notification of when such event happens through the vm_event subsystem.
>>
>> This patch will likely needs to be broken up into several smaller
patches.
>> Right now what this patch adds (and could be broken into smaller patches
>> accordingly):
>>      - Implement monitor_op domctl handler for SOFTWARE_BREAKPOINTs on
ARM
>>      - Implement vm_event register fill/set routines for ARM. This
required
>>          removing the function from common as the function prototype now
>>          differs on the two archs.
>>      - Sending notification as SOFTWARE_BREAKPOINT vm_event from the SMC
trap
>>          handlers.
>>      - Extend the xen-access test tool to receive SMC notification and
step
>>          the PC manually in the reply.
>>
>> I'm sending it as an RFC to gather feedback on what has been overlooked
in this
>> revision. This patch has been tested on a Cubietruck board and works
fine,
>> but would probably not work on 64-bit boards.
>
>
> Hi Tamas,
>
> If I may, I'm still unable to work at the moment, being ill, but I'm
checking the xen-devel lists from time to time.
> Your patch caught my attention, reminding me of the conversation we had
some time ago on this matter.
> The only real reason I don't see SMC (secure-monitor-call) as being an
ideal candidate for this is that, according to the ARM manuals, SMC should
directly cause undefined exception if executed from user-mode (EL0),
instead of a hypervisor trap - isn't that the case on the machine you
tested this on or is this really only for the EL1 of domains?

That's correct, it can only be issued by the kernel. So as long as you want
to monitor the kernel it can be used just fine. I can also envision
trampoline-like traps (syscalls injected into EL0 to trigger SMC) but
that's beyond the scope I intend this for now.

>
> Also:
> - SMC, by definition, is a call to the secure side, it doesn't relate to
debugging directly (it's a syscall to the 'secure' side). There is a viable
INT3 equivalent on ARM, that being the BKPT/BRK instruction, using that
instead would require a bit more effort (but would, conceptually, be more
correct) and might be less performant, I suppose that's why you didn't go
for that?

I would have to double check but AFAIK those instructions can't be
configured to trap to the hypervisor directly. So while SMC was not
intended to be a breakpoint, conceptually it's the closest thing we have an
on ARM to the INT3 instruction when configured to trap to the VMM.

> - SMC can be disabled by the secure side (over which Xen doesn't have
control) - not really a problem on though, since the hypervisor trap
happens before that check
> But these 2 are conceptual problems, they don't impede usage of SMC as
you intend in practice.

Sure, the TrustZone is more privileged then the hypervisor so you need to
take that into account as well when you consider your threat model. If the
TZ is malicious though IMHO there isn't much you can do on the hypervisor
side anyway. So in the usecase I have for this I control the TZ as well.

Tamas

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