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Re: [Xen-devel] [PATCH] vm_event: Implement ARM SMC events


On 12/04/2016 16:01, Tamas K Lengyel wrote:

On Apr 12, 2016 01:51, "Corneliu ZUZU" <czuzu@xxxxxxxxxxxxxxx
<mailto:czuzu@xxxxxxxxxxxxxxx>> wrote:
 > On 4/11/2016 10:47 PM, Tamas K Lengyel wrote:
 >> From: Tamas K Lengyel <tklengyel@xxxxxxxxxxxxx
 >> 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
 >> notification of when such event happens through the vm_event subsystem.
 >> This patch will likely needs to be broken up into several smaller
 >> 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
 >>          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
 >> 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?

This paragraph is part of Corneliu's answer but it gives the impression you wrote it. Can you configure your e-mail client to quote properly?

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?

BKPT/BRK could be used by the guest for debugging. You would need to differentiate a breakpoint inserted by Xen or by a debugger in the guest.

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.

Whilst any access to SMC currently results to inject an undefined exception, it may not be the case in the future. There have been discussion to allow guest issuing SMC call (see [1]).

I think the safest instruction would be HVC #imm. Xen is only using a small number of immediate. You could allocate a specific value for software debugging.

 > - 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.


[1] http://lists.xen.org/archives/html/xen-devel/2015-07/txtwZfvJnXlYG.txt

Julien Grall

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