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

On 4/12/2016 8:24 PM, Tamas K Lengyel wrote:

On Tue, Apr 12, 2016 at 11:05 AM, Corneliu ZUZU <czuzu@xxxxxxxxxxxxxxx> wrote:
On 4/12/2016 7:24 PM, Julien Grall wrote:

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.

Then indeed SMC is the -easiest- way to go, @ least until user-mode breakpoints are implemented.

 > 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.

Isn't that also the case for X86's INT3? I guess differentiation is done based on the bkpt address/privilege level? On ARM that could also (partially) be done by looking @ the immediate value of the BKPT/BRK instruction. Another thing I realized might be troublesome with NOT using BKPT/BRK would be that on ARMv7 BKPT is always unconditional, even in IT blocks. IDK if that applies to SMC, but if it doesn't you'd have to check for that as well to make sure the breakpoint is actually executed.

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.

Please see AArch32 HDCR.TDE and AArch64 MDCR_EL2.TDE bits. Since activating this bit would imply additional (in this context -unwanted-) traps, the performance hit of having this bit set might be significant.

Right, actually I believe KVM already implemented this, I was just under the impression it was only for aarch64. As for performance overhead I'm not that worried about it, rather I need to make sure the presence of the monitoring can remain stealthy. I know on KVM for example this type of trapping renders the guest to be unable to use singlestepping, which would easily reveal the presence of the external monitor (see https://lists.cs.columbia.edu/pipermail/kvmarm/2015-May/014589.html). So this will need to be looked at carefully.

That seems to apply to single-stepping only, which would be a different matter. As for stealthiness or not limiting the guest, IMO that shouldn't be a problem with BKPT/BRK, since I believe you can inject the breakpoint exception into the guest as if no hypervisor trap occured in between (of course, once you decide whether that breakpoint is Xen's or guest-internal). But what about X86? How is stealthiness achieved there? Is INT3 entirely not available for the guest anymore when guest-debugging is enabled or are ALL INT3's reported by Xen as software breakpoint vm-events?


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 don't see a problem with that, as long as it's configurable whether SMC calls are trapped or pass-through.

Pass-through meaning "not trapped at all"? If yes, the problem would be that you won't be able to set breakpoints when SMC is configured to "completely" pass-through. But there's also the option of emulating the SMC, instead of not trapping it at all, when pass-through is needed, although IDK how complex that emulation would be.


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.

Another issue came to my mind: "HVC #imm", if handled through the hvm-ops code, currently requires setting other registers to predefined values before the HVC is actually issued. That would imply additional effort to save/restore those registers if an external privileged domain would want to set guest breakpoints. Given that, if we were to use HVC for sw-bkpts, IMO it would be nice if the hvm-ops code architecture would be slightly changed such that -lone- "HVM #imm" calls would be achievable for some use cases, such as this.

IMHO that would also be better conceptually, although it would still suffer from the limitation of not being available from user-space (and potentially from the above IT block issue).

Sure, HVC would also be a possibility but I do see use-cases for trapping SMC calls and forwarding them to a guest instead of the TZ. For example, if malware tries to exploit TZ vulnerabilities, it would be a lot easier to contain and monitor such exploits if the TZ is virtualized rather then just crashing the guest or forwarding the calls to a real TZ. So trapping SMC would allow us to use the real TZ for other purposes and maintain a barrier between malicious guests and the TZ.

Then you'd have to differentiate between a genuine guest SMC and a software-breakpoint SMC. IDK how much of a problem that would be.

So what I will do instead of issuing a software_breakpoint vm_event for SMCs, I'll introduce a new type, say VM_EVENT_REASON_PRIVILEGED_CALL, that can be used to forward both hypercalls and SMCs to a monitoring guest. This would also allow us to use the software_breakpoint type for the actual software breakpoint events in the future.


Isn't the HVC-part already achieved by guest-request vm-events? Maybe tying this vm-event specifically to SMC (in which case the name could be something like VM_EVENT_REASON_SECURE_CALL) and thus making it ARM-specific would avoid that redundancy?

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