Query regarding extracting instruction which caused a data-abort exception

Wed Jun 14 14:40:49 CEST 2017

Thanks Stefan!

We are actually trying to do device emulation in the secure world genode,
for peripherals marked as secure (UART or ESDHC) in csu_config.h. We got
encouraged by the following discussion at
http://genode.org/documentation/articles/trustzone

==========================================================================
The basic idea of emulating device access is to let the hypervisor pass
control to the VMM as soon as the non-secure OS accesses an address
outside the permitted physical address ranges. The VMM can then inspect
the address in question and the program counter of the non-secure OS that
raised the access violation. Given the program counter value, the VMM can
fetch and decode the faulting instruction and emulate it in software.
Because ARM is a RISC architecture, the instruction decoding is rather
simple. The instruction in question can only be a load or a store
instruction. No other instruction would raise an access fault. For read
operations, the VMM would provide the result of the operation by changing
the corresponding entry of the VM state structure.

That said, we found that the trap-and-execute emulation model is not
possible to implement with the TrustZone protection mechanisms in general.
Dependent on the concrete platform, the CPU will not immediately enter the
hypervisor when the fault occurs but attempts to perform the bus
transaction. This transaction will trigger an external data abort. This
abort is similar to a device interrupt. It principally raises an exception
(so the violation can be detected) but not always immediately. Therefore,
there is no way to uniquely reconstruct what happened in between the
invalid access and the reception of the external abort exception in the
hypervisor. Neither can the hypervisor recover the non-secure world to a
useful state.

A noteworthy advantage of the CSU compared to the ARM TZ protection
controller within the ARM Cortex A9 reference board is the way of how
access violations are handled. As stated in Device emulation, the ARM TZ
protection controller responds to invalid accesses with an asynchronous
external abort exception, similar to a device interrupt. Upon the
execution of an offending instruction, the TZ protection controller
detects the violation, yet the CPU would continue the execution of further
instructions until the flagged violation eventually reaches the CPU,
triggering an external abort exception. This scheme effectively rules out
any attempt to emulate device accesses. In contrast, the i.MX CSU responds
to access violations by synchronously yielding control to the exception
handler. So when such an exception occurs, the offending instruction can
be determined and emulated in software. However, even though device
emulation using the CSU is principally possible, we haven't investigated
this opportunity further.
===========================================================================

We want to read the instruction faulting in NW linux in tz_vmm,
disassemble it, emulate it in genode code and restart the VM at the next
instruction of the normal world. Do you think this is feasible, or your
comments about "synchronous data abort in IMX53 vs. asynchronous aborts in
Versatile Express" don't hold always?

Thanks!
Riju

> Hello,
>
> On 06/13/2017 11:17 AM, Abhishek Kumar wrote:
>> Hello
>> I am trying to modify genode trustzone. I want to read the instruction
>> which lead to data abort exception in normal world, in the `dump`
>> function in tz_vmm. I have value of all the registers through `_state`
>> register. We tried with `_state->ip`. On converting 16 bits stored at
>> the address pointed by _state->ip, we got ARM Thumb instruction:
>>
>>     STRH    R0, [R0, #6]
>>
>>
>>
>> But the value (R0) + 6, doesn't match dfar. We're not sure if _state->ip
>> is the register to go with. We tried with _state->mode[2].lr which is
>> lr_abt register. But the address stored in lr_abt, lr_abt-16, lr_abt-32
>> all have 0s.
>>
>> Which is right register to get the address of the instruction which
>> caused the data-abort exception?
>
> As long as you get an synchronous data-abort from the normal world,
> reading the current instruction pointer of the 'state' structure is
> perfectly fine. The mode-specific lr register is useful for the handling
> of MMU faults within the "normal" world itself. They are not modified,
> as long as the "normal" world MMU can resolve an access, but some bus
> resp. CSU is answering that the access is not allowed. This will not
> change the "normal" world register set.
>
> On the other hand, in general a bus fault triggered by unallowed access
> of the "normal" world does not necessarily mean a synchronous
> data-abort, although on i.MX53 I only observed those. In general, it can
> also provoke an asynchronous external data-abort, which means that the
> instruction pointer is not necessarily pointing to the instruction that
> triggered the fault.
>
> Moreover, looking at the "normal" world's memory from the secure side is
> troublesome. Because the normal and secure world's memory view is not
> cache-coherent. Cache entries are always tagged by the NS bit. That
> means you have to take care to flush caches yourself. If you want to
> debug instructions, you should instead look at the Linux binary itself
> and not into the memory on the secure side. To me it looks strange that
> you identify a Thumb instruction in the kernel here.
>
> Btw. these kind of TrustZone/i.MX53 questions were asked repeatedly in
> the past, and are mostly answered in our TrustZone report:
>
>   https://genode.org/documentation/articles/trustzone
>
> and in the discussions of our mailing list:
>
>   https://sourceforge.net/p/genode/mailman/search/?q=trustzone
>
> Regards
> Stefan
>
>>
>> Thanks
>> Abhishek
>>
>>
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>
> --
> Stefan Kalkowski
> Genode Labs
>
> https://github.com/skalk · http://genode.org/
>
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