Re: [PATCH v4 01/15] KVM: arm: plug guest debug exploit

[Zhichao Huang <zhichao.huang@linaro.org>]

On 2015/9/2 19:38, Christoffer Dall wrote:
> 
> I really think that we should read the register, clear the bits you care
> about (MDBGen and HDBGen) and then write back the register.
> 
> So, if I recall correctly, this is to avoid having to set HDCR_TDE
> below?
> 
> Given Will's concerns about touching this register, I'm thinking if we
> shouldn't start with the HDCR_TDE enabled (and a handler in KVM) and
> then see if we want to add this optimization later?
> 
> At the very least, you should do as Will pointed out and predicate
> writes to this register based on whether the reset code in
> hw_breakpoint.c successfully reset the debug regs.  I think checking the
> debug_err_mask variable from the C code and pass this on to the Hyp code
> would be the right way to go.
> 
> But as I said, I think we should just trap debug exceptions to begin
> with (to plug the hole) and then add the more intelligent stuff later.
> 

OK, I will set HDCR_TDE, and ignore all the debug exceptions in
KVM handlers to prevent the guest to mess with the host states.

>> +	.endif
>> +
>> +	mcr	p14, 0, r2, c0, c2, 2	@ DBGDSCR
>>  .endm
>>  
>>  /*
>> @@ -620,7 +633,7 @@ ARM_BE8(rev	r6, r6  )
>>   * (hardware reset value is 0) */
>>  .macro set_hdcr operation
>>  	mrc	p15, 4, r2, c1, c1, 1
>> -	ldr	r3, =(HDCR_TPM|HDCR_TPMCR)
>> +	ldr	r3, =(HDCR_TPM|HDCR_TPMCR|HDCR_TDRA|HDCR_TDOSA|HDCR_TDA)
> 
> 
> 
>>  	.if \operation == vmentry
>>  	orr	r2, r2, r3		@ Trap some perfmon accesses
>>  	.else
>> -- 
>> 1.7.12.4
>>
> 
> Thanks,
> -Christoffer
>