Re: [PATCH] arm64: KVM: Reject non-compliant HVC calls from guest kernel

[Marc Zyngier]

On 08/08/17 08:36, Christoffer Dall wrote:
> Hi Shanker,
> 
> On Mon, Aug 07, 2017 at 02:03:28PM -0500, Shanker Donthineni wrote:
>> The SMC/HVC instructions with an immediate value non-zero are not compliant
>> according to 'SMC calling convention system software document'. Add a
>> validation check in handle_hvc() to avoid malicious HVC calls from VM, and
> 
> 
> Why do the HVC calls become malicious if they have non-zero immediate
> values --- can it actually break something today?

More importantly, the *architecture* allows non-zero values. The SMCCC
is a *convention*, and not part of the architecture (which is what KVM
is concerned about).

> 
> 
>> inject an undefined instruction for those calls.
>>
>> http://infocenter.arm.com/help/topic/com.arm.doc.den0028b/ARM_DEN0028B_SMC_Calling_Convention.pdf
>>
>> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
>> ---
>>  arch/arm64/include/asm/esr.h |  4 ++++
>>  arch/arm64/kvm/handle_exit.c | 12 +++++++-----
>>  2 files changed, 11 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
>> index 8cabd57..fa988e5 100644
>> --- a/arch/arm64/include/asm/esr.h
>> +++ b/arch/arm64/include/asm/esr.h
>> @@ -107,6 +107,9 @@
>>  #define ESR_ELx_AR 		(UL(1) << 14)
>>  #define ESR_ELx_CM 		(UL(1) << 8)
>>  
>> +/* ISS field definitions for HVC/SVC instruction execution traps */
>> +#define ESR_HVC_IMMEDIATE(esr)	((esr) & 0xFFFF)
>> +
>>  /* ISS field definitions for exceptions taken in to Hyp */
>>  #define ESR_ELx_CV		(UL(1) << 24)
>>  #define ESR_ELx_COND_SHIFT	(20)
>> @@ -114,6 +117,7 @@
>>  #define ESR_ELx_WFx_ISS_WFE	(UL(1) << 0)
>>  #define ESR_ELx_xVC_IMM_MASK	((1UL << 16) - 1)
>>  
>> +
>>  /* ESR value templates for specific events */
>>  
>>  /* BRK instruction trap from AArch64 state */
>> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
>> index 17d8a16..a900dcd 100644
>> --- a/arch/arm64/kvm/handle_exit.c
>> +++ b/arch/arm64/kvm/handle_exit.c
>> @@ -42,13 +42,15 @@ static int handle_hvc(struct kvm_vcpu *vcpu, struct kvm_run *run)
>>  			    kvm_vcpu_hvc_get_imm(vcpu));
>>  	vcpu->stat.hvc_exit_stat++;
>>  
>> -	ret = kvm_psci_call(vcpu);
>> -	if (ret < 0) {
>> -		kvm_inject_undefined(vcpu);
>> -		return 1;
>> +	/* HVC immediate value must be zero for all compliant calls */
>> +	if (!ESR_HVC_IMMEDIATE(kvm_vcpu_get_hsr(vcpu))) {
>> +		ret = kvm_psci_call(vcpu);
>> +		if (ret >= 0)
>> +			return ret;
> 
> Out of curiosity, have you seen guests or any bad behavior with a
> non-zero PSCI value, or are we just making sure we only support callers
> that follow the SMC calling convention?
> 
> I hope we don't break any existing guests out there with this change,
> including UEFI, old versions of Linux, etc.
> 
> (I do have this feeling that this check should be inside kvm_psci_call
> instead, but it really doesn't matter at this point and we can always
> move things around later if we start using other types of hypercalls
> for anything.)

Exactly. PSCI doesn't rely on SMCCC (the initial version pre-dates it),
though it mandates the HVC immediate to be zero. So let's check it
there, and not as a blanket statement forbidding a reasonable use of the
architecture.

Eventually, I'd like to be able to forward such HVC/SMC calls to
userspace (and ultimately deprecate PSCI in the kernel), so that it can
apply whatever policy it decides.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...