Re: [PATCH 2/5] booke: exit to guest userspace for unimplemented hcalls in kvm

[Scott Wood <scottwood@freescale.com>]

On 07/15/2013 06:11:16 AM, Bharat Bhushan wrote:
> Exit to guest user space if kvm does not implement the hcall.
> 
> Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
> ---
>  arch/powerpc/kvm/booke.c   |   47  
> +++++++++++++++++++++++++++++++++++++------
>  arch/powerpc/kvm/powerpc.c |    1 +
>  include/uapi/linux/kvm.h   |    1 +
>  3 files changed, 42 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/booke.c b/arch/powerpc/kvm/booke.c
> index 17722d8..c8b41b4 100644
> --- a/arch/powerpc/kvm/booke.c
> +++ b/arch/powerpc/kvm/booke.c
> @@ -1005,9 +1005,25 @@ int kvmppc_handle_exit(struct kvm_run *run,  
> struct kvm_vcpu *vcpu,
>  		break;
> 
>  #ifdef CONFIG_KVM_BOOKE_HV
> -	case BOOKE_INTERRUPT_HV_SYSCALL:
> +	case BOOKE_INTERRUPT_HV_SYSCALL: {
> +		int i;
>  		if (!(vcpu->arch.shared->msr & MSR_PR)) {
> -			kvmppc_set_gpr(vcpu, 3, kvmppc_kvm_pv(vcpu));
> +			r = kvmppc_kvm_pv(vcpu);
> +			if (r != EV_UNIMPLEMENTED) {
> +				/* except unimplemented return to guest  
> */
> +				kvmppc_set_gpr(vcpu, 3, r);
> +				kvmppc_account_exit(vcpu,  
> SYSCALL_EXITS);
> +				r = RESUME_GUEST;
> +				break;
> +			}
> +			/* Exit to userspace for unimplemented hcalls  
> in kvm */
> +			run->epapr_hcall.nr = kvmppc_get_gpr(vcpu, 11);
> +			run->epapr_hcall.ret = 0;
> +			for (i = 0; i < 8; i++)
> +				run->epapr_hcall.args[i] =  
> kvmppc_get_gpr(vcpu, 3 + i);

You need to clear the upper half of each register if CONFIG_PPC64=y and  
MSR_CM is not set.

> +			vcpu->arch.hcall_needed = 1;

The existing code for hcall_needed restores 9 return arguments, rather  
than the 8 that are defined for this interface.  Thus, you'll be  
restoring one word of padding into the guest -- which could be  
arbitrary userspace data that shouldn't be leaked.  r12 is volatile in  
the ePAPR hcall ABI so simply clobbering it isn't a problem, though.

-Scott