Re: [PATCH] KVM: x86: Revert "check CS.DPL against RPL during task switch"

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/08/2014 23:09, Paolo Bonzini ha scritto:
> Il 17/08/2014 21:32, Nadav Amit ha scritto:
>> This reverts commit 5045b468037dfe1c848827ce10e99d87f5669160.  Although the
>> cs.dpl=cs.rpl check is mentioned in table 7-1 of the SDM as causing a #TSS
>> exception, it is not mentioned in table 6-6 that lists "invalid TSS conditions"
>> which cause #TSS exceptions. As it causes some tests, which pass on bare-metal,
>> to fail - it should be reverted.
> 
> Right.  However, I think reverting the patch is too big a hammer.  We 
> still need in_task_switch to raise TS_VECTOR instead of GP_VECTOR, so I 
> propose instead something like:
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 56657b0bb3bb..cd230b035514 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -1468,7 +1468,7 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
>  		return ret;
>  
>  	err_code = selector & 0xfffc;
> -	err_vec = GP_VECTOR;
> +	err_vec = in_task_switch ? TS_VECTOR : GP_VECTOR;
>  
>  	/* can't load system descriptor into segment selector */
>  	if (seg <= VCPU_SREG_GS && !seg_desc.s)
> @@ -1491,9 +1491,6 @@ static int __load_segment_descriptor(struct x86_emulate_ctxt *ctxt,
>  			goto exception;
>  		break;
>  	case VCPU_SREG_CS:
> -		if (in_task_switch && rpl != dpl)
> -			goto exception;
> -
>  		if (!(seg_desc.type & 8))
>  			goto exception;
>  

Also, what about the rpl > cpl test below, for non-conforming code
segments?  It is not mentioned in table 6-6 either.

Paolo