Re: [PATCH 2/3] KVM: x86: set ctxt->have_exception in x86_decode_insn()

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Aug 27, 2019 at 01:07:08PM +0000, Jan Dakinevich wrote:
> x86_emulate_instruction() takes into account ctxt->have_exception flag
> during instruction decoding, but in practice this flag is never set in
> x86_decode_insn().
> 
> Fixes: 6ea6e84 ("KVM: x86: inject exceptions produced by x86_decode_insn")
> Cc: Denis Lunev <den@virtuozzo.com>
> Cc: Roman Kagan <rkagan@virtuozzo.com>
> Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
> Signed-off-by: Jan Dakinevich <jan.dakinevich@virtuozzo.com>
> ---
>  arch/x86/kvm/emulate.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 6170ddf..f93880f 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -5395,6 +5395,8 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
>  					ctxt->memopp->addr.mem.ea + ctxt->_eip);
>  
>  done:
> +	if (rc == X86EMUL_PROPAGATE_FAULT)
> +		ctxt->have_exception = true;

We should add a sanity check or two on the vector since the emulator code
goes all over the place, e.g. #UD should not be injected/propagated, and
trap-like exceptions should not be handled/encountered during decode.
Note, exception_type() also warns on illegal vectors.

  WARN_ON_ONCE(ctxt->exception.vector == UD_VECTOR ||
	       exception_type(ctxt->exception.vector) == EXCPT_TRAP);

>  	return (rc != X86EMUL_CONTINUE) ? EMULATION_FAILED : EMULATION_OK;
>  }
>  
> -- 
> 2.1.4
>