Re: [PATCH] KVM: x86: fix emulation of "movzbl %bpl, %eax"

[Gleb Natapov <gleb@redhat.com>]

On Thu, Oct 31, 2013 at 11:05:24PM +0100, Paolo Bonzini wrote:
> When I was looking at RHEL5.9's failure to start with
> unrestricted_guest=0/emulate_invalid_guest_state=1, I got it working with a
> slightly older tree than kvm.git.  I now debugged the remaining failure,
> which was introduced by commit 660696d1 (KVM: X86 emulator: fix
> source operand decoding for 8bit mov[zs]x instructions, 2013-04-24)
> introduced a similar mis-emulation to the one in commit 8acb4207 (KVM:
> fix sil/dil/bpl/spl in the mod/rm fields, 2013-05-30).  The incorrect
> decoding occurs in 8-bit movzx/movsx instructions whose 8-bit operand
> is sil/dil/bpl/spl.
> 
> Needless to say, "movzbl %bpl, %eax" does occur in RHEL5.9's decompression
> prolog, just a handful of instructions before finally giving control to
> the decompressed vmlinux and getting out of the invalid guest state.
> 
> Because OpMem8 bypasses decode_modrm, the same handling of the REX prefix
> must be applied to OpMem8.
> 
> Reported-by: Michele Baldessari <michele@redhat.com>
> Cc: stable@vger.kernel.org
> Cc: Gleb Natapov <gleb@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>

> ---
>  arch/x86/kvm/emulate.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 16c037e7db7d..282d28cb9931 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4117,7 +4117,10 @@ static int decode_operand(struct x86_emulate_ctxt *ctxt, struct operand *op,
>  	case OpMem8:
>  		ctxt->memop.bytes = 1;
>  		if (ctxt->memop.type == OP_REG) {
> -			ctxt->memop.addr.reg = decode_register(ctxt, ctxt->modrm_rm, 1);
> +			int highbyte_regs = ctxt->rex_prefix == 0;
> +
> +			ctxt->memop.addr.reg = decode_register(ctxt, ctxt->modrm_rm,
> +					       highbyte_regs);
>  			fetch_register_operand(&ctxt->memop);
>  		}
>  		goto mem_common;
> -- 
> 1.8.3.1

--
			Gleb.