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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Gleb Natapov <gleb@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Reviewed-by: Gleb Natapov <gleb@xxxxxxxxxx>
> ---
> 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.
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