Re: [PATCH 2/6] KVM: x86: Wrong emulation on 'xadd X, X'

Bandan Das
2014-06-16T17:38:51Z

Nadav Amit <namit@cs.technion.ac.il> writes:

> The emulator does not emulate the xadd instruction correctly if the two
> operands are the same. In this (unlikely) situation the result should be the
> sum of X and X (2X) when it is currently X. The solution is to first perform
> writeback to the source, before writing to the destination. The only
> instruction which should be affected is xadd, as the other instructions that
> perform writeback to the source use the extended accumlator (e.g., RAX:RDX).
>
> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
> ---
> arch/x86/kvm/emulate.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index f0b0a10..3c8d867 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4711,17 +4711,17 @@ special_insn:
> goto done;
>
> writeback:
> - if (!(ctxt->d & NoWrite)) {
> - rc = writeback(ctxt, &ctxt->dst);
> - if (rc != X86EMUL_CONTINUE)
> - goto done;
> - }
> if (ctxt->d & SrcWrite) {
> BUG_ON(ctxt->src.type == OP_MEM || ctxt->src.type == OP_MEM_STR);
While we are here, I think we should replace this BUG_ON with a warning
and return X86EMUL_UNHANDLEABLE if the condition is true.

> rc = writeback(ctxt, &ctxt->src);
> if (rc != X86EMUL_CONTINUE)
> goto done;
> }
> + if (!(ctxt->d & NoWrite)) {
> + rc = writeback(ctxt, &ctxt->dst);
> + if (rc != X86EMUL_CONTINUE)
> + goto done;
> + }
>
> /*
> * restore dst type in case the decoding will be reused