Commit ee212297 authored Jun 15, 2014 by Nadav Amit Committed by Paolo Bonzini Jun 19, 2014

KVM: x86: Wrong emulation on 'xadd X, X'

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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 7dec5603

arch/x86/kvm/emulate.c

+5 −5

Original line number	Diff line number	Diff line
		@@ -4736,17 +4736,17 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
		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);
		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