[PATCH] KVM: emulator: Handle wraparound in (cs_base + offset) when fetching.

[Nelson Elhage]

Currently, setting a large (i.e. negative) base address for %cs does not work on
a 64-bit host. The "JOS" teaching operating system, used by MIT and other
universities, relies on such segments while bootstrapping its way to full
virtual memory management.

Signed-off-by: Nelson Elhage <nelhage@xxxxxxxxxxx>
---
 arch/x86/kvm/emulate.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 0ad47b8..54e84b2 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -505,9 +505,12 @@ static int do_fetch_insn_byte(struct x86_emulate_ctxt *ctxt,
 	int size, cur_size;
 
 	if (eip == fc->end) {
+		unsigned long linear = eip + ctxt->cs_base;
+		if (ctxt->mode != X86EMUL_MODE_PROT64)
+			linear &= (u32)-1;
 		cur_size = fc->end - fc->start;
 		size = min(15UL - cur_size, PAGE_SIZE - offset_in_page(eip));
-		rc = ops->fetch(ctxt->cs_base + eip, fc->data + cur_size,
+		rc = ops->fetch(linear, fc->data + cur_size,
 				size, ctxt->vcpu, &ctxt->exception);
 		if (rc != X86EMUL_CONTINUE)
 			return rc;
-- 
1.7.2.43.g36c08.dirty

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