[PATCH 3.16 05/12] KVM: x86: Introduce segmented_write_std

[Ben Hutchings]

3.16.41-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Steve Rutherford <srutherford@xxxxxxxxxx>

commit 129a72a0d3c8e139a04512325384fe5ac119e74d upstream.

Introduces segemented_write_std.

Switches from emulated reads/writes to standard read/writes in fxsave,
fxrstor, sgdt, and sidt.  This fixes CVE-2017-2584, a longstanding
kernel memory leak.

Since commit 283c95d0e389 ("KVM: x86: emulate FXSAVE and FXRSTOR",
2016-11-09), which is luckily not yet in any final release, this would
also be an exploitable kernel memory *write*!

Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Fixes: 96051572c819194c37a8367624b285be10297eca
Fixes: 283c95d0e3891b64087706b344a4b545d04a6e62
Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Steve Rutherford <srutherford@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[bwh: Backported to 3.16: drop changes to em_fxsave(), em_fxrstor()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -744,6 +744,20 @@ static int segmented_read_std(struct x86
 	return ctxt->ops->read_std(ctxt, linear, data, size, &ctxt->exception);
 }
 
+static int segmented_write_std(struct x86_emulate_ctxt *ctxt,
+			       struct segmented_address addr,
+			       void *data,
+			       unsigned int size)
+{
+	int rc;
+	ulong linear;
+
+	rc = linearize(ctxt, addr, size, true, &linear);
+	if (rc != X86EMUL_CONTINUE)
+		return rc;
+	return ctxt->ops->write_std(ctxt, linear, data, size, &ctxt->exception);
+}
+
 /*
  * Fetch the next byte of the instruction being emulated which is pointed to
  * by ctxt->_eip, then increment ctxt->_eip.
@@ -3268,8 +3282,8 @@ static int emulate_store_desc_ptr(struct
 	}
 	/* Disable writeback. */
 	ctxt->dst.type = OP_NONE;
-	return segmented_write(ctxt, ctxt->dst.addr.mem,
-			       &desc_ptr, 2 + ctxt->op_bytes);
+	return segmented_write_std(ctxt, ctxt->dst.addr.mem,
+				   &desc_ptr, 2 + ctxt->op_bytes);
 }
 
 static int em_sgdt(struct x86_emulate_ctxt *ctxt)