[PATCH] Reload CS when startup_64 is used.

In long mode the %cs is largely a relic.  However there are a few cases
like iret where it matters that we have a valid value.  Without this
patch it is possible to enter the kernel in startup_64 without setting
%cs to a valid value.  With this patch we don't care what %cs value
we enter the kernel with, so long as the cs shadow register indicates
it is a privileged code segment.

Thanks to Magnus Damm for finding this problem and posting the
first workable patch.  I have moved the jump to set %cs down a
few instructions so we don't need to take an extra jump.  Which
keeps the code simpler.

Signed-of-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andi Kleen <ak@suse.de>

arch/x86_64/kernel/head.S

@@ -185,12 +185,15 @@ startup_64:
 
 	/* Finally jump to run C code and to be on real kernel address
 	 * Since we are running on identity-mapped space we have to jump
-	 * to the full 64bit address , this is only possible as indirect
-	 * jump
+	 * to the full 64bit address, this is only possible as indirect
+	 * jump.  In addition we need to ensure %cs is set so we make this
+	 * a far return.
 	 */
 	movq	initial_code(%rip),%rax
-	pushq	$0		# fake return address
-	jmp	*%rax
+	pushq	$0		# fake return address to stop unwinder
+	pushq	$__KERNEL_CS	# set correct cs
+	pushq	%rax		# target address in negative space
+	lretq
 
 	/* SMP bootup changes these two */
 	.align	8