[PATCH 2/2] x86/Xen/32: simplify xen_iret_crit_fixup's ring check

[Jan Beulich <jbeulich@suse.com>]

This can be had with two instead of six insns, by just checking the high
CS.RPL bit.

Also adjust the comment - there would be no #GP in the mentioned cases,
as there's no segment limit violation or alike. Instead there'd be #PF,
but that one reports the target EIP of said branch, not the address of
the branch insn itself.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
An alternative would be to keep using SEGMENT_RPL_MASK, but follow it
with "jpe".

--- a/arch/x86/xen/xen-asm_32.S
+++ b/arch/x86/xen/xen-asm_32.S
@@ -153,22 +153,15 @@ hyper_iret:
  * it's still on stack), we need to restore its value here.
  */
 ENTRY(xen_iret_crit_fixup)
-	pushl %ecx
 	/*
 	 * Paranoia: Make sure we're really coming from kernel space.
 	 * One could imagine a case where userspace jumps into the
 	 * critical range address, but just before the CPU delivers a
-	 * GP, it decides to deliver an interrupt instead.  Unlikely?
-	 * Definitely.  Easy to avoid?  Yes.  The Intel documents
-	 * explicitly say that the reported EIP for a bad jump is the
-	 * jump instruction itself, not the destination, but some
-	 * virtual environments get this wrong.
+	 * PF, it decides to deliver an interrupt instead.  Unlikely?
+	 * Definitely.  Easy to avoid?  Yes.
 	 */
-	movl 3*4(%esp), %ecx		/* nested CS */
-	andl $SEGMENT_RPL_MASK, %ecx
-	cmpl $USER_RPL, %ecx
-	popl %ecx
-	je 2f
+	testb $2, 2*4(%esp)		/* nested CS */
+	jnz 2f
 
 	/*
 	 * If eip is before iret_restore_end then stack