[tip: x86/urgent] x86/xen/32: Simplify ring check in xen_iret_crit_fixup()

[tip-bot2 for Jan Beulich]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     922eea2ce5c799228d9ff1be9890e6873ce8fff6
Gitweb:        https://git.kernel.org/tip/922eea2ce5c799228d9ff1be9890e6873ce8fff6
Author:        Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate:    Mon, 11 Nov 2019 15:32:59 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 19 Nov 2019 21:58:28 +01:00

x86/xen/32: Simplify ring check in xen_iret_crit_fixup()

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@xxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Link: https://lkml.kernel.org/r/a5986837-01eb-7bf8-bf42-4d3084d6a1f5@xxxxxxxx

---
 arch/x86/xen/xen-asm_32.S | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/arch/x86/xen/xen-asm_32.S b/arch/x86/xen/xen-asm_32.S
index 392e033..cd17777 100644
--- 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