[PATCH] i386: Allow a kernel not to be in ring 0

We allow for the fact that the guest kernel may not run in ring 0.  This
requires some abstraction in a few places when setting %cs or checking
privilege level (user vs kernel).

This is Chris' [RFC PATCH 15/33] move segment checks to subarch, except rather
than using #define USER_MODE_MASK which depends on a config option, we use
Zach's more flexible approach of assuming ring 3 == userspace.  I also used
"get_kernel_rpl()" over "get_kernel_cs()" because I think it reads better in
the code...

1) Remove the hardcoded 3 and introduce #define SEGMENT_RPL_MASK 3 2) Add a
get_kernel_rpl() macro, and don't assume it's zero.

And:

Clean up of patch for letting kernel run other than ring 0:

a. Add some comments about the SEGMENT_IS_*_CODE() macros.
b. Add a USER_RPL macro.  (Code was comparing a value to a mask
   in some places and to the magic number 3 in other places.)
c. Add macros for table indicator field and use them.
d. Change the entry.S tests for LDT stack segment to use the macros

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 5 insertions, 4 deletions

diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
index 3872fca5c74a..284f2e908ad0 100644
--- a/arch/i386/kernel/entry.S
+++ b/arch/i386/kernel/entry.S
@@ -240,8 +240,9 @@ ret_from_intr:
 check_userspace:
         movl EFLAGS(%esp), %eax         # mix EFLAGS and CS
         movb CS(%esp), %al
-        testl $(VM_MASK | 3), %eax
-        jz resume_kernel
+        andl $(VM_MASK | SEGMENT_RPL_MASK), %eax
+        cmpl $USER_RPL, %eax
+        jb resume_kernel                # not returning to v8086 or userspace
 ENTRY(resume_userspace)
         DISABLE_INTERRUPTS              # make sure we don't miss an interrupt
                                         # setting need_resched or sigpending
@@ -377,8 +378,8 @@ restore_all:
         # See comments in process.c:copy_thread() for details.
         movb OLDSS(%esp), %ah
         movb CS(%esp), %al
-        andl $(VM_MASK | (4 << 8) | 3), %eax
-        cmpl $((4 << 8) | 3), %eax
+        andl $(VM_MASK | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
+        cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
         CFI_REMEMBER_STATE
         je ldt_ss                       # returning to user-space with LDT SS
 restore_nocheck: