Re: [patch 3/8] Allow a kernel to not be in ring 0.

From: Chuck Ebbert
Date: Fri Aug 04 2006 - 20:46:16 EST

In-Reply-To: <20060803002518.190834642@xxxxxxxxxxxxx>

On Wed, 02 Aug 2006 17:25:13 -0700, Jeremy Fitzhardinge wrote:

> 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).

I made some changes:

a. Added some comments about the SEGMENT_IS_*_CODE() macros.
b. Added a USER_RPL macro. (You were comparing a value to a mask
in some places and to the magic number 3 in other places.)
c. Changed the entry.S tests for LDT stack segment to use the macros.



From: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>

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.
3) Use USER_RPL macro instead of hardcoded 3

Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Signed-off-by: Zachary Amsden <zach@xxxxxxxxxx>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xxxxxxxxxxxxx>
Signed-off-by: Chuck Ebbert <76306.1226@xxxxxxxxxxxxxx>

---

arch/i386/kernel/entry.S | 9 +++++----
arch/i386/kernel/process.c | 2 +-
arch/i386/mm/extable.c | 2 +-
arch/i386/mm/fault.c | 11 ++++-------
include/asm-i386/ptrace.h | 5 +++--
include/asm-i386/segment.h | 12 ++++++++++++
6 files changed, 26 insertions(+), 15 deletions(-)

--- 2.6.18-rc3-32.orig/arch/i386/kernel/entry.S
+++ 2.6.18-rc3-32/arch/i386/kernel/entry.S
@@ -229,8 +229,9 @@ 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)
cli # make sure we don't miss an interrupt
# setting need_resched or sigpending
@@ -367,8 +368,8 @@ 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 | (4 << 8) | SEGMENT_RPL_MASK), %eax
+ cmpl $((4 << 8) | USER_RPL), %eax
CFI_REMEMBER_STATE
je ldt_ss # returning to user-space with LDT SS
restore_nocheck:
--- 2.6.18-rc3-32.orig/arch/i386/kernel/process.c
+++ 2.6.18-rc3-32/arch/i386/kernel/process.c
@@ -346,7 +346,7 @@ int kernel_thread(int (*fn)(void *), voi
regs.xes = __USER_DS;
regs.orig_eax = -1;
regs.eip = (unsigned long) kernel_thread_helper;
- regs.xcs = __KERNEL_CS;
+ regs.xcs = __KERNEL_CS | get_kernel_rpl();
regs.eflags = X86_EFLAGS_IF | X86_EFLAGS_SF | X86_EFLAGS_PF | 0x2;

/* Ok, create the new process.. */
--- 2.6.18-rc3-32.orig/arch/i386/mm/extable.c
+++ 2.6.18-rc3-32/arch/i386/mm/extable.c
@@ -11,7 +11,7 @@ int fixup_exception(struct pt_regs *regs
const struct exception_table_entry *fixup;

#ifdef CONFIG_PNPBIOS
- if (unlikely((regs->xcs & ~15) == (GDT_ENTRY_PNPBIOS_BASE << 3)))
+ if (unlikely(SEGMENT_IS_PNP_CODE(regs->xcs)))
{
extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
extern u32 pnp_bios_is_utter_crap;
--- 2.6.18-rc3-32.orig/arch/i386/mm/fault.c
+++ 2.6.18-rc3-32/arch/i386/mm/fault.c
@@ -27,6 +27,7 @@
#include <asm/uaccess.h>
#include <asm/desc.h>
#include <asm/kdebug.h>
+#include <asm/segment.h>

extern void die(const char *,struct pt_regs *,long);

@@ -119,10 +120,10 @@ static inline unsigned long get_segment_
}

/* The standard kernel/user address space limit. */
- *eip_limit = (seg & 3) ? USER_DS.seg : KERNEL_DS.seg;
+ *eip_limit = user_mode(regs) ? USER_DS.seg : KERNEL_DS.seg;

/* By far the most common cases. */
- if (likely(seg == __USER_CS || seg == __KERNEL_CS))
+ if (likely(SEGMENT_IS_FLAT_CODE(seg)))
return eip;

/* Check the segment exists, is within the current LDT/GDT size,
@@ -436,11 +437,7 @@ good_area:
write = 0;
switch (error_code & 3) {
default: /* 3: write, present */
-#ifdef TEST_VERIFY_AREA
- if (regs->cs == KERNEL_CS)
- printk("WP fault at %08lx\n", regs->eip);
-#endif
- /* fall through */
+ /* fall through */
case 2: /* write, not present */
if (!(vma->vm_flags & VM_WRITE))
goto bad_area;
--- 2.6.18-rc3-32.orig/include/asm-i386/ptrace.h
+++ 2.6.18-rc3-32/include/asm-i386/ptrace.h
@@ -60,6 +60,7 @@ struct pt_regs {
#ifdef __KERNEL__

#include <asm/vm86.h>
+#include <asm/segment.h>

struct task_struct;
extern void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs, int error_code);
@@ -73,11 +74,11 @@ extern void send_sigtrap(struct task_str
*/
static inline int user_mode(struct pt_regs *regs)
{
- return (regs->xcs & 3) != 0;
+ return (regs->xcs & SEGMENT_RPL_MASK) == USER_RPL;
}
static inline int user_mode_vm(struct pt_regs *regs)
{
- return ((regs->xcs & 3) | (regs->eflags & VM_MASK)) != 0;
+ return ((regs->xcs & SEGMENT_RPL_MASK) | (regs->eflags & VM_MASK)) >= USER_RPL;
}
#define instruction_pointer(regs) ((regs)->eip)
#if defined(CONFIG_SMP) && defined(CONFIG_FRAME_POINTER)
--- 2.6.18-rc3-32.orig/include/asm-i386/segment.h
+++ 2.6.18-rc3-32/include/asm-i386/segment.h
@@ -83,6 +83,11 @@

#define GDT_SIZE (GDT_ENTRIES * 8)

+/* Matches __KERNEL_CS and __USER_CS (they must be 2 entries apart) */
+#define SEGMENT_IS_FLAT_CODE(x) (((x) & 0xec) == GDT_ENTRY_KERNEL_CS * 8)
+/* Matches PNP_CS32 and PNP_CS16 (they must be consecutive) */
+#define SEGMENT_IS_PNP_CODE(x) (((x) & 0xf4) == GDT_ENTRY_PNPBIOS_BASE * 8)
+
/* Simple and small GDT entries for booting only */

#define GDT_ENTRY_BOOT_CS 2
@@ -112,4 +117,11 @@
*/
#define IDT_ENTRIES 256

+/* Bottom three bits of xcs give the ring privilege level */
+#define SEGMENT_RPL_MASK 0x3
+
+/* User mode is privilege level 3 */
+#define USER_RPL 0x3
+
+#define get_kernel_rpl() 0
#endif
--
Chuck
-
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