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

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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>
This commit is contained in:
Rusty Russell 2006-09-26 10:52:39 +02:00 committed by Andi Kleen
parent 0da5db3133
commit 78be3706b2
6 changed files with 31 additions and 15 deletions
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9
arch/i386/kernel/entry.S
View File

@ -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:

2
arch/i386/kernel/process.c
View File

@ -338,7 +338,7 @@ int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
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
arch/i386/mm/extable.c
View File

@ -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;

11
arch/i386/mm/fault.c
View File

@ -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);
@ -113,10 +114,10 @@ static inline unsigned long get_segment_eip(struct pt_regs *regs,
}
/* 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,
@ -430,11 +431,7 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs,
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;

5
include/asm-i386/ptrace.h
View File

@ -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_struct *tsk, struct pt_regs *regs, int erro
*/
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)
extern unsigned long profile_pc(struct pt_regs *regs);

17
include/asm-i386/segment.h
View File

@ -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 +117,16 @@
*/
#define IDT_ENTRIES 256
/* Bottom two bits of selector give the ring privilege level */
#define SEGMENT_RPL_MASK 0x3
/* Bit 2 is table indicator (LDT/GDT) */
#define SEGMENT_TI_MASK 0x4
/* User mode is privilege level 3 */
#define USER_RPL 0x3
/* LDT segment has TI set, GDT has it cleared */
#define SEGMENT_LDT 0x4
#define SEGMENT_GDT 0x0
#define get_kernel_rpl() 0
#endif