[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>
diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
index 3872fca..284f2e9 100644
--- a/arch/i386/kernel/entry.S
+++ b/arch/i386/kernel/entry.S
@@ -240,8 +240,9 @@
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 @@
# 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:
diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index 220aeca..8c190ca 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -338,7 +338,7 @@
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.. */
diff --git a/arch/i386/mm/extable.c b/arch/i386/mm/extable.c
index de03c54..0ce4f22 100644
--- a/arch/i386/mm/extable.c
+++ b/arch/i386/mm/extable.c
@@ -11,7 +11,7 @@
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;
diff --git a/arch/i386/mm/fault.c b/arch/i386/mm/fault.c
index 0ce86168..5e17a3f 100644
--- a/arch/i386/mm/fault.c
+++ b/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);
@@ -113,10 +114,10 @@
}
/* 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 @@
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;
diff --git a/include/asm-i386/ptrace.h b/include/asm-i386/ptrace.h
index 30a442e..21bb916 100644
--- a/include/asm-i386/ptrace.h
+++ b/include/asm-i386/ptrace.h
@@ -60,6 +60,7 @@
#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 @@
*/
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);
diff --git a/include/asm-i386/segment.h b/include/asm-i386/segment.h
index faf9953..b7ab596 100644
--- a/include/asm-i386/segment.h
+++ b/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 +117,16 @@
*/
#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