Commit 78be3706 authored by Rusty Russell's avatar Rusty Russell Committed by Andi Kleen
Browse files

[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: default avatarRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: default avatarZachary Amsden <zach@vmware.com>
Signed-off-by: default avatarJeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarAndi Kleen <ak@suse.de>
parent 0da5db31

arch/i386/kernel/entry.S

+5 −4
Original line number Diff line number Diff line
@@ -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:

arch/i386/kernel/process.c

+1 −1
Original line number Diff line number Diff line
@@ -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.. */

arch/i386/mm/extable.c

+1 −1
Original line number Diff line number Diff line
@@ -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;

arch/i386/mm/fault.c

+4 −7
Original line number Diff line number Diff line
@@ -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,
@@ -430,10 +431,6 @@ 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 */

		case 2:		/* write, not present */

			if (!(vma->vm_flags & VM_WRITE))

include/asm-i386/ptrace.h

+3 −2
Original line number Diff line number Diff line
@@ -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);

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