Re: [RFC PATCH 15/33] move segment checks to subarch

From: Rusty Russell
Date: Tue Jul 18 2006 - 15:05:22 EST

On Tue, 2006-07-18 at 00:00 -0700, Chris Wright wrote:
> plain text document attachment (i386-segments)
> 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).

Zach had an alternate patch for this, which didn't assume the kernel ran
in a compile-time known ring, but is otherwise very similar. I've put
it below for discussion (but Zach now tells me the asm parts are not
required: Zach, can you mod this patch and comment?).

Your patch #16 finishes the job you started here, by doing the mods to
entry.S. I think it's cleaner to have all this in one patch (and it can
go in almost immediately AFAICT).

Comments?
Rusty.

Name: Kernel Ring Cleanups
Status: Booted on 2.6.18-rc1
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

This is Zach's patch to clean up assumptions about the kernel running
in ring 0 (which it doesn't when running paravirtualized).

1) Remove the hardcoded 3 and introduce #define SEGMENT_RPL_MASK 3
2) Add a get_kernel_rpl() function
3) Create COMPARE_SEGMENT_STACK and COMPARE_SEGMENT_REG macros which
can mask out the bottom two bits (RPL) when comparing for
paravirtualization.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .27795-linux-2.6.18-rc1/arch/i386/kernel/entry.S .27795-linux-2.6.18-rc1.updated/arch/i386/kernel/entry.S
--- .27795-linux-2.6.18-rc1/arch/i386/kernel/entry.S 2006-07-07 10:46:38.000000000 +1000
+++ .27795-linux-2.6.18-rc1.updated/arch/i386/kernel/entry.S 2006-07-07 12:11:01.000000000 +1000
@@ -228,9 +228,11 @@ ret_from_intr:
GET_THREAD_INFO(%ebp)
check_userspace:
movl EFLAGS(%esp), %eax # mix EFLAGS and CS
+ andl $VM_MASK, %eax
movb CS(%esp), %al
- testl $(VM_MASK | 3), %eax
- jz resume_kernel
+ andb $SEGMENT_RPL_MASK, %al
+ cmpl $SEGMENT_RPL_MASK, %eax
+ jb resume_kernel # returning to kernel or vm86-space
ENTRY(resume_userspace)
cli # make sure we don't miss an interrupt
# setting need_resched or sigpending
@@ -517,22 +519,16 @@ syscall_badsys:
/* put ESP to the proper location */ \
movl %eax, %esp;
#define UNWIND_ESPFIX_STACK \
- pushl %eax; \
- CFI_ADJUST_CFA_OFFSET 4; \
- movl %ss, %eax; \
- /* see if on 16bit stack */ \
- cmpw $__ESPFIX_SS, %ax; \
- je 28f; \
-27: popl %eax; \
- CFI_ADJUST_CFA_OFFSET -4; \
+ COMPARE_SEGMENT_REG(__ESPFIX_SS, %ss); \
+ jne 28f; \
.section .fixup,"ax"; \
-28: movl $__KERNEL_DS, %eax; \
+ movl $__KERNEL_DS, %eax; \
movl %eax, %ds; \
movl %eax, %es; \
/* switch to 32bit stack */ \
FIXUP_ESPFIX_STACK; \
- jmp 27b; \
-.previous
+.previous \
+28: ;

/*
* Build the entry stubs and pointer table with
@@ -629,6 +625,7 @@ error_code:
popl %ecx
CFI_ADJUST_CFA_OFFSET -4
/*CFI_REGISTER es, ecx*/
+ movl EAX(%esp), %eax
movl ES(%esp), %edi # get the function address
movl ORIG_EAX(%esp), %edx # get the error code
movl %eax, ORIG_EAX(%esp)
@@ -694,12 +691,12 @@ device_not_available_emulate:
* the instruction that would have done it for sysenter.
*/
#define FIX_STACK(offset, ok, label) \
- cmpw $__KERNEL_CS,4(%esp); \
+ COMPARE_SEGMENT_STACK(__KERNEL_CS, 4); \
jne ok; \
label: \
movl TSS_sysenter_esp0+offset(%esp),%esp; \
pushfl; \
- pushl $__KERNEL_CS; \
+ push %cs; \
pushl $sysenter_past_esp

KPROBE_ENTRY(debug)
@@ -727,12 +724,7 @@ debug_stack_correct:
*/
ENTRY(nmi)
RING0_INT_FRAME
- pushl %eax
- CFI_ADJUST_CFA_OFFSET 4
- movl %ss, %eax
- cmpw $__ESPFIX_SS, %ax
- popl %eax
- CFI_ADJUST_CFA_OFFSET -4
+ COMPARE_SEGMENT_REG(__ESPFIX_SS, %ss)
je nmi_16bit_stack
cmpl $sysenter_entry,(%esp)
je nmi_stack_fixup
@@ -763,7 +755,7 @@ nmi_stack_fixup:
FIX_STACK(12,nmi_stack_correct, 1)
jmp nmi_stack_correct
nmi_debug_stack_check:
- cmpw $__KERNEL_CS,16(%esp)
+ COMPARE_SEGMENT_STACK(__KERNEL_CS, 16)
jne nmi_stack_correct
cmpl $debug,(%esp)
jb nmi_stack_correct
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .27795-linux-2.6.18-rc1/arch/i386/kernel/process.c .27795-linux-2.6.18-rc1.updated/arch/i386/kernel/process.c
--- .27795-linux-2.6.18-rc1/arch/i386/kernel/process.c 2006-07-07 10:46:38.000000000 +1000
+++ .27795-linux-2.6.18-rc1.updated/arch/i386/kernel/process.c 2006-07-07 11:10:30.000000000 +1000
@@ -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.. */
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .27795-linux-2.6.18-rc1/arch/i386/kernel/traps.c .27795-linux-2.6.18-rc1.updated/arch/i386/kernel/traps.c
--- .27795-linux-2.6.18-rc1/arch/i386/kernel/traps.c 2006-07-07 10:46:38.000000000 +1000
+++ .27795-linux-2.6.18-rc1.updated/arch/i386/kernel/traps.c 2006-07-07 11:10:30.000000000 +1000
@@ -1034,10 +1034,10 @@ fastcall void setup_x86_bogus_stack(unsi
memcpy((void *)(stack_bot + iret_frame16_off), &regs->eip, 20);
/* fill in the switch pointers */
switch16_ptr[0] = (regs->esp & 0xffff0000) | iret_frame16_off;
- switch16_ptr[1] = __ESPFIX_SS;
+ switch16_ptr[1] = __ESPFIX_SS | get_kernel_rpl();
switch32_ptr[0] = (unsigned long)stk + sizeof(struct pt_regs) +
8 - CPU_16BIT_STACK_SIZE;
- switch32_ptr[1] = __KERNEL_DS;
+ switch32_ptr[1] = __KERNEL_DS | get_kernel_rpl();
}

fastcall unsigned char * fixup_x86_bogus_stack(unsigned short sp)
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .27795-linux-2.6.18-rc1/include/asm-i386/ptrace.h .27795-linux-2.6.18-rc1.updated/include/asm-i386/ptrace.h
--- .27795-linux-2.6.18-rc1/include/asm-i386/ptrace.h 2006-03-23 12:44:59.000000000 +1100
+++ .27795-linux-2.6.18-rc1.updated/include/asm-i386/ptrace.h 2006-07-07 11:10:30.000000000 +1000
@@ -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) == 3;
}
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)) >= 3);
}
#define instruction_pointer(regs) ((regs)->eip)
#if defined(CONFIG_SMP) && defined(CONFIG_FRAME_POINTER)
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .27795-linux-2.6.18-rc1/include/asm-i386/segment.h .27795-linux-2.6.18-rc1.updated/include/asm-i386/segment.h
--- .27795-linux-2.6.18-rc1/include/asm-i386/segment.h 2006-03-23 12:44:59.000000000 +1100
+++ .27795-linux-2.6.18-rc1.updated/include/asm-i386/segment.h 2006-07-07 11:49:39.000000000 +1000
@@ -112,4 +112,20 @@
*/
#define IDT_ENTRIES 256

+/* Bottom three bits of xcs give the ring privilege level */
+#define SEGMENT_RPL_MASK 0x3
+
+#define get_kernel_rpl() 0
+
+#define COMPARE_SEGMENT_STACK(segment, offset) \
+ cmpw $segment, offset(%esp);
+
+#define COMPARE_SEGMENT_REG(segment, reg) \
+ pushl %eax; \
+ CFI_ADJUST_CFA_OFFSET 4; \
+ mov reg, %eax; \
+ cmpw $segment,%ax; \
+ popl %eax; \
+ CFI_ADJUST_CFA_OFFSET -4
+
#endif

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