Privilege checking cleanup. Originally, these diffs were much greater,
but recent cleanups in Linux have already done much of the cleanup. I
added some explanatory comments in places where the reasoning behind
certain tests is rather subtle.
Also, in traps.c, we can skip the user_mode check in handle_BUG(). The
reason is, there are only two call chains - one via die_if_kernel() and
one via do_page_fault(), both entering from die(). Both of these paths
already ensure that a kernel mode failure has happened. Also,
the original check here, if (user_mode(regs)) was insufficient anyways,
since it would not rule out BUG faults from V8086 mode execution.
Saving the %ss segment in show_regs() rather than assuming a fixed value
also gives better information about the current kernel state in the
register dump.
Signed-off-by: Zachary Amsden <[email protected]>
Index: linux-2.6.13/arch/i386/kernel/signal.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/signal.c 2005-08-02 17:06:21.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/signal.c 2005-08-02 17:09:54.000000000 -0700
@@ -603,7 +603,9 @@
* We want the common case to go fast, which
* is why we may in certain cases get here from
* kernel mode. Just return without doing anything
- * if so.
+ * if so. vm86 regs switched out by assembly code
+ * before reaching here, so testing against kernel
+ * CS suffices.
*/
if (!user_mode(regs))
return 1;
Index: linux-2.6.13/arch/i386/kernel/traps.c
===================================================================
--- linux-2.6.13.orig/arch/i386/kernel/traps.c 2005-08-02 17:06:21.000000000 -0700
+++ linux-2.6.13/arch/i386/kernel/traps.c 2005-08-02 17:09:54.000000000 -0700
@@ -245,7 +245,7 @@
unsigned short ss;
esp = (unsigned long) (®s->esp);
- ss = __KERNEL_DS;
+ savesegment(ss, ss);
if (user_mode(regs)) {
in_kernel = 0;
esp = regs->esp;
@@ -302,9 +302,6 @@
char c;
unsigned long eip;
- if (user_mode(regs))
- goto no_bug; /* Not in kernel */
-
eip = regs->eip;
if (eip < PAGE_OFFSET)
Index: linux-2.6.13/include/asm-i386/ptrace.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/ptrace.h 2005-08-02 17:04:23.000000000 -0700
+++ linux-2.6.13/include/asm-i386/ptrace.h 2005-08-02 17:09:54.000000000 -0700
@@ -61,6 +61,13 @@
struct task_struct;
extern void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs, int error_code);
+/*
+ * user_mode_vm(regs) determines whether a register set came from user mode.
+ * This is true if V8086 mode was enabled OR if the register set was from
+ * protected mode with RPL-3 CS value. This tricky test checks that with
+ * one comparison. Many places in the kernel can bypass this full check
+ * if they have already ruled out V8086 mode, so user_mode(regs) can be used.
+ */
static inline int user_mode(struct pt_regs *regs)
{
return (regs->xcs & 3) != 0;
-
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