[PATCH] x86: privilege cleanup

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 <zach@vmware.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

3 files changed, 11 insertions, 5 deletions

diff --git a/arch/i386/kernel/signal.c b/arch/i386/kernel/signal.c
index 7bcda6d69d87..61eb0c8a6e47 100644
--- a/arch/i386/kernel/signal.c
+++ b/arch/i386/kernel/signal.c
@@ -604,7 +604,9 @@ int fastcall do_signal(struct pt_regs *regs, sigset_t *oldset)
          * 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;
diff --git a/arch/i386/kernel/traps.c b/arch/i386/kernel/traps.c
index 1144df0c48d6..b2b4bb890bb7 100644
--- a/arch/i386/kernel/traps.c
+++ b/arch/i386/kernel/traps.c
@@ -210,7 +210,7 @@ void show_registers(struct pt_regs *regs)
         unsigned short ss;
 
         esp = (unsigned long) (&regs->esp);
-        ss = __KERNEL_DS;
+        savesegment(ss, ss);
         if (user_mode(regs)) {
                 in_kernel = 0;
                 esp = regs->esp;
@@ -267,9 +267,6 @@ static void handle_BUG(struct pt_regs *regs)
         char c;
         unsigned long eip;
 
-        if (user_mode(regs))
-                goto no_bug;            /* Not in kernel */
-
         eip = regs->eip;
 
         if (eip < PAGE_OFFSET)
diff --git a/include/asm-i386/ptrace.h b/include/asm-i386/ptrace.h
index 05532875e39e..7e0f2945d17d 100644
--- a/include/asm-i386/ptrace.h
+++ b/include/asm-i386/ptrace.h
@@ -61,6 +61,13 @@ struct pt_regs {
 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;