MIPS: Avoid accidental raw backtrace

BugLink: http://bugs.launchpad.net/bugs/1702863

commit 8542363 upstream.

Since commit 81a76d7 ("MIPS: Avoid using unwind_stack() with
usermode") show_backtrace() invokes the raw backtracer when
cp0_status & ST0_KSU indicates user mode to fix issues on EVA kernels
where user and kernel address spaces overlap.

However this is used by show_stack() which creates its own pt_regs on
the stack and leaves cp0_status uninitialised in most of the code paths.
This results in the non deterministic use of the raw back tracer
depending on the previous stack content.

show_stack() deals exclusively with kernel mode stacks anyway, so
explicitly initialise regs.cp0_status to KSU_KERNEL (i.e. 0) to ensure
we get a useful backtrace.

Fixes: 81a76d7 ("MIPS: Avoid using unwind_stack() with usermode")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16656/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

arch/mips/kernel/traps.c

@@ -194,6 +194,8 @@ void show_stack(struct task_struct *task, unsigned long *sp)
 {
 	struct pt_regs regs;
 	mm_segment_t old_fs = get_fs();
+
+	regs.cp0_status = KSU_KERNEL;
 	if (sp) {
 		regs.regs[29] = (unsigned long)sp;
 		regs.regs[31] = 0;