MIPS: Avoid clobbering struct pt_regs in kthreads

The resume() implementation octeon_switch.S examines the saved cp0_status
register.  We were clobbering the entire pt_regs structure in kernel
threads leading to random crashes.

When switching away from a kernel thread, the saved cp0_status is examined
and if bit 30 is set it is cleared and the CP2 state saved into the pt_regs
structure.  Since the kernel thread stack overlaid the pt_regs structure
this resulted in a corrupt stack.  When the kthread with the corrupt stack
was resumed, it could crash if it used any of the data in the stack that was
clobbered.

We fix it by moving the kernel thread stack down so it doesn't overlay
pt_regs.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/processor.h

@@ -311,8 +311,9 @@ extern void start_thread(struct pt_regs * regs, unsigned long pc, unsigned long
 
 unsigned long get_wchan(struct task_struct *p);
 
-#define __KSTK_TOS(tsk) ((unsigned long)task_stack_page(tsk) + THREAD_SIZE - 32)
-#define task_pt_regs(tsk) ((struct pt_regs *)__KSTK_TOS(tsk) - 1)
+#define __KSTK_TOS(tsk) ((unsigned long)task_stack_page(tsk) + \
+			 THREAD_SIZE - 32 - sizeof(struct pt_regs))
+#define task_pt_regs(tsk) ((struct pt_regs *)__KSTK_TOS(tsk))
 #define KSTK_EIP(tsk) (task_pt_regs(tsk)->cp0_epc)
 #define KSTK_ESP(tsk) (task_pt_regs(tsk)->regs[29])
 #define KSTK_STATUS(tsk) (task_pt_regs(tsk)->cp0_status)

arch/mips/kernel/head.S

@@ -188,7 +188,8 @@ NESTED(kernel_entry, 16, sp)			# kernel entry point
 
 	MTC0		zero, CP0_CONTEXT	# clear context register
 	PTR_LA		$28, init_thread_union
-	PTR_LI		sp, _THREAD_SIZE - 32
+	/* Set the SP after an empty pt_regs.  */
+	PTR_LI		sp, _THREAD_SIZE - 32 - PT_SIZE
 	PTR_ADDU	sp, $28
 	set_saved_sp	sp, t0, t1
 	PTR_SUBU	sp, 4 * SZREG		# init stack pointer

arch/mips/kernel/process.c

@@ -115,7 +115,7 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
 {
 	struct thread_info *ti = task_thread_info(p);
 	struct pt_regs *childregs;
-	long childksp;
+	unsigned long childksp;
 	p->set_child_tid = p->clear_child_tid = NULL;
 
 	childksp = (unsigned long)task_stack_page(p) + THREAD_SIZE - 32;
@@ -132,6 +132,8 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
 
 	/* set up new TSS. */
 	childregs = (struct pt_regs *) childksp - 1;
+	/*  Put the stack after the struct pt_regs.  */
+	childksp = (unsigned long) childregs;
 	*childregs = *regs;
 	childregs->regs[7] = 0;	/* Clear error flag */