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>

3 files changed, 8 insertions, 4 deletions

diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
index 0f926aa0cb47..087a8884ef06 100644
--- a/arch/mips/include/asm/processor.h
+++ b/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)
diff --git a/arch/mips/kernel/head.S b/arch/mips/kernel/head.S
index 492a0a8d70fb..531ce7b16124 100644
--- a/arch/mips/kernel/head.S
+++ b/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
diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index c09d681b7181..f3d73e1831c1 100644
--- a/arch/mips/kernel/process.c
+++ b/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 */