[POWERPC] Harden validate_sp against stack corruption

If something has overflowed or corrupted the stack and causes an oops,
and we try to print a stack trace, that will call validate_sp, which
can itself cause an oops if the cpu field of the thread_info struct at
the bottom of the stack has been corrupted (if CONFIG_IRQSTACKS is
set).  This makes debugging harder.

To avoid the second oops, this adds a check to make sure that the cpu
number is reasonable before using it to check whether the stack is on
the softirq or hardirq stack.

Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 30 insertions, 13 deletions

diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index f3d4dd580dd6..972b2acbe713 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -818,6 +818,35 @@ out:
         return error;
 }
 
+#ifdef CONFIG_IRQSTACKS
+static inline int valid_irq_stack(unsigned long sp, struct task_struct *p,
+                                  unsigned long nbytes)
+{
+        unsigned long stack_page;
+        unsigned long cpu = task_cpu(p);
+
+        /*
+         * Avoid crashing if the stack has overflowed and corrupted
+         * task_cpu(p), which is in the thread_info struct.
+         */
+        if (cpu < NR_CPUS && cpu_possible(cpu)) {
+                stack_page = (unsigned long) hardirq_ctx[cpu];
+                if (sp >= stack_page + sizeof(struct thread_struct)
+                    && sp <= stack_page + THREAD_SIZE - nbytes)
+                        return 1;
+
+                stack_page = (unsigned long) softirq_ctx[cpu];
+                if (sp >= stack_page + sizeof(struct thread_struct)
+                    && sp <= stack_page + THREAD_SIZE - nbytes)
+                        return 1;
+        }
+        return 0;
+}
+
+#else
+#define valid_irq_stack(sp, p, nb)      0
+#endif /* CONFIG_IRQSTACKS */
+
 int validate_sp(unsigned long sp, struct task_struct *p,
                        unsigned long nbytes)
 {
@@ -827,19 +856,7 @@ int validate_sp(unsigned long sp, struct task_struct *p,
             && sp <= stack_page + THREAD_SIZE - nbytes)
                 return 1;
 
-#ifdef CONFIG_IRQSTACKS
-        stack_page = (unsigned long) hardirq_ctx[task_cpu(p)];
-        if (sp >= stack_page + sizeof(struct thread_struct)
-            && sp <= stack_page + THREAD_SIZE - nbytes)
-                return 1;
-
-        stack_page = (unsigned long) softirq_ctx[task_cpu(p)];
-        if (sp >= stack_page + sizeof(struct thread_struct)
-            && sp <= stack_page + THREAD_SIZE - nbytes)
-                return 1;
-#endif
-
-        return 0;
+        return valid_irq_stack(sp, p, nbytes);
 }
 
 #ifdef CONFIG_PPC64