stackprotector: use canary at end of stack to indicate overruns at oops time

(Updated with a common max-stack-used checker that knows about
the canary, as suggested by Joe Perches)

Use a canary at the end of the stack to clearly indicate
at oops time whether the stack has ever overflowed.

This is a very simple implementation with a couple of
drawbacks:

1) a thread may legitimately use exactly up to the last
   word on the stack

 -- but the chances of doing this and then oopsing later seem slim

2) it's possible that the stack usage isn't dense enough
   that the canary location could get skipped over

 -- but the worst that happens is that we don't flag the overrun
 -- though this happens fairly often in my testing :(

With the code in place, an intentionally-bloated stack oops might
do:

BUG: unable to handle kernel paging request at ffff8103f84cc680
IP: [<ffffffff810253df>] update_curr+0x9a/0xa8
PGD 8063 PUD 0
Thread overran stack or stack corrupted
Oops: 0000 [1] SMP
CPU 0
...

... unless the stack overrun is so bad that it corrupts some other
thread.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/magic.h b/include/linux/magic.h
index 1fa0c2ce4dec..74e68e201166 100644
--- a/include/linux/magic.h
+++ b/include/linux/magic.h
@@ -42,4 +42,5 @@
 #define FUTEXFS_SUPER_MAGIC     0xBAD1DEA
 #define INOTIFYFS_SUPER_MAGIC   0x2BAD1DEA
 
+#define STACK_END_MAGIC         0x57AC6E9D
 #endif /* __LINUX_MAGIC_H__ */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d6a515158783..c5181e77f305 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1969,6 +1969,19 @@ static inline unsigned long *end_of_stack(struct task_struct *p)
 
 extern void thread_info_cache_init(void);
 
+#ifdef CONFIG_DEBUG_STACK_USAGE
+static inline unsigned long stack_not_used(struct task_struct *p)
+{
+        unsigned long *n = end_of_stack(p);
+
+        do {    /* Skip over canary */
+                n++;
+        } while (!*n);
+
+        return (unsigned long)n - (unsigned long)end_of_stack(p);
+}
+#endif
+
 /* set thread flags in other task's structures
  * - see asm/thread_info.h for TIF_xxxx flags available
  */