[PATCH v13 03/12] scs: add support for stack usage debugging

[Sami Tolvanen]

Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks. When enabled,
also prints out the highest shadow stack usage per process.

Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
---
 kernel/scs.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/kernel/scs.c b/kernel/scs.c
index 8769016c714c..2a96573f2b1b 100644
--- a/kernel/scs.c
+++ b/kernel/scs.c
@@ -68,6 +68,43 @@ int scs_prepare(struct task_struct *tsk, int node)
 	return 0;
 }
 
+#ifdef CONFIG_DEBUG_STACK_USAGE
+static unsigned long __scs_used(struct task_struct *tsk)
+{
+	unsigned long *p = task_scs(tsk);
+	unsigned long *end = __scs_magic(p);
+	unsigned long s = (unsigned long)p;
+
+	while (p < end && READ_ONCE_NOCHECK(*p))
+		p++;
+
+	return (unsigned long)p - s;
+}
+
+static void scs_check_usage(struct task_struct *tsk)
+{
+	static unsigned long highest;
+	unsigned long used = __scs_used(tsk);
+	unsigned long prev;
+	unsigned long curr = highest;
+
+	while (used > curr) {
+		prev = cmpxchg_relaxed(&highest, curr, used);
+
+		if (prev == curr) {
+			pr_info("%s (%d): highest shadow stack usage: "
+				"%lu bytes\n",
+				tsk->comm, task_pid_nr(tsk), used);
+			break;
+		}
+
+		curr = prev;
+	}
+}
+#else
+static inline void scs_check_usage(struct task_struct *tsk) {}
+#endif
+
 void scs_release(struct task_struct *tsk)
 {
 	void *s;
@@ -77,6 +114,7 @@ void scs_release(struct task_struct *tsk)
 		return;
 
 	WARN_ON(scs_corrupted(tsk));
+	scs_check_usage(tsk);
 
 	scs_account(tsk, -1);
 	scs_free(s);
-- 
2.26.2.303.gf8c07b1a785-goog