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

Sami Tolvanen
2020-04-27T16:01:14Z

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@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
---
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