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Date:   Thu, 31 Oct 2019 09:46:27 -0700
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Subject: [PATCH v3 07/17] scs: add support for stack usage debugging
From:   samitolvanen@google.com
To:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux@googlegroups.com,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Sami Tolvanen <samitolvanen@google.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
 kernel/scs.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/kernel/scs.c b/kernel/scs.c
index 7780fc4e29ac..67c43af627d1 100644
--- a/kernel/scs.c
+++ b/kernel/scs.c
@@ -167,6 +167,44 @@ int scs_prepare(struct task_struct *tsk, int node)
 	return 0;
 }
 
+#ifdef CONFIG_DEBUG_STACK_USAGE
+static inline unsigned long scs_used(struct task_struct *tsk)
+{
+	unsigned long *p = __scs_base(tsk);
+	unsigned long *end = scs_magic(tsk);
+	uintptr_t s = (uintptr_t)p;
+
+	while (p < end && *p)
+		p++;
+
+	return (uintptr_t)p - s;
+}
+
+static void scs_check_usage(struct task_struct *tsk)
+{
+	static DEFINE_SPINLOCK(lock);
+	static unsigned long highest;
+	unsigned long used = scs_used(tsk);
+
+	if (used <= highest)
+		return;
+
+	spin_lock(&lock);
+
+	if (used > highest) {
+		pr_info("%s: highest shadow stack usage %lu bytes\n",
+			__func__, used);
+		highest = used;
+	}
+
+	spin_unlock(&lock);
+}
+#else
+static inline void scs_check_usage(struct task_struct *tsk)
+{
+}
+#endif
+
 bool scs_corrupted(struct task_struct *tsk)
 {
 	return *scs_magic(tsk) != SCS_END_MAGIC;
@@ -181,6 +219,7 @@ void scs_release(struct task_struct *tsk)
 		return;
 
 	WARN_ON(scs_corrupted(tsk));
+	scs_check_usage(tsk);
 
 	scs_account(tsk, -1);
 	task_set_scs(tsk, NULL);
-- 
2.24.0.rc0.303.g954a862665-goog