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Date: Thu, 16 Apr 2020 09:12:36 -0700
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Will Deacon <will@...nel.org>, Catalin Marinas <catalin.marinas@....com>, 
	James Morse <james.morse@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ard Biesheuvel <ard.biesheuvel@...aro.org>, Mark Rutland <mark.rutland@....com>, 
	Masahiro Yamada <masahiroy@...nel.org>, Michal Marek <michal.lkml@...kovi.net>, 
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dave Martin <Dave.Martin@....com>, Kees Cook <keescook@...omium.org>, 
	Laura Abbott <labbott@...hat.com>, Marc Zyngier <maz@...nel.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Jann Horn <jannh@...gle.com>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, 
	clang-built-linux@...glegroups.com, kernel-hardening@...ts.openwall.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH v11 03/12] scs: add support for stack usage debugging

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@...gle.com>
Reviewed-by: Kees Cook <keescook@...omium.org>
---
 kernel/scs.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/kernel/scs.c b/kernel/scs.c
index 5245e992c692..ad74d13f2c0f 100644
--- a/kernel/scs.c
+++ b/kernel/scs.c
@@ -184,6 +184,44 @@ 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(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 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 (%d): highest shadow stack usage: %lu bytes\n",
+			tsk->comm, task_pid_nr(tsk), 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)
 {
 	unsigned long *magic = scs_magic(__scs_base(tsk));
@@ -200,6 +238,7 @@ 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);
-- 
2.26.1.301.g55bc3eb7cb9-goog

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