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Date:   Mon, 27 Apr 2020 09:00:09 -0700
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Subject: [PATCH v13 03/12] scs: add support for stack usage debugging
From:   Sami Tolvanen <samitolvanen@google.com>
To:     Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.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>
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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