linux-kernel - [PATCH 2/2] kasan: unpoison stack of idle task on cpu online

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Date:	Wed, 2 Mar 2016 16:51:59 +0300
From:	Andrey Ryabinin <aryabinin@...tuozzo.com>
To:	<linux-kernel@...r.kernel.org>
CC:	Mark Rutland <mark.rutland@....com>,
	Alexander Potapenko <glider@...gle.com>,
	Andrey Konovalov <adech.fo@...il.com>,
	Dmitriy Vyukov <dvyukov@...gle.com>, <will.deacon@....com>,
	<catalin.marinas@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<kasan-dev@...glegroups.com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: [PATCH 2/2] kasan: unpoison stack of idle task on cpu online

KASAN poisons stack redzones on function's entrance and unpoisons prior
return. So when cpu goes offline the stack of idle task left poisoned.
When cpu goes back online it re-enters the kernel via another path and
starts using idle task's stack. Hence it's possible to hit stale poison
values which results in false-positive KASAN splats.

This patch registers cpu hotplug notifier which unpoisons idle task's
stack prior to onlining cpu.

Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
---
 include/linux/sched.h |  6 ++++++
 kernel/smpboot.h      |  2 --
 mm/kasan/kasan.c      | 33 +++++++++++++++++++++++++++------
 3 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index a10494a..18e526d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -337,6 +337,12 @@ extern asmlinkage void schedule_tail(struct task_struct *prev);
 extern void init_idle(struct task_struct *idle, int cpu);
 extern void init_idle_bootup_task(struct task_struct *idle);
 
+#ifdef CONFIG_GENERIC_SMP_IDLE_THREAD
+extern struct task_struct *idle_thread_get(unsigned int cpu);
+#else
+static inline struct task_struct *idle_thread_get(unsigned int cpu) { return NULL; }
+#endif
+
 extern cpumask_var_t cpu_isolated_map;
 
 extern int runqueue_is_locked(int cpu);
diff --git a/kernel/smpboot.h b/kernel/smpboot.h
index 72415a0..eebf9ec 100644
--- a/kernel/smpboot.h
+++ b/kernel/smpboot.h
@@ -4,11 +4,9 @@
 struct task_struct;
 
 #ifdef CONFIG_GENERIC_SMP_IDLE_THREAD
-struct task_struct *idle_thread_get(unsigned int cpu);
 void idle_thread_set_boot_cpu(void);
 void idle_threads_init(void);
 #else
-static inline struct task_struct *idle_thread_get(unsigned int cpu) { return NULL; }
 static inline void idle_thread_set_boot_cpu(void) { }
 static inline void idle_threads_init(void) { }
 #endif
diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index bc0a8d8..c4ffd82 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -16,6 +16,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #define DISABLE_BRANCH_PROFILING
 
+#include <linux/cpu.h>
 #include <linux/export.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
@@ -537,16 +538,36 @@ static int kasan_mem_notifier(struct notifier_block *nb,
 {
 	return (action == MEM_GOING_ONLINE) ? NOTIFY_BAD : NOTIFY_OK;
 }
+#endif
+
+static int kasan_cpu_callback(struct notifier_block *nfb,
+			unsigned long action, void *hcpu)
+{
+	unsigned int cpu = (unsigned long)hcpu;
+
+	if ((action == CPU_UP_PREPARE) || (action == CPU_UP_PREPARE_FROZEN)) {
+		struct task_struct *tidle = idle_thread_get(cpu);
+		kasan_unpoison_shadow(task_stack_page(tidle), THREAD_SIZE);
+	}
+	return NOTIFY_OK;
+}
 
-static int __init kasan_memhotplug_init(void)
+static struct notifier_block kasan_cpu_notifier =
 {
-	pr_err("WARNING: KASAN doesn't support memory hot-add\n");
-	pr_err("Memory hot-add will be disabled\n");
+	.notifier_call = kasan_cpu_callback,
+};
 
-	hotplug_memory_notifier(kasan_mem_notifier, 0);
+static int __init kasan_notifiers_init(void)
+{
+	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG)) {
+		pr_err("WARNING: KASAN doesn't support memory hot-add\n");
+		pr_err("Memory hot-add will be disabled\n");
+		hotplug_memory_notifier(kasan_mem_notifier, 0);
+	}
+
+	register_hotcpu_notifier(&kasan_cpu_notifier);
 
 	return 0;
 }
 
-module_init(kasan_memhotplug_init);
-#endif
+module_init(kasan_notifiers_init);
-- 
2.4.10