linux-kernel - [PATCH v2 2/8] sched: Add try_get_task_stack() and put_task

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Message-Id: <17a434f50ad3d77000104f21666575e10a9c1fbd.1474003868.git.luto@kernel.org>
Date:   Thu, 15 Sep 2016 22:45:43 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org,
        Brian Gerst <brgerst@...il.com>, Jann Horn <jann@...jh.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2 2/8] sched: Add try_get_task_stack() and put_task_stack()

There are a few places in the kernel that access stack memory
belonging to a different task.  Before we can start freeing task
stacks before the task_struct is freed, we need a way for those code
paths to pin the stack.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 include/linux/sched.h | 16 ++++++++++++++++
 init/Kconfig          |  3 +++
 2 files changed, 19 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index a287e8b13549..a95867267e9f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -3094,11 +3094,19 @@ static inline struct thread_info *task_thread_info(struct task_struct *task)
 {
 	return &task->thread_info;
 }
+
+/*
+ * When accessing the stack of a non-current task that might exit, use
+ * try_get_task_stack() instead.  task_stack_page will return a pointer
+ * that could get freed out from under you.
+ */
 static inline void *task_stack_page(const struct task_struct *task)
 {
 	return task->stack;
 }
+
 #define setup_thread_stack(new,old)	do { } while(0)
+
 static inline unsigned long *end_of_stack(const struct task_struct *task)
 {
 	return task->stack;
@@ -3134,6 +3142,14 @@ static inline unsigned long *end_of_stack(struct task_struct *p)
 }
 
 #endif
+
+static inline void *try_get_task_stack(struct task_struct *tsk)
+{
+	return task_stack_page(tsk);
+}
+
+static inline void put_task_stack(struct task_struct *tsk) {}
+
 #define task_stack_end_corrupted(task) \
 		(*(end_of_stack(task)) != STACK_END_MAGIC)
 
diff --git a/init/Kconfig b/init/Kconfig
index ec8d43894b02..3b9a47fe843b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -33,6 +33,9 @@ config THREAD_INFO_IN_TASK
 	  make this work, an arch will need to remove all thread_info fields
 	  except flags and fix any runtime bugs.
 
+	  One subtle change that will be needed is to use try_get_task_stack()
+	  and put_task_stack() in save_thread_stack_tsk() and get_wchan().
+
 menu "General setup"
 
 config BROKEN
-- 
2.7.4