[PATCH 4.4 076/162] sched/core: Add try_get_task_stack() and put_task_stack()

[Greg Kroah-Hartman]

From: Andy Lutomirski <luto@xxxxxxxxxx>

commit c6c314a613cd7d03fb97713e0d642b493de42e69 upstream.

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@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Jann Horn <jann@xxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/17a434f50ad3d77000104f21666575e10a9c1fbd.1474003868.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: zhangyi (F) <yi.zhang@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/linux/sched.h |   16 ++++++++++++++++
 init/Kconfig          |    3 +++
 2 files changed, 19 insertions(+)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2858,11 +2858,19 @@ static inline struct thread_info *task_t
 {
 	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;
@@ -2898,6 +2906,14 @@ static inline unsigned long *end_of_stac
 }
 
 #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)
 
--- 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