Commit-ID:  c6c314a613cd7d03fb97713e0d642b493de42e69
Gitweb:     http://git.kernel.org/tip/c6c314a613cd7d03fb97713e0d642b493de42e69
Author:     Andy Lutomirski <l...@kernel.org>
AuthorDate: Thu, 15 Sep 2016 22:45:43 -0700
Committer:  Ingo Molnar <mi...@kernel.org>
CommitDate: Fri, 16 Sep 2016 09:18:53 +0200

sched/core: 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 <l...@kernel.org>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Brian Gerst <brge...@gmail.com>
Cc: Denys Vlasenko <dvlas...@redhat.com>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Jann Horn <j...@thejh.net>
Cc: Josh Poimboeuf <jpoim...@redhat.com>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Thomas Gleixner <t...@linutronix.de>
Link: 
http://lkml.kernel.org/r/17a434f50ad3d77000104f21666575e10a9c1fbd.1474003868.git.l...@kernel.org
Signed-off-by: Ingo Molnar <mi...@kernel.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 a287e8b..a958672 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 ec8d438..3b9a47f 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

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