[tip:sched/urgent] sched/core: Fix oops in sched_show_task()

[tip-bot for Tetsuo Handa <tipbot@zytor.com>]

Commit-ID:  382005027fedc50b28d40ae64ef1461cca38953e
Gitweb:     http://git.kernel.org/tip/382005027fedc50b28d40ae64ef1461cca38953e
Author:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
AuthorDate: Wed, 2 Nov 2016 19:50:29 +0900
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Thu, 3 Nov 2016 07:27:34 +0100

sched/core: Fix oops in sched_show_task()

When CONFIG_THREAD_INFO_IN_TASK=y, it is possible that an exited thread
remains in the task list after its stack pointer was already set to NULL.

Therefore, thread_saved_pc() and stack_not_used() in sched_show_task()
will trigger NULL pointer dereference if an attempt to dump such thread's
traces (e.g. SysRq-t, khungtaskd) is made.

Since show_stack() in sched_show_task() calls try_get_task_stack() and
sched_show_task() is called from interrupt context, calling
try_get_task_stack() from sched_show_task() will be safe as well.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bp@alien8.de
Cc: brgerst@gmail.com
Cc: jann@thejh.net
Cc: keescook@chromium.org
Cc: linux-api@vger.kernel.org
Cc: tycho.andersen@canonical.com
Link: http://lkml.kernel.org/r/201611021950.FEJ34368.HFFJOOMLtQOVSF@I-love.SAKURA.ne.jp
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 42d4027..9abf66b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5192,6 +5192,8 @@ void sched_show_task(struct task_struct *p)
 	int ppid;
 	unsigned long state = p->state;
 
+	if (!try_get_task_stack(p))
+		return;
 	if (state)
 		state = __ffs(state) + 1;
 	printk(KERN_INFO "%-15.15s %c", p->comm,
@@ -5221,6 +5223,7 @@ void sched_show_task(struct task_struct *p)
 
 	print_worker_info(KERN_INFO, p);
 	show_stack(p, NULL);
+	put_task_stack(p);
 }
 
 void show_state_filter(unsigned long state_filter)