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>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 42d4027f9e2656b763aa3050e38c2ae1803e1890..9abf66b6c271e98f011fa93cf5a69f27c36a8e2b 100644 (file)
--- 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)