x86: Pin task-stack in __get_wchan()

When commit 5d1ceb39 ("x86: Fix __get_wchan() for !STACKTRACE")
moved from stacktrace to native unwind_*() usage, the
try_get_task_stack() got lost, leading to use-after-free issues for
dying tasks.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Fixes: 5d1ceb39 ("x86: Fix __get_wchan() for !STACKTRACE")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=215031
Link: https://lore.kernel.org/stable/YZV02RCRVHIa144u@fedora64.linuxtx.org/Reported-by: NJustin Forbes <jmforbes@linuxtx.org>
Reported-by: NHolger Hoffstätte <holger@applied-asynchrony.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/process.c

@@ -964,6 +964,9 @@ unsigned long __get_wchan(struct task_struct *p)
 	struct unwind_state state;
 	unsigned long addr = 0;
 
+	if (!try_get_task_stack(p))
+		return 0;
+
 	for (unwind_start(&state, p, NULL, NULL); !unwind_done(&state);
 	     unwind_next_frame(&state)) {
 		addr = unwind_get_return_address(&state);
@@ -974,6 +977,8 @@ unsigned long __get_wchan(struct task_struct *p)
 		break;
 	}
 
+	put_task_stack(p);
+
 	return addr;
 }