[PATCH v2 5/8] x86/process: Pin the target stack in get_wchan()

[Andy Lutomirski <luto@kernel.org>]

This will prevent a crash if get_wchan() runs after the task stack
is freed.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/kernel/process.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 0b9ed8ec5226..4002b475171c 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -532,15 +532,18 @@ unsigned long thread_saved_pc(struct task_struct *tsk)
  */
 unsigned long get_wchan(struct task_struct *p)
 {
-	unsigned long start, bottom, top, sp, fp, ip;
+	unsigned long start, bottom, top, sp, fp, ip, ret = 0;
 	int count = 0;
 
 	if (!p || p == current || p->state == TASK_RUNNING)
 		return 0;
 
+	if (!try_get_task_stack(p))
+		return 0;
+
 	start = (unsigned long)task_stack_page(p);
 	if (!start)
-		return 0;
+		goto out;
 
 	/*
 	 * Layout of the stack page:
@@ -564,16 +567,21 @@ unsigned long get_wchan(struct task_struct *p)
 
 	sp = READ_ONCE(p->thread.sp);
 	if (sp < bottom || sp > top)
-		return 0;
+		goto out;
 
 	fp = READ_ONCE_NOCHECK(((struct inactive_task_frame *)sp)->bp);
 	do {
 		if (fp < bottom || fp > top)
-			return 0;
+			goto out;
 		ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
-		if (!in_sched_functions(ip))
-			return ip;
+		if (!in_sched_functions(ip)) {
+			ret = ip;
+			goto out;
+		}
 		fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
 	} while (count++ < 16 && p->state != TASK_RUNNING);
-	return 0;
+
+out:
+	put_task_stack(p);
+	return ret;
 }
-- 
2.7.4