[PATCH 4.9] can: bcm: fix UAF of bcm op

[Ziyang Xuan <william.xuanziyang@huawei.com>]

Stopping tasklet and hrtimer rely on the active state of tasklet and
hrtimer sequentially in bcm_remove_op(), the op object will be freed
if they are all unactive. Assume the hrtimer timeout is short, the
hrtimer cb has been excuted after tasklet conditional judgment which
must be false after last round tasklet_kill() and before condition
hrtimer_active(), it is false when execute to hrtimer_active(). Bug
is triggerd, because the stopping action is end and the op object
will be freed, but the tasklet is scheduled. The resources of the op
object will occur UAF bug.

Move hrtimer_cancel() behind tasklet_kill() and switch 'while () {...}'
to 'do {...} while ()' to fix the op UAF problem.

Fixes: a06393ed0316 ("can: bcm: fix hrtimer/tasklet termination in bcm op removal")
Reported-by: syzbot+5ca851459ed04c778d1d@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
---
 net/can/bcm.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/net/can/bcm.c b/net/can/bcm.c
index 369326715b9c..bfb507223468 100644
--- a/net/can/bcm.c
+++ b/net/can/bcm.c
@@ -761,21 +761,21 @@ static struct bcm_op *bcm_find_op(struct list_head *ops,
 static void bcm_remove_op(struct bcm_op *op)
 {
 	if (op->tsklet.func) {
-		while (test_bit(TASKLET_STATE_SCHED, &op->tsklet.state) ||
-		       test_bit(TASKLET_STATE_RUN, &op->tsklet.state) ||
-		       hrtimer_active(&op->timer)) {
-			hrtimer_cancel(&op->timer);
+		do {
 			tasklet_kill(&op->tsklet);
-		}
+			hrtimer_cancel(&op->timer);
+		} while (test_bit(TASKLET_STATE_SCHED, &op->tsklet.state) ||
+			 test_bit(TASKLET_STATE_RUN, &op->tsklet.state) ||
+			 hrtimer_active(&op->timer));
 	}
 
 	if (op->thrtsklet.func) {
-		while (test_bit(TASKLET_STATE_SCHED, &op->thrtsklet.state) ||
-		       test_bit(TASKLET_STATE_RUN, &op->thrtsklet.state) ||
-		       hrtimer_active(&op->thrtimer)) {
-			hrtimer_cancel(&op->thrtimer);
+		do {
 			tasklet_kill(&op->thrtsklet);
-		}
+			hrtimer_cancel(&op->thrtimer);
+		} while (test_bit(TASKLET_STATE_SCHED, &op->thrtsklet.state) ||
+			 test_bit(TASKLET_STATE_RUN, &op->thrtsklet.state) ||
+			 hrtimer_active(&op->thrtimer));
 	}
 
 	if ((op->frames) && (op->frames != &op->sframe))
-- 
2.25.1