blktrace: Protect q->blk_trace with RCU

KASAN is reporting that __blk_add_trace() has a use-after-free issue
when accessing q->blk_trace. Indeed the switching of block tracing (and
thus eventual freeing of q->blk_trace) is completely unsynchronized with
the currently running tracing and thus it can happen that the blk_trace
structure is being freed just while __blk_add_trace() works on it.
Protect accesses to q->blk_trace by RCU during tracing and make sure we
wait for the end of RCU grace period when shutting down tracing. Luckily
that is rare enough event that we can afford that. Note that postponing
the freeing of blk_trace to an RCU callback should better be avoided as
it could have unexpected user visible side-effects as debugfs files
would be still existing for a short while block tracing has been shut
down.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=205711
CC: stable@vger.kernel.org
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reported-by: Tristan Madani <tristmd@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 13 insertions, 5 deletions

diff --git a/include/linux/blktrace_api.h b/include/linux/blktrace_api.h
index 7bb2d8de9f30..3b6ff5902edc 100644
--- a/include/linux/blktrace_api.h
+++ b/include/linux/blktrace_api.h
@@ -51,9 +51,13 @@ void __trace_note_message(struct blk_trace *, struct blkcg *blkcg, const char *f
  **/
 #define blk_add_cgroup_trace_msg(q, cg, fmt, ...)			\
 	do {								\
-		struct blk_trace *bt = (q)->blk_trace;			\
+		struct blk_trace *bt;					\
+									\
+		rcu_read_lock();					\
+		bt = rcu_dereference((q)->blk_trace);			\
 		if (unlikely(bt))					\
 			__trace_note_message(bt, cg, fmt, ##__VA_ARGS__);\
+		rcu_read_unlock();					\
 	} while (0)
 #define blk_add_trace_msg(q, fmt, ...)					\
 	blk_add_cgroup_trace_msg(q, NULL, fmt, ##__VA_ARGS__)
@@ -61,10 +65,14 @@ void __trace_note_message(struct blk_trace *, struct blkcg *blkcg, const char *f
 
 static inline bool blk_trace_note_message_enabled(struct request_queue *q)
 {
-	struct blk_trace *bt = q->blk_trace;
-	if (likely(!bt))
-		return false;
-	return bt->act_mask & BLK_TC_NOTIFY;
+	struct blk_trace *bt;
+	bool ret;
+
+	rcu_read_lock();
+	bt = rcu_dereference(q->blk_trace);
+	ret = bt && (bt->act_mask & BLK_TC_NOTIFY);
+	rcu_read_unlock();
+	return ret;
 }
 
 extern void blk_add_driver_data(struct request_queue *q, struct request *rq,