tracing: Move mutex to protect against resetting of seq data

commit 1245800c upstream.

The iter->seq can be reset outside the protection of the mutex. So can
reading of user data. Move the mutex up to the beginning of the function.

Fixes: d7350c3f ("tracing/core: make the read callbacks reentrants")
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/trace/trace.c

@@ -3331,13 +3331,6 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 	static struct tracer *old_tracer;
 	ssize_t sret;
 
-	/* return any leftover data */
-	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
-	if (sret != -EBUSY)
-		return sret;
-
-	trace_seq_init(&iter->seq);
-
 	/* copy the tracer to avoid using a global lock all around */
 	mutex_lock(&trace_types_lock);
 	if (unlikely(old_tracer != current_trace && current_trace)) {
@@ -3352,6 +3345,14 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 	 * is protected.
 	 */
 	mutex_lock(&iter->mutex);
+
+	/* return any leftover data */
+	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
+	if (sret != -EBUSY)
+		goto out;
+
+	trace_seq_init(&iter->seq);
+
 	if (iter->trace->read) {
 		sret = iter->trace->read(iter, filp, ubuf, cnt, ppos);
 		if (sret)