[RFC PATCH] sound: line6: Fix race condition in line6_probe

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

syzbot reported general protection fault in midibuf_is_full.
the cause is linemidi pointer in struct usb_line6 isn't properly
initialized.

the pointer isn't initialized because there is race condition
in line6_probe. it calls line6_init_cap_control first, which submits urb.
and then it initializes it's data using private_init function.

so it's possible line6_data_received is called before it's
data isn't initialized.

Link: https://lkml.org/lkml/2021/5/17/543
Reported-by: syzbot+0d2b3feb0a2887862e06@syzkallerlkml..appspotmail.com
Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
---
 sound/usb/line6/driver.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/sound/usb/line6/driver.c b/sound/usb/line6/driver.c
index a030dd65eb28..2c183a2a30f0 100644
--- a/sound/usb/line6/driver.c
+++ b/sound/usb/line6/driver.c
@@ -788,17 +788,17 @@ int line6_probe(struct usb_interface *interface,
 
 	line6_get_usb_properties(line6);
 
+	/* initialize device data based on device: */
+	ret = private_init(line6, id);
+	if (ret < 0)
+		goto error;
+
 	if (properties->capabilities & LINE6_CAP_CONTROL) {
 		ret = line6_init_cap_control(line6);
 		if (ret < 0)
 			goto error;
 	}
 
-	/* initialize device data based on device: */
-	ret = private_init(line6, id);
-	if (ret < 0)
-		goto error;
-
 	/* creation of additional special files should go here */
 
 	dev_info(&interface->dev, "Line 6 %s now attached\n",
-- 
2.25.1