[PATCH AUTOSEL 4.9 6/7] ALSA: line6: Fix endless MIDI read loop

[Sasha Levin]

From: Takashi Iwai <tiwai at suse.de>

[ Upstream commit d683469b3c93d7e2afd39e6e1970f24700eb7a68 ]

The MIDI input event parser of the LINE6 driver may enter into an
endless loop when the unexpected data sequence is given, as it tries
to continue the secondary bytes without termination.  Also, when the
input data is too short, the parser returns a negative error, while
the caller doesn't handle it properly.  This would lead to the
unexpected behavior as well.

This patch addresses those issues by checking the return value
correctly and handling the one-byte event in the parser properly.

The bug was reported by syzkaller.

Reported-by: syzbot+cce32521ee0a824c21f7 at syzkaller.appspotmail.com
Cc: <stable at vger.kernel.org>
Link: https://lore.kernel.org/r/000000000000033087059f8f8fa3@google.com
Link: https://lore.kernel.org/r/20200309095922.30269-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai at suse.de>
Signed-off-by: Sasha Levin <sashal at kernel.org>
---
 sound/usb/line6/driver.c  | 2 +-
 sound/usb/line6/midibuf.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/sound/usb/line6/driver.c b/sound/usb/line6/driver.c
index 09189249d0d19..ea3a9bd05e687 100644
--- a/sound/usb/line6/driver.c
+++ b/sound/usb/line6/driver.c
@@ -306,7 +306,7 @@ static void line6_data_received(struct urb *urb)
 				line6_midibuf_read(mb, line6->buffer_message,
 						LINE6_MIDI_MESSAGE_MAXLEN);
 
-			if (done == 0)
+			if (done <= 0)
 				break;
 
 			line6->message_length = done;
diff --git a/sound/usb/line6/midibuf.c b/sound/usb/line6/midibuf.c
index 36a610ba342ec..c931d48801ebe 100644
--- a/sound/usb/line6/midibuf.c
+++ b/sound/usb/line6/midibuf.c
@@ -163,7 +163,7 @@ int line6_midibuf_read(struct midi_buffer *this, unsigned char *data,
 			int midi_length_prev =
 			    midibuf_message_length(this->command_prev);
 
-			if (midi_length_prev > 0) {
+			if (midi_length_prev > 1) {
 				midi_length = midi_length_prev - 1;
 				repeat = 1;
 			} else
-- 
2.20.1