[PATCH] ALSA: usb-audio: Fix out-of-bound error

[Jaejoong Kim <climbbb.kim@gmail.com>]

The snd_usb_copy_string_desc() retrieves the usb string corresponding to
the index number thought the usb_string(). And for NULL-terminated, insert
'0' by using the return value of usb_string() as the index of buffer to
hold the string.

The problem is that the usb_string() also returns the length of the string
read(>= 0), but it can also return a negative value, the error or status
value of usb_control_msg(). If iClockSource is '0' as shown below,
usb_string() will return -EINVAL. This will result in '0' being inserted
into buf[-22], and the following KASAN out-of-bound error message will be
output.

AudioControl Interface Descriptor:
  bLength                 8
  bDescriptorType        36
  bDescriptorSubtype     10 (CLOCK_SOURCE)
  bClockID                1
  bmAttributes         0x07 Internal programmable Clock (synced to SOF)
  bmControls           0x07
  Clock Frequency Control (read/write)
  Clock Validity Control (read-only)
  bAssocTerminal          0
  iClockSource            0

To fix out-of-bound error, insert 0 only if the return value of
usb_string() is greater than 0.

==================================================================
BUG: KASAN: stack-out-of-bounds in parse_audio_unit+0x1327/0x1960 [snd_usb_audio]
Write of size 1 at addr ffff88007e66735a by task systemd-udevd/18376

CPU: 0 PID: 18376 Comm: systemd-udevd Not tainted 4.13.0+ #3
Hardware name: LG Electronics                   15N540-RFLGL/White Tip Mountain, BIOS 15N5
Call Trace:
dump_stack+0x63/0x8d
print_address_description+0x70/0x290
? parse_audio_unit+0x1327/0x1960 [snd_usb_audio]
kasan_report+0x265/0x350
__asan_store1+0x4a/0x50
parse_audio_unit+0x1327/0x1960 [snd_usb_audio]
? save_stack+0xb5/0xd0
? save_stack_trace+0x1b/0x20
? save_stack+0x46/0xd0
? kasan_kmalloc+0xad/0xe0
? kmem_cache_alloc_trace+0xff/0x230
? snd_usb_create_mixer+0xb0/0x4b0 [snd_usb_audio]
? usb_audio_probe+0x4de/0xf40 [snd_usb_audio]
? usb_probe_interface+0x1f5/0x440
? driver_probe_device+0x3ed/0x660
? build_feature_ctl+0xb10/0xb10 [snd_usb_audio]
? save_stack_trace+0x1b/0x20
? init_object+0x69/0xa0
? snd_usb_find_csint_desc+0xa8/0xf0 [snd_usb_audio]
snd_usb_mixer_controls+0x1dc/0x370 [snd_usb_audio]
? build_audio_procunit+0x890/0x890 [snd_usb_audio]
? snd_usb_create_mixer+0xb0/0x4b0 [snd_usb_audio]
? kmem_cache_alloc_trace+0xff/0x230
? usb_ifnum_to_if+0xbd/0xf0
snd_usb_create_mixer+0x25b/0x4b0 [snd_usb_audio]
? snd_usb_create_stream+0x255/0x2c0 [snd_usb_audio]
usb_audio_probe+0x4de/0xf40 [snd_usb_audio]
? snd_usb_autosuspend.part.7+0x30/0x30 [snd_usb_audio]
? __pm_runtime_idle+0x90/0x90
? kernfs_activate+0xa6/0xc0
? usb_match_one_id_intf+0xdc/0x130
? __pm_runtime_set_status+0x2d4/0x450
usb_probe_interface+0x1f5/0x440

Cc: <stable@vger.kernel.org>
Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
---

The AudioControl Interface Descriptor in commit message is from
lsusb output with real usb audio DAC.

The usb audio product causing the OOB are as follows:
http://www.lg.com/uk/lg-friends/lg-AFD-1200

It only prints OOB error and usb audio works well. :)

 sound/usb/mixer.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
index e630813..5a83c2c 100644
--- a/sound/usb/mixer.c
+++ b/sound/usb/mixer.c
@@ -204,7 +204,8 @@ static int snd_usb_copy_string_desc(struct mixer_build *state,
 				    int index, char *buf, int maxlen)
 {
 	int len = usb_string(state->chip->dev, index, buf, maxlen - 1);
-	buf[len] = 0;
+	if (len > 0)
+		buf[len] = 0;
 	return len;
 }
 
-- 
2.7.4