ALSA: line6: reject oversized playback packets

In the Linux kernel, the following vulnerability has been resolved:

ALSA: line6: reject oversized playback packets

Each playback URB is handed a slice of out.buffer that is exactly
LINE6_ISO_PACKETS * max_packet_size_out bytes, sized from the OUT
endpoint, but the number of bytes written into that slice is never
compared against it.

submit_audio_out_urb() takes the frame count from prev_fsize, which
audio_in_callback() derived from the *IN* endpoint's received packet
length, and rescales it with the playback frame size; when prev_fsize is
still zero it synthesizes a length from the sample rate instead. On a
device declaring a large IN wMaxPacketSize and a small OUT
wMaxPacketSize, the resulting memcpy(), or the memset() when the
playback stream is idle, runs past its slice and, as the reproducer
below shows, beyond the allocation.

usb_submit_urb() is not a backstop. max_packet_size_out comes from
usb_maxpacket(), which returns only the low 11 bits of wMaxPacketSize,
while USB core validates a high-speed isochronous length against that
base scaled by usb_endpoint_maxp_mult(). A length of up to three times
the allocated slice is therefore accepted, and even a rejected URB is
only rejected after the write.

Reject a packet that does not fit the slice the driver allocated for it.
Clamping it instead would silently shorten the capture-derived rate
feedback and desynchronize the two streams.

BUG: KASAN: slab-out-of-bounds in submit_audio_out_urb (sound/usb/line6/playback.c:229)
Write of size 1024 at addr ffff888100bbd400 by task kworker/1:2/5002
Workqueue: events line6_startup_work
Call Trace:
__asan_memcpy (mm/kasan/shadow.c:106)
submit_audio_out_urb (sound/usb/line6/playback.c:229)
line6_submit_audio_out_all_urbs (sound/usb/line6/playback.c:291)
line6_stream_start (sound/usb/line6/pcm.c:194)
line6_pcm_acquire (sound/usb/line6/pcm.c:337)
line6_startup_work (sound/usb/line6/driver.c:728)
process_one_work (kernel/workqueue.c:3396)
worker_thread (kernel/workqueue.c:3479 kernel/workqueue.c:3560)
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)

The buggy address belongs to the object at ffff888100bbd400
which belongs to the cache kmalloc-256 of size 256
The buggy address is located 0 bytes inside of
allocated 256-byte region [ffff888100bbd400, ffff888100bbd500)