Re: [PATCH v3 2/2] ALSA: usx2y: Guard the playback copy

[Sinan Turunc <enscrpt@pm.me>]

> If actual_length has a non-zero remainder modulo usx2y->stride, can this
> still read past the end of subs->tmpbuf or runtime->dma_area?

Example 1 (a descriptor of 203 bytes on a 200-byte endpoint) does not
reach the host controller: usb_submit_urb() compares every ISO
descriptor length with the endpoint maximum and returns -EMSGSIZE
(drivers/usb/core/urb.c, the number_of_packets loop), and for the
full-speed devices this driver serves that maximum is maxpacksize. The
driver then stops the streams on the submit error.

Example 2 is valid and pre-existing. The descriptors are filled from the
raw actual_length while transfer_buffer_length and both copies use
count * stride, so a capture packet that is not a whole number of frames
makes the descriptors add up to more than the buffer, by at most
stride - 1 bytes per packet. The excess is read by the host controller
and sent to the device; the driver itself copies only whole frames.

v4 sizes each descriptor as counts * stride, so the descriptors add up
to transfer_buffer_length and, with the maxpacksize check, each stays
within the endpoint maximum, which also removes example 1 without
relying on the core check.

I went through every value in this file's data paths that the device
controls (packet status, capture and playback actual_length, start_frame
and wMaxPacketSize) and where each is bounded. The descriptor length
was the only one left unchecked after v3. With an emulated device
sending 195-byte capture packets, the v3 descriptors add up to 780
bytes for 768 bytes of data and USB/IP's completion check refuses the
URB; with v4 every descriptor is 192 bytes and the stream runs.

Thanks,
Sinan