ALSA: FCP: do not copy out an uninitialised init response

fcp_ioctl_init() allocates its response buffer with kmalloc() and copies
the whole buffer back to userspace:

	buf_size = init.step0_resp_size + init.step2_resp_size;

	void *resp __free(kfree) =
		kmalloc(buf_size, GFP_KERNEL);
	...
	if (copy_to_user(arg->resp, resp, buf_size))
		return -EFAULT;

Nothing clears the buffer, and the only writer of its leading
step0_resp_size bytes is the step-0 control transfer:

	err = snd_usb_ctl_msg(dev, usb_rcvctrlpipe(dev, 0),
		FCP_USB_REQ_STEP0,
		USB_RECIP_INTERFACE | USB_TYPE_CLASS | USB_DIR_IN,
		0, private->bInterfaceNumber,
		step0_resp, private->step0_resp_size);
	if (err < 0)
		return err;

usb_fill_control_urb() does not set URB_SHORT_NOT_OK, so a short or
zero-length data stage completes with status 0 and snd_usb_ctl_msg()
returns a small actual_length.  The only check is err < 0, so a short
transfer is accepted as success.

snd_usb_ctl_msg() copies the full size back unconditionally:

	buf = kmemdup(data, size, GFP_KERNEL);
	...
	memcpy(data, buf, size);

Bytes the device never wrote are therefore restored into resp unchanged
and copied to userspace.  step0_resp_size and step2_resp_size are each
validated only to 1..255, so the caller also picks the slab cache, from
kmalloc-8 up to kmalloc-512.

On 7.2.0-rc5 (arm64), device answering step 0 with a zero-length data
stage, s0 = s2 = 255:

  # init_on_alloc off, no spray
  step0 window [0,255): nonzero=94/255
  000: 00 80 60 06 00 00 ff ff 18 00 00 00 57 01 ea 01
  010: 08 78 22 13 00 00 ff ff a8 c4 5f 80 00 80 ff ff

  # same kernel, kmalloc-512 pre-seeded with an 8-byte tag
  step0 window [0,255): nonzero=219/255  tagbytes=232

  # identical run, init_on_alloc=1
  step0 window [0,255): nonzero=0/255  tagbytes=0

  # all three runs
  step2 window [255,510): device words matched=62/62

a8 c4 5f 80 00 80 ff ff is the little-endian kernel text address
ffff8000805fc4a8.  The step-2 window is unaffected, so the disclosure is
exactly the step-0 region.

Zero the buffer, and require the step-0 transfer to deliver the full
step0_resp_size bytes so a short data stage is reported as an error.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 46757a3e7d50 ("ALSA: FCP: Add Focusrite Control Protocol driver")
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
Link: https://patch.msgid.link/20260805013804.38839-1-baul.lee@xbow.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>