Affected: Linux

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

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_a…