Code: static ssize_t f_hidg_write(struct file *file, const char __user *buffer,
			    size_t count, loff_t *offp)
{
	struct f_hidg *hidg  = file->private_data;
	struct usb_request *req;
	unsigned long flags;
	ssize_t status = -ENOMEM;

	if (!access_ok(buffer, count))
		return -EFAULT;

	spin_lock_irqsave(&hidg->write_spinlock, flags);

#define WRITE_COND (!hidg->write_pending)
try_again:
	/* write queue */
	while (!WRITE_COND) {
		spin_unlock_irqrestore(&hidg->write_spinlock, flags);
		if (file->f_flags & O_NONBLOCK)
			return -EAGAIN;

		if (wait_event_interruptible_exclusive(
				hidg->write_queue, WRITE_COND))
			return -ERESTARTSYS;

		spin_lock_irqsave(&hidg->write_spinlock, flags);
	}

	hidg->write_pending = 1;
	req = hidg->req;
	count  = min_t(unsigned, count, hidg->report_length);

	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
	status = copy_from_user(req->buf, buffer, count);

	if (status != 0) {
		ERROR(hidg->func.config->cdev,
			"copy_from_user error\n");
		status = -EINVAL;
		goto release_write_pending;
	}

	spin_lock_irqsave(&hidg->write_spinlock, flags);

	/* when our function has been disabled by host */
	if (!hidg->req) {
		free_ep_req(hidg->in_ep, req);
		/*
		 * TODO
		 * Should we fail with error here?
		 */
		goto try_again;
	}

	req->status   = 0;
	req->zero     = 0;
	req->length   = count;
 	req->complete = f_hidg_req_complete;
 	req->context  = hidg;
 
 	status = usb_ep_queue(hidg->in_ep, req, GFP_ATOMIC);
 	if (status < 0) {
 		ERROR(hidg->func.config->cdev,
 			"usb_ep_queue error on int endpoint %zd\n", status);
		goto release_write_pending_unlocked;
 	} else {
 		status = count;
 	}
	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
 
 	return status;
 release_write_pending:
 	spin_lock_irqsave(&hidg->write_spinlock, flags);
release_write_pending_unlocked:
 	hidg->write_pending = 0;
 	spin_unlock_irqrestore(&hidg->write_spinlock, flags);
 
	wake_up(&hidg->write_queue);

	return status;
}

Vulnerability Type: 
CWE ID: CWE-189
Summary: In the Linux kernel before 4.16.4, a double-locking error in drivers/usb/dwc3/gadget.c may potentially cause a deadlock with f_hid.
Commit Message: USB: gadget: f_hid: fix deadlock in f_hidg_write()

In f_hidg_write() the write_spinlock is acquired before calling
usb_ep_queue() which causes a deadlock when dummy_hcd is being used.
This is because dummy_queue() callbacks into f_hidg_req_complete() which
tries to acquire the same spinlock. This is (part of) the backtrace when
the deadlock occurs:

  0xffffffffc06b1410 in f_hidg_req_complete
  0xffffffffc06a590a in usb_gadget_giveback_request
  0xffffffffc06cfff2 in dummy_queue
  0xffffffffc06a4b96 in usb_ep_queue
  0xffffffffc06b1eb6 in f_hidg_write
  0xffffffff8127730b in __vfs_write
  0xffffffff812774d1 in vfs_write
  0xffffffff81277725 in SYSC_write

Fix this by releasing the write_spinlock before calling usb_ep_queue()

Reviewed-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Tested-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: stable@vger.kernel.org # 4.11+
Fixes: 749494b6bdbb ("usb: gadget: f_hid: fix: Move IN request allocation to set_alt()")
Signed-off-by: Radoslav Gerganov <rgerganov@vmware.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>