Code: static long vbg_misc_device_ioctl(struct file *filp, unsigned int req,
				  unsigned long arg)
{
	struct vbg_session *session = filp->private_data;
	size_t returned_size, size;
	struct vbg_ioctl_hdr hdr;
	bool is_vmmdev_req;
	int ret = 0;
	void *buf;

	if (copy_from_user(&hdr, (void *)arg, sizeof(hdr)))
		return -EFAULT;

	if (hdr.version != VBG_IOCTL_HDR_VERSION)
		return -EINVAL;

	if (hdr.size_in < sizeof(hdr) ||
	    (hdr.size_out && hdr.size_out < sizeof(hdr)))
		return -EINVAL;

	size = max(hdr.size_in, hdr.size_out);
	if (_IOC_SIZE(req) && _IOC_SIZE(req) != size)
		return -EINVAL;
	if (size > SZ_16M)
		return -E2BIG;

	/*
	 * IOCTL_VMMDEV_REQUEST needs the buffer to be below 4G to avoid
	 * the need for a bounce-buffer and another copy later on.
	 */
	is_vmmdev_req = (req & ~IOCSIZE_MASK) == VBG_IOCTL_VMMDEV_REQUEST(0) ||
			 req == VBG_IOCTL_VMMDEV_REQUEST_BIG;

	if (is_vmmdev_req)
		buf = vbg_req_alloc(size, VBG_IOCTL_HDR_TYPE_DEFAULT);
	else
		buf = kmalloc(size, GFP_KERNEL);
 	if (!buf)
 		return -ENOMEM;
 
	if (copy_from_user(buf, (void *)arg, hdr.size_in)) {
 		ret = -EFAULT;
 		goto out;
 	}
	if (hdr.size_in < size)
		memset(buf + hdr.size_in, 0, size -  hdr.size_in);

	ret = vbg_core_ioctl(session, req, buf);
	if (ret)
		goto out;

	returned_size = ((struct vbg_ioctl_hdr *)buf)->size_out;
	if (returned_size > size) {
		vbg_debug("%s: too much output data %zu > %zu\n",
			  __func__, returned_size, size);
		returned_size = size;
	}
	if (copy_to_user((void *)arg, buf, returned_size) != 0)
		ret = -EFAULT;

out:
	if (is_vmmdev_req)
		vbg_req_free(buf, size);
	else
		kfree(buf);

	return ret;
}

Vulnerability Type: DoS +Info 
CWE ID: CWE-362
Summary: An issue was discovered in the Linux kernel through 4.17.2. vbg_misc_device_ioctl() in drivers/virt/vboxguest/vboxguest_linux.c reads the same user data twice with copy_from_user. The header part of the user data is double-fetched, and a malicious user thread can tamper with the critical variables (hdr.size_in and hdr.size_out) in the header between the two fetches because of a race condition, leading to severe kernel errors, such as buffer over-accesses. This bug can cause a local denial of service and information leakage.
Commit Message: virt: vbox: Only copy_from_user the request-header once

In vbg_misc_device_ioctl(), the header of the ioctl argument is copied from
the userspace pointer 'arg' and saved to the kernel object 'hdr'. Then the
'version', 'size_in', and 'size_out' fields of 'hdr' are verified.

Before this commit, after the checks a buffer for the entire request would
be allocated and then all data including the verified header would be
copied from the userspace 'arg' pointer again.

Given that the 'arg' pointer resides in userspace, a malicious userspace
process can race to change the data pointed to by 'arg' between the two
copies. By doing so, the user can bypass the verifications on the ioctl
argument.

This commit fixes this by using the already checked copy of the header
to fill the header part of the allocated buffer and only copying the
remainder of the data from userspace.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>