LoadPin: Return EFAULT on copy_from_user() failures

The copy_from_user() function returns the number of bytes remaining to
be copied on a failure. Such failures should return -EFAULT to high
levels.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3f805f8 ("LoadPin: Enable loading from trusted dm-verity devices")
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

security/loadpin/loadpin.c

@@ -356,13 +356,11 @@ static long dm_verity_ioctl(struct file *filp, unsigned int cmd, unsigned long a
 {
 	void __user *uarg = (void __user *)arg;
 	unsigned int fd;
-	int rc;
 
 	switch (cmd) {
 	case LOADPIN_IOC_SET_TRUSTED_VERITY_DIGESTS:
-		rc = copy_from_user(&fd, uarg, sizeof(fd));
-		if (rc)
-			return rc;
+		if (copy_from_user(&fd, uarg, sizeof(fd)))
+			return -EFAULT;
 
 		return read_trusted_verity_root_digests(fd);