vfs: read file_handle only once in handle_to_path

We used to read file_handle twice.  Once to get the amount of extra
bytes, and once to fetch the entire structure.

This may be problematic since we do size verifications only after the
first read, so if the number of extra bytes changes in userspace between
the first and second calls, we'll have an incoherent view of
file_handle.

Instead, read the constant size once, and copy that over to the final
structure without having to re-read it again.

Git-commit: 161f873b89136eb1e69477c847d5a5033239d9ba
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git

Change-Id: I5d7a076e0345004e436f5496272e826e8e6d9ef2
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Zhoulu Luo <zluo@codeaurora.org>

fs/fhandle.c

@@ -198,8 +198,9 @@ static int handle_to_path(int mountdirfd, struct file_handle __user *ufh,
 		goto out_err;
 	}
 	/* copy the full handle */
-	if (copy_from_user(handle, ufh,
-			   sizeof(struct file_handle) +
+	*handle = f_handle;
+	if (copy_from_user(&handle->f_handle,
+			   &ufh->f_handle,
 			   f_handle.handle_bytes)) {
 		retval = -EFAULT;
 		goto out_handle;