cifs: fix dentry refcount leak when opening a FIFO on lookup

The cifs code will attempt to open files on lookup under certain
circumstances. What happens though if we find that the file we opened
was actually a FIFO or other special file?

Currently, the open filehandle just ends up being leaked leading to
a dentry refcount mismatch and oops on umount. Fix this by having the
code close the filehandle on the server if it turns out not to be a
regular file. While we're at it, change this spaghetti if statement
into a switch too.

Cc: stable@vger.kernel.org
Reported-by: CAI Qian <caiqian@redhat.com>
Tested-by: CAI Qian <caiqian@redhat.com>
Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/dir.c

@@ -584,10 +584,26 @@ cifs_lookup(struct inode *parent_dir_inode, struct dentry *direntry,
 			 * If either that or op not supported returned, follow
 			 * the normal lookup.
 			 */
-			if ((rc == 0) || (rc == -ENOENT))
+			switch (rc) {
+			case 0:
+				/*
+				 * The server may allow us to open things like
+				 * FIFOs, but the client isn't set up to deal
+				 * with that. If it's not a regular file, just
+				 * close it and proceed as if it were a normal
+				 * lookup.
+				 */
+				if (newInode && !S_ISREG(newInode->i_mode)) {
+					CIFSSMBClose(xid, pTcon, fileHandle);
+					break;
+				}
+			case -ENOENT:
 				posix_open = true;
-			else if ((rc == -EINVAL) || (rc != -EOPNOTSUPP))
+			case -EOPNOTSUPP:
+				break;
+			default:
 				pTcon->broken_posix_open = true;
+			}
 		}
 		if (!posix_open)
 			rc = cifs_get_inode_info_unix(&newInode, full_path,