cifs: fix panic in smb2_reconnect

RH Bugzilla: 1702264

We need to protect so that the call to smb2_reconnect() in
smb2_reconnect_server() does not end up freeing the session
because it can lead to a use after free and crash.
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>

fs/cifs/smb2pdu.c

@@ -3114,9 +3114,14 @@ void smb2_reconnect_server(struct work_struct *work)
 				tcon_exist = true;
 			}
 		}
+		/*
+		 * IPC has the same lifetime as its session and uses its
+		 * refcount.
+		 */
 		if (ses->tcon_ipc && ses->tcon_ipc->need_reconnect) {
 			list_add_tail(&ses->tcon_ipc->rlist, &tmp_list);
 			tcon_exist = true;
+			ses->ses_count++;
 		}
 	}
 	/*
@@ -3135,7 +3140,10 @@ void smb2_reconnect_server(struct work_struct *work)
 		else
 			resched = true;
 		list_del_init(&tcon->rlist);
-		cifs_put_tcon(tcon);
+		if (tcon->ipc)
+			cifs_put_smb_ses(tcon->ses);
+		else
+			cifs_put_tcon(tcon);
 	}
 
 	cifs_dbg(FYI, "Reconnecting tcons finished\n");