From patchwork Mon Nov 15 02:24:28 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ian Kent X-Patchwork-Id: 12618573 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 72003C433F5 for ; Mon, 15 Nov 2021 02:25:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 55DA260D42 for ; Mon, 15 Nov 2021 02:25:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230395AbhKOC1w (ORCPT ); Sun, 14 Nov 2021 21:27:52 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33512 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229686AbhKOC1q (ORCPT ); Sun, 14 Nov 2021 21:27:46 -0500 Received: from smtp02.aussiebb.com.au (smtp02.aussiebb.com.au [IPv6:2403:5800:3:25::1002]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A4C63C061746; Sun, 14 Nov 2021 18:24:46 -0800 (PST) Received: from localhost (localhost.localdomain [127.0.0.1]) by smtp02.aussiebb.com.au (Postfix) with ESMTP id D3C1D102D82; Mon, 15 Nov 2021 13:24:29 +1100 (AEDT) X-Virus-Scanned: Debian amavisd-new at smtp02.aussiebb.com.au Received: from smtp02.aussiebb.com.au ([127.0.0.1]) by localhost (smtp02.aussiebb.com.au [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id UqFNLGMF9cvO; Mon, 15 Nov 2021 13:24:29 +1100 (AEDT) Received: by smtp02.aussiebb.com.au (Postfix, from userid 116) id B36D3102D70; Mon, 15 Nov 2021 13:24:29 +1100 (AEDT) Received: from mickey.themaw.net (unknown [100.72.131.210]) by smtp02.aussiebb.com.au (Postfix) with ESMTP id 3E3B6102D70; Mon, 15 Nov 2021 13:24:28 +1100 (AEDT) Subject: [PATCH 2 2/2] xfs: make sure link path does not go away at access From: Ian Kent To: xfs , "Darrick J. Wong" , Christoph Hellwig Cc: Miklos Szeredi , Brian Foster , Al Viro , David Howells , linux-fsdevel , Kernel Mailing List Date: Mon, 15 Nov 2021 10:24:28 +0800 Message-ID: <163694306800.229789.11812765289669370510.stgit@mickey.themaw.net> In-Reply-To: <163694289979.229789.1176392639284347792.stgit@mickey.themaw.net> References: <163694289979.229789.1176392639284347792.stgit@mickey.themaw.net> User-Agent: StGit/0.23 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org When following an inline symlink in rcu-walk mode it's possible to succeed in getting the ->get_link() method pointer but the link path string be deallocated while it's being used. This is becuase of the xfs inode reclaim mechanism. While rcu freeing the link path can prevent it from being freed during use the inode reclaim could assign a new value to the field at any time outside of the path walk and result in an invalid link path pointer being returned. Admittedly a very small race window but possible. The best way to mitigate this risk is to return -ECHILD to the VFS if the inline symlink method, ->get_link(), is called in rcu-walk mode so the VFS can switch to ref-walk mode or redo the walk if the inode has become invalid. If it's discovered that staying in rcu-walk mode gives a worth while performance improvement (unlikely) then the link path could be freed under rcu once potential side effects of the xfs inode reclaim sub-system have been analysed and dealt with if needed. Signed-off-by: Ian Kent Reviewed-by: Brian Foster --- fs/xfs/xfs_iops.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c index a607d6aca5c4..0a96183c5381 100644 --- a/fs/xfs/xfs_iops.c +++ b/fs/xfs/xfs_iops.c @@ -520,6 +520,9 @@ xfs_vn_get_link_inline( struct xfs_inode *ip = XFS_I(inode); char *link; + if (!dentry) + return ERR_PTR(-ECHILD); + ASSERT(ip->i_df.if_format == XFS_DINODE_FMT_LOCAL); /*