From mboxrd@z Thu Jan 1 00:00:00 1970 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 31FEBC04A68 for ; Wed, 27 Jul 2022 11:34:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231983AbiG0LeS (ORCPT ); Wed, 27 Jul 2022 07:34:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:50606 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229604AbiG0LeP (ORCPT ); Wed, 27 Jul 2022 07:34:15 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E892B13CDE; Wed, 27 Jul 2022 04:34:14 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 8FF94B82022; Wed, 27 Jul 2022 11:34:13 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8D119C433C1; Wed, 27 Jul 2022 11:34:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1658921652; bh=db3UoE/JVeKIkJbNmRDPqnol/NodJCjMUZp8vPWicCs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=iqBNMAw4d8h6D2FW8pNMl/xzkWqHEMCuV4bviEGmRGIT4dJ5u4m5B77JxMTp0zYwm Gtrq8OUXh6pYU0QtNfK4gEEOWKBPa3Z2XD5iR4Eknd7/yviaLd2NAFsTD+5mb8ifY6 GeJFS4qjfrmv92fFRtlx+qu48rTzv0+RAq3cD4hFAwpIWWjx2gIrW7SrWdgEKgEw/x vOJL+XTk4y2VyyrZk++kqwTei9MCpBEdJOHAFhyv3AMCOSnktC5R/PV62sRb8b6uCs loqefTIB6JbzdZbTeEwyxRsIIUxXbjQ3C84er3lkzLctezVdAxMri2l5dlPmocv2BA KgIDl2URQW1mQ== Date: Wed, 27 Jul 2022 13:34:06 +0200 From: Christian Brauner To: Jeff Layton Cc: viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org, Yongchen Yang Subject: Re: [RFC PATCH] vfs: don't check may_create_in_sticky if the file is already open/created Message-ID: <20220727113406.ewu4kzsoo643cf66@wittgenstein> References: <20220726202333.165490-1-jlayton@kernel.org> <8e4d498a3e8ed80ada2d3da01e7503e082be31a3.camel@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <8e4d498a3e8ed80ada2d3da01e7503e082be31a3.camel@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 26, 2022 at 04:27:56PM -0400, Jeff Layton wrote: > On Tue, 2022-07-26 at 16:23 -0400, Jeff Layton wrote: > > NFS server is exporting a sticky directory (mode 01777) with root > > squashing enabled. Client has protect_regular enabled and then tries to > > open a file as root in that directory. File is created (with ownership > > set to nobody:nobody) but the open syscall returns an error. > > > > The problem is may_create_in_sticky, which rejects the open even though > > the file has already been created/opened. Only call may_create_in_sticky > > if the file hasn't already been opened or created. > > > > Cc: Christian Brauner > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829 > > Reported-by: Yongchen Yang > > Signed-off-by: Jeff Layton > > --- > > fs/namei.c | 13 +++++++++---- > > 1 file changed, 9 insertions(+), 4 deletions(-) > > > > diff --git a/fs/namei.c b/fs/namei.c > > index 1f28d3f463c3..7480b6dc8d27 100644 > > --- a/fs/namei.c > > +++ b/fs/namei.c > > @@ -3495,10 +3495,15 @@ static int do_open(struct nameidata *nd, > > return -EEXIST; > > if (d_is_dir(nd->path.dentry)) > > return -EISDIR; > > - error = may_create_in_sticky(mnt_userns, nd, > > - d_backing_inode(nd->path.dentry)); > > - if (unlikely(error)) > > - return error; > > + if (!(file->f_mode & (FMODE_OPENED | FMODE_CREATED))) { > > + error = may_create_in_sticky(mnt_userns, nd, > > + d_backing_inode(nd->path.dentry)); > > + if (unlikely(error)) { > > + printk("%s: f_mode=0x%x oflag=0x%x\n", > > + __func__, file->f_mode, open_flag); > > + return error; > > + } > > + } > > } > > if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry)) > > return -ENOTDIR; > > I'm pretty sure this patch is the wrong approach, actually, since it > doesn't fix the regular (non-atomic) open codepath. Any thoughts on what Hey Jeff, I haven't quite understood why that won't work for the regular open codepaths. I'm probably missing something obvious. > the right fix might be? When an actual creation has taken place - and not just a lookup - then may_create_in_sticky() assumes that the owner of the inode matches current_fsuid(). That'd would also be problematic on fat or in fact on any fs where the actual inode->i_{g,u}id are based on e.g. uid/gid mount options and not on current_fsuid(), I think? So in order to improve this we would need to work around that assumption in some way. Either by skipping may_create_in_sticky() if the file got created or by adapting may_create_in_sticky(). I only wonder whether skipping may_create_in_sticky() altogether might be a bit too lax. One possibility that came to my mind might be to relax this assumption when the file has been created and the creator has CAP_FOWNER. So (not compile tested or in any way) sm like: diff --git a/fs/namei.c b/fs/namei.c index 1f28d3f463c3..239e9f423346 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1221,7 +1221,8 @@ int may_linkat(struct user_namespace *mnt_userns, struct path *link) * Returns 0 if the open is allowed, -ve on error. */ static int may_create_in_sticky(struct user_namespace *mnt_userns, - struct nameidata *nd, struct inode *const inode) + struct nameidata *nd, struct inode *const inode, + bool created) { umode_t dir_mode = nd->dir_mode; kuid_t dir_uid = nd->dir_uid; @@ -1230,7 +1231,9 @@ static int may_create_in_sticky(struct user_namespace *mnt_userns, (!sysctl_protected_regular && S_ISREG(inode->i_mode)) || likely(!(dir_mode & S_ISVTX)) || uid_eq(i_uid_into_mnt(mnt_userns, inode), dir_uid) || - uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode))) + uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)) || + (created && + capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FOWNER))) return 0; if (likely(dir_mode & 0002) || @@ -3496,7 +3499,8 @@ static int do_open(struct nameidata *nd, if (d_is_dir(nd->path.dentry)) return -EISDIR; error = may_create_in_sticky(mnt_userns, nd, - d_backing_inode(nd->path.dentry)); + d_backing_inode(nd->path.dentry), + (file->f_mode & FMODE_CREATED)); if (unlikely(error)) return error; }