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 9C5EBC433EF for ; Tue, 10 May 2022 15:13:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345449AbiEJPRg (ORCPT ); Tue, 10 May 2022 11:17:36 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42574 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345610AbiEJPQM (ORCPT ); Tue, 10 May 2022 11:16:12 -0400 Received: from mail.hallyn.com (mail.hallyn.com [178.63.66.53]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 97C329B1A2; Tue, 10 May 2022 07:51:35 -0700 (PDT) Received: by mail.hallyn.com (Postfix, from userid 1001) id 712D514B7; Tue, 10 May 2022 09:51:34 -0500 (CDT) Date: Tue, 10 May 2022 09:51:34 -0500 From: "Serge E. Hallyn" To: Christian Brauner Cc: Amir Goldstein , "Serge E. Hallyn" , Stefan Berger , linux-integrity , Mimi Zohar , Christian Brauner , containers@lists.linux.dev, Dmitry Kasatkin , "Eric W. Biederman" , krzysztof.struczynski@huawei.com, roberto.sassu@huawei.com, mpeters@redhat.com, lhinds@redhat.com, lsturman@redhat.com, puiterwi@redhat.com, jejb@linux.ibm.com, jamjoom@us.ibm.com, linux-kernel , Paul Moore , Richard Guy Briggs , LSM List , James Morris , jpenumak@redhat.com, John Johansen , Matthew Garrett , Micah Morton , Kentaro Takeda , Jarkko Sakkinen Subject: Re: [PATCH v12 01/26] securityfs: rework dentry creation Message-ID: <20220510145134.GA7974@mail.hallyn.com> References: <20220420140633.753772-1-stefanb@linux.ibm.com> <20220420140633.753772-2-stefanb@linux.ibm.com> <20220509195414.GA30894@mail.hallyn.com> <20220509203618.GA31408@mail.hallyn.com> <20220510103817.jalhkw4a2oyqhxhm@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220510103817.jalhkw4a2oyqhxhm@wittgenstein> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, May 10, 2022 at 12:38:17PM +0200, Christian Brauner wrote: > On Tue, May 10, 2022 at 11:43:13AM +0300, Amir Goldstein wrote: > > On Mon, May 9, 2022 at 11:36 PM Serge E. Hallyn wrote: > > > > > > On Mon, May 09, 2022 at 02:54:14PM -0500, Serge E. Hallyn wrote: > > > > On Wed, Apr 20, 2022 at 10:06:08AM -0400, Stefan Berger wrote: > > > > > From: Christian Brauner > > > > > > > > > > When securityfs creates a new file or directory via > > > > > securityfs_create_dentry() it will take an additional reference on the > > > > > newly created dentry after it has attached the new inode to the new > > > > > dentry and added it to the hashqueues. > > > > > If we contrast this with debugfs which has the same underlying logic as > > > > > securityfs. It uses a similar pairing as securityfs. Where securityfs > > > > > has the securityfs_create_dentry() and securityfs_remove() pairing, > > > > > debugfs has the __debugfs_create_file() and debugfs_remove() pairing. > > > > > > > > > > In contrast to securityfs, debugfs doesn't take an additional reference > > > > > on the newly created dentry in __debugfs_create_file() which would need > > > > > to be put in debugfs_remove(). > > > > > > > > > > The additional dget() isn't a problem per se. In the current > > > > > implementation of securityfs each created dentry pins the filesystem via > > > > > > > > Is 'via' an extra word here or is there a missing word? > > > > > > > > I'll delay the rest of my response as the missing word may answer my > > > > remaining question :) > > > > > > > > > until it is removed. Since it is virtually guaranteed that there is at > > > > > least one user of securityfs that has created dentries the initial > > > > > securityfs mount cannot go away until all dentries have been removed. > > > > > > > > > > Since most of the users of the initial securityfs mount don't go away > > > > > until the system is shutdown the initial securityfs won't go away when > > > > > unmounted. Instead a mount will usually surface the same superblock as > > > > > before. The additional dget() doesn't matter in this scenario since it > > > > > is required that all dentries have been cleaned up by the respective > > > > > users before the superblock can be destroyed, i.e. superblock shutdown > > > > > is tied to the lifetime of the associated dentries. > > > > > > > > > > However, in order to support ima namespaces we need to extend securityfs > > > > > to support being mounted outside of the initial user namespace. For > > > > > namespaced users the pinning logic doesn't make sense. Whereas in the > > > > > initial namespace the securityfs instance and the associated data > > > > > structures of its users can't go away for reason explained earlier users > > > > > of non-initial securityfs instances do go away when the last users of > > > > > the namespace are gone. > > > > > > > > > > So for those users we neither want to duplicate the pinning logic nor > > > > > make the global securityfs instance display different information based > > > > > on the namespace. Both options would be really messy and hacky. > > > > > > > > > > Instead we will simply give each namespace its own securityfs instance > > > > > similar to how each ipc namespace has its own mqueue instance and all > > > > > entries in there are cleaned up on umount or when the last user of the > > > > > associated namespace is gone. > > > > > > > > > > This means that the superblock's lifetime isn't tied to the dentries. > > > > > Instead the last umount, without any fds kept open, will trigger a clean > > > > > shutdown. But now the additional dget() gets in the way. Instead of > > > > > being able to rely on the generic superblock shutdown logic we would > > > > > need to drop the additional dentry reference during superblock shutdown > > > > > for all associated users. That would force the use of a generic > > > > > coordination mechanism for current and future users of securityfs which > > > > > is unnecessary. Simply remove the additional dget() in > > > > > securityfs_dentry_create(). > > > > > > > > > > In securityfs_remove() we will call dget() to take an additional > > > > > reference on the dentry about to be removed. After simple_unlink() or > > > > > simple_rmdir() have dropped the dentry refcount we can call d_delete() > > > > > which will either turn the dentry into negative dentry if our earlier > > > > > dget() is the only reference to the dentry, i.e. it has no other users, > > > > > or remove it from the hashqueues in case there are additional users. > > > > > > > > > The first case (turn negative) cannot happen because the function is > > entered with at least 1 refcount and increments it by 1. > > So you can follow commit 46c46f8df9aa ("devpts_pty_kill(): don't bother > > with d_delete()") and use d_drop() instead. > > > > > > > All of these changes should not have any effect on the userspace > > > > > semantics of the initial securityfs mount. > > > > > > > > > > Signed-off-by: Christian Brauner > > > > > Cc: John Johansen > > > > > Cc: Matthew Garrett > > > > > Cc: Micah Morton > > > > > Cc: Kentaro Takeda > > > > > Cc: James Morris > > > > > Cc: Jarkko Sakkinen > > > > > Signed-off-by: Stefan Berger > > > > > Reviewed-by: Mimi Zohar > > > > > --- > > > > > security/inode.c | 3 ++- > > > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > > > > > diff --git a/security/inode.c b/security/inode.c > > > > > index 6c326939750d..13e6780c4444 100644 > > > > > --- a/security/inode.c > > > > > +++ b/security/inode.c > > > > > @@ -159,7 +159,6 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode, > > > > > inode->i_fop = fops; > > > > > } > > > > > d_instantiate(dentry, inode); > > > > > - dget(dentry); > > > > > inode_unlock(dir); > > > > > return dentry; > > > > > > > > > > @@ -302,10 +301,12 @@ void securityfs_remove(struct dentry *dentry) > > > > > dir = d_inode(dentry->d_parent); > > > > > inode_lock(dir); > > > > > if (simple_positive(dentry)) { > > > > > + dget(dentry); > > > > > if (d_is_dir(dentry)) > > > > > simple_rmdir(dir, dentry); > > > > > > Hm, so I realize your patch isn't introducing this, but is the > > > fact that we ignore the possible -ENOTEMPTY return value of > > > simple_rmdir() not a problem? > > > > As long as we are using debugfs as a reference code, wouldn't > > securityfs need to use simple_recursive_removal()? > > Can we guaranty that modules always cleanup all entries in > > correct order? > > We could but that seems like a separate cleanup patch. Yes, I'm not saying this set should fix it, just something that caught my eye. Thanks. > This patch became part of the series because we want non-initial ima > namespaces to guarantee cleanup on securityfs umount. That's different > for the initial securityfs mount which is alwasy going to be around. The > patch is intended to this a little cleaner to implement.