Re: [PATCH v10 04/27] securityfs: rework dentry creation

From: Mimi Zohar <zohar@linux.ibm.com>
To: Stefan Berger <stefanb@linux.ibm.com>, linux-integrity@vger.kernel.org
Cc: serge@hallyn.com, containers@lists.linux.dev,
	dmitry.kasatkin@gmail.com, ebiederm@xmission.com,
	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@vger.kernel.org, paul@paul-moore.com,
	rgb@redhat.com, linux-security-module@vger.kernel.org,
	jmorris@namei.org, Christian Brauner <brauner@kernel.org>,
	John Johansen <john.johansen@canonical.com>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	Micah Morton <mortonm@chromium.org>,
	Kentaro Takeda <takedakn@nttdata.co.jp>
Subject: Re: [PATCH v10 04/27] securityfs: rework dentry creation
Date: Thu, 10 Feb 2022 07:03:28 -0500	[thread overview]
Message-ID: <39ec72c0776f67c89fc80a0305c75ec4290590ae.camel@linux.ibm.com> (raw)
In-Reply-To: <20220201203735.164593-5-stefanb@linux.ibm.com>

[Cc'ing JJ, Matthew, Micah, Kentaro, Casey - maintainers of securityfs
usages, not already cc'ed]

On Tue, 2022-02-01 at 15:37 -0500, Stefan Berger wrote:
> From: Christian Brauner <brauner@kernel.org>
> 
> 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
> 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.
> 
> All of these changes should not have any effect on the userspace
> semantics of the initial securityfs mount.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>

Thanks, Christian, Stefan.

Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>

This change is really independent of the IMA namespacing.  Based on
Greg's request of unification of where platform specific
variables/keys/etc are stored, the consensus so far seems to be
'securityfs/secrets'.  Although this patch isn't a bug fix, let's try
and get this upstreamed.

The current securityfs usages are apparmor, lockdown, safesetid,
tomoyo, core LSM ("security/lsm"), and the TPM.

Only on failure to create securityfs files or directories, are
previously created securityfs files/directories removed.  The one
exception seems to be the TPM, which may be built as a kernel module.

-- 
thanks,

Mimi