Re: [PATCH v5 15/16] ima: Move dentries into ima_namespace

[Stefan Berger <stefanb@linux.ibm.com>]

On 12/9/21 14:38, James Bottomley wrote:
> On Thu, 2021-12-09 at 10:30 -0500, James Bottomley wrote:
>> On Thu, 2021-12-09 at 15:37 +0100, Christian Brauner wrote:
>>> On Thu, Dec 09, 2021 at 03:34:28PM +0100, Christian Brauner wrote:
>>>> On Wed, Dec 08, 2021 at 05:18:17PM -0500, Stefan Berger wrote:
>>>>> Move the dentries into the ima_namespace for reuse by
>>>>> virtualized
>>>>> SecurityFS. Implement function freeing the dentries in order of
>>>>> files and symlinks before directories.
>>>>>
>>>>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>>>>> ---
>>>> This doesn't work as implemented, I think.
>>>>
>>>> What I would have preferred and what I tried to explain in the
>>>> earlier review was:
>>>> Keep the dentry stashing global since it is only needed for
>>>> init_ima_ns.
>>>> Then struct ima_namespace becomes way smaller and simpler.
>>>> If you do that then it makes sense to remove the additional
>>>> dget() in securityfs_create_dentry() for non-init_ima_ns.
>>>> Then you can rely on auto-cleanup in .kill_sb() or on
>>>> ima_securityfs_init() failure and you only need to call
>>>> ima_fs_ns_free_dentries() if ns != init_ima_ns.
>>>>
>>>> IIuc, it seems you're currently doing one dput() too many since
>>>> you're calling securityfs_remove() in the error path for non-
>>>> init_ima_ns which relies on the previous increased dget() which
>>>> we removed.
>>> If you really want to move the dentry stashing into struct
>>> ima_namespace even though it's really unnecessary then you may as
>>> well not care about the auto-cleanup and keep that additional
>>> ima_fs_ns_free_dentries(ns) call in .kill_sb(). But I really think
>>> not dragging dentry stashing into struct ima_namespace is the
>>> correct way to go about this.
>> We, unfortunately, do have one case we can't avoid stashing for the
>> policy file.  It's this code in ima_release_policy:
>>
>>> #if !defined(CONFIG_IMA_WRITE_POLICY) &&
>>> !defined(CONFIG_IMA_READ_POLICY)
>>> 	securityfs_remove(ns->dentry[IMAFS_DENTRY_IMA_POLICY]);
>>> 	ns->dentry[IMAFS_DENTRY_IMA_POLICY] = NULL;
>>>
>> What it does is that in certain config options, the policy file entry
>> gets removed from the securityfs ima directory after you write to it.
> This is what I have incremental to v5 that corrects all of this.  It
> actually keeps every dentry reference (including init_user_ns ones) at
> 1 so they can be reaped on unmount.  For the remove case it does
> d_delete and then puts the only reference.  This means
> securityfs_remove() works for the namespaced policy file as well.
I fixed it now as well but do another dget() on securityfs_removed() for 
ns != init_user_ns. Ok, I will add what you have below.
>
> I also got rid of the spurious initialized check in ima_securityfs_init
> because it prevents you doing a mount;umount;mount on securityfs within
> a namespace.
>
> There's still the problem that if you write the policy, making the file
> disappear then unmount and remount securityfs it will come back.  My
> guess for fixing this is that we only stash the policy file reference,
> create it if NULL but then set the pointer to PTR_ERR(-EINVAL) or
> something and refuse to create it for that value.

What about boolean to remember this? I just added this.


>
> James
>
> ---
>
>  From 7de285a81ff06b6e0eb2c6db24810aeef9f6dd17 Mon Sep 17 00:00:00 2001
> From: James Bottomley <James.Bottomley@HansenPartnership.com>
> Date: Thu, 9 Dec 2021 19:33:49 +0000
> Subject: [PATCH] fix dentry ref counting
>
> ---
>   security/inode.c                | 12 ++----------
>   security/integrity/ima/ima_fs.c |  4 ----
>   2 files changed, 2 insertions(+), 14 deletions(-)
>
> diff --git a/security/inode.c b/security/inode.c
> index eaccba7017d9..b53152f7a625 100644
> --- a/security/inode.c
> +++ b/security/inode.c
> @@ -178,8 +178,6 @@ static struct dentry *securityfs_create_dentry(const char *name, umode_t mode,
>   		inode->i_fop = fops;
>   	}
>   	d_instantiate(dentry, inode);
> -	if (ns == &init_user_ns)
> -		dget(dentry);
>   	inode_unlock(dir);
>   	return dentry;
>   
> @@ -317,21 +315,15 @@ EXPORT_SYMBOL_GPL(securityfs_create_symlink);
>   void securityfs_remove(struct dentry *dentry)
>   {
>   	struct user_namespace *ns = dentry->d_sb->s_user_ns;
> -	struct inode *dir;
>   
>   	if (!dentry || IS_ERR(dentry))
>   		return;
>   
> -	dir = d_inode(dentry->d_parent);
> -	inode_lock(dir);
>   	if (simple_positive(dentry)) {
> -		if (d_is_dir(dentry))
> -			simple_rmdir(dir, dentry);
> -		else
> -			simple_unlink(dir, dentry);
> +		d_delete(dentry);
>   		dput(dentry);
>   	}
> -	inode_unlock(dir);
> +
>   	if (ns == &init_user_ns)
>   		simple_release_fs(&init_securityfs_mount,
>   				  &init_securityfs_mount_count);
> diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
> index 778983fd9a73..077a6ff46858 100644
> --- a/security/integrity/ima/ima_fs.c
> +++ b/security/integrity/ima/ima_fs.c
> @@ -466,10 +466,6 @@ int ima_securityfs_init(struct user_namespace *user_ns, struct dentry *root)
>   	struct ima_namespace *ns = user_ns->ima_ns;
>   	struct dentry *ima_dir;
>   
> -	/* already initialized? */
> -	if (ns->dentry[IMAFS_DENTRY_INTEGRITY_DIR])
> -		return 0;
> -
>   	/* FIXME: update when evm and integrity are namespaced */
>   	if (user_ns != &init_user_ns) {
>   		ns->dentry[IMAFS_DENTRY_INTEGRITY_DIR] =