linux-kernel - Re: [PATCH v4 16/16] ima: Setup securityfs for IMA namespace

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Message-ID: <711ce320-25c7-5ace-a026-89a55dc7c068@linux.ibm.com>
Date:   Wed, 8 Dec 2021 10:39:48 -0500
From:   Stefan Berger <stefanb@...ux.ibm.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-integrity@...r.kernel.org, zohar@...ux.ibm.com,
        serge@...lyn.com, containers@...ts.linux.dev,
        dmitry.kasatkin@...il.com, ebiederm@...ssion.com,
        krzysztof.struczynski@...wei.com, roberto.sassu@...wei.com,
        mpeters@...hat.com, lhinds@...hat.com, lsturman@...hat.com,
        puiterwi@...hat.com, jejb@...ux.ibm.com, jamjoom@...ibm.com,
        linux-kernel@...r.kernel.org, paul@...l-moore.com, rgb@...hat.com,
        linux-security-module@...r.kernel.org, jmorris@...ei.org,
        James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: [PATCH v4 16/16] ima: Setup securityfs for IMA namespace


On 12/8/21 07:58, Christian Brauner wrote:
> On Tue, Dec 07, 2021 at 03:21:27PM -0500, Stefan Berger wrote:
>>   
>>   #if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
>> diff --git a/security/inode.c b/security/inode.c
>> index 121ac1874dde..10ee20917f42 100644
>> --- a/security/inode.c
>> +++ b/security/inode.c
>> @@ -16,6 +16,7 @@
>>   #include <linux/fs_context.h>
>>   #include <linux/mount.h>
>>   #include <linux/pagemap.h>
>> +#include <linux/ima.h>
>>   #include <linux/init.h>
>>   #include <linux/namei.h>
>>   #include <linux/security.h>
>> @@ -41,6 +42,7 @@ static const struct super_operations securityfs_super_operations = {
>>   static int securityfs_fill_super(struct super_block *sb, struct fs_context *fc)
>>   {
>>   	static const struct tree_descr files[] = {{""}};
>> +	struct user_namespace *ns = fc->user_ns;
>>   	int error;
>>   
>>   	error = simple_fill_super(sb, SECURITYFS_MAGIC, files);
>> @@ -49,7 +51,10 @@ static int securityfs_fill_super(struct super_block *sb, struct fs_context *fc)
>>   
>>   	sb->s_op = &securityfs_super_operations;
>>   
>> -	return 0;
>> +	if (ns != &init_user_ns)
>> +		error = ima_fs_ns_init(ns, sb->s_root);
>> +
>> +	return error;
>>   }
>>   
>>   static int securityfs_get_tree(struct fs_context *fc)
>> @@ -69,6 +74,11 @@ static int securityfs_init_fs_context(struct fs_context *fc)
>>   
>>   static void securityfs_kill_super(struct super_block *sb)
>>   {
>> +	struct user_namespace *ns = sb->s_fs_info;
>> +
>> +	if (ns != &init_user_ns)
>> +		ima_fs_ns_free_dentries(ns);
> Say securityfs is unmounted. Then all the inodes and dentries become
> invalid. It's not allowed to hold on to any dentries or inodes after the
> super_block is shut down. So I just want to be sure that nothing in ima
> can access these dentries after securityfs is unmounted.

> To put it another way: why are they stored in struct ima_namespace in
> the first place? If you don't pin a filesystem when creating files or
> directories like you do for securityfs in init_ima_ns then you don't
> need to hold on to them as they will be automatically be wiped during
> umount.


The reason was so that securityfs for init_ima_ns and IMA namespaces 
could share the code assigning to dentries to keep around and can clean 
up if an error occurs while creating a dentry.

What about this: We keep the dentries in the ima_namespace, modify the 
code creating the dentries in securityfs_create_dentry() to only take 
the additional reference in case of init_user_ns (I suppose this is what 
you suggest) and then keep 'static void ima_fs_ns_free_dentries()' only 
for removing the dentries for the error case and never call it from 
securityfs_kill_super()? Would that be acceptable?