linux-kernel - Re: [PATCH v4 10/16] ima: Implement hierarchical processing of file accesses

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Date:   Wed, 8 Dec 2021 13:09:54 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Stefan Berger <stefanb@...ux.ibm.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
Subject: Re: [PATCH v4 10/16] ima: Implement hierarchical processing of file
 accesses

On Tue, Dec 07, 2021 at 03:21:21PM -0500, Stefan Berger wrote:
> Implement hierarchical processing of file accesses in IMA namespaces by
> walking the list of IMA namespaces towards the init_ima_ns. This way
> file accesses can be audited in an IMA namespace and also be evaluated
> against the IMA policies of parent IMA namespaces.
> 
> Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
> ---
>  security/integrity/ima/ima_main.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 2121a831f38a..e9fa46eedd27 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -200,10 +200,10 @@ void ima_file_free(struct file *file)
>  	ima_check_last_writer(iint, inode, file);
>  }
>  
> -static int process_measurement(struct ima_namespace *ns,
> -			       struct file *file, const struct cred *cred,
> -			       u32 secid, char *buf, loff_t size, int mask,
> -			       enum ima_hooks func)
> +static int _process_measurement(struct ima_namespace *ns,

Hm, it's much more common to use double underscores then single
underscores to

__process_measurement()

reads a lot more natural to people perusing kernel code quite often.

> +				struct file *file, const struct cred *cred,
> +				u32 secid, char *buf, loff_t size, int mask,
> +				enum ima_hooks func)
>  {
>  	struct inode *inode = file_inode(file);
>  	struct integrity_iint_cache *iint = NULL;
> @@ -405,6 +405,27 @@ static int process_measurement(struct ima_namespace *ns,
>  	return 0;
>  }
>  
> +static int process_measurement(struct ima_namespace *ns,
> +			       struct file *file, const struct cred *cred,
> +			       u32 secid, char *buf, loff_t size, int mask,
> +			       enum ima_hooks func)
> +{
> +	int ret = 0;
> +	struct user_namespace *user_ns;
> +
> +	do {
> +		ret = _process_measurement(ns, file, cred, secid, buf, size, mask, func);
> +		if (ret)
> +			break;
> +		user_ns = ns->user_ns->parent;
> +		if (!user_ns)
> +			break;
> +		ns = user_ns->ima_ns;
> +	} while (1);

I'd rather write this as:

	struct user_namespace *user_ns = ns->user_ns;

	while (user_ns) {
		ns = user_ns->ima_ns;

   		ret = __process_measurement(ns, file, cred, secid, buf, size, mask, func);
   		if (ret)
   			break;
		user_ns = user_ns->parent;
		
	}

because the hierarchy is only an implicit property inherited by ima
namespaces from the implementation of user namespaces. In other words,
we're only indirectly walking a hierarchy of ima namespaces because
we're walking a hierarchy of user namespaces. So the ima ns actually
just gives us the entrypoint into the userns hierarchy which the double
deref writing it with a while() makes obvious.

But that's just how I'd conceptualize it. This you should do however you
prefer.