linux-kernel - Re: [PATCH v4 14/16] ima: Use mac_admin_ns_capable() to check corresponding capability

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Message-ID: <20211208124031.lbutcvt3ns73vgsw@wittgenstein>
Date:   Wed, 8 Dec 2021 13:40:31 +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,
        Denis Semakin <denis.semakin@...wei.com>
Subject: Re: [PATCH v4 14/16] ima: Use mac_admin_ns_capable() to check
 corresponding capability

On Tue, Dec 07, 2021 at 03:21:25PM -0500, Stefan Berger wrote:
> Use mac_admin_ns_capable() to check corresponding capability to allow
> read/write IMA policy without CAP_SYS_ADMIN but with CAP_MAC_ADMIN.
> 
> Signed-off-by: Denis Semakin <denis.semakin@...wei.com>
> Signed-off-by: Stefan Berger <stefanb@...ux.ibm.com>
> ---
>  include/linux/capability.h      | 6 ++++++
>  security/integrity/ima/ima_fs.c | 2 +-
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 65efb74c3585..991579178f32 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -270,6 +270,12 @@ static inline bool checkpoint_restore_ns_capable(struct user_namespace *ns)
>  		ns_capable(ns, CAP_SYS_ADMIN);
>  }
>  
> +static inline bool mac_admin_ns_capable(struct user_namespace *ns)
> +{
> +	return ns_capable(ns, CAP_MAC_ADMIN) ||
> +		ns_capable(ns, CAP_SYS_ADMIN);
> +}
> +
>  /* audit system wants to get cap info from files as well */
>  int get_vfs_caps_from_disk(struct user_namespace *mnt_userns,
>  			   const struct dentry *dentry,
> diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
> index 0e582ceecc7f..a749a3e79304 100644
> --- a/security/integrity/ima/ima_fs.c
> +++ b/security/integrity/ima/ima_fs.c
> @@ -394,7 +394,7 @@ static int ima_open_policy(struct inode *inode, struct file *filp)
>  #else
>  		if ((filp->f_flags & O_ACCMODE) != O_RDONLY)
>  			return -EACCES;
> -		if (!capable(CAP_SYS_ADMIN))
> +		if (!mac_admin_ns_capable(ns->user_ns))
>  			return -EPERM;

Hm, couldn't this rather just be:

		if (ns_capable(ns, CAP_MAC_ADMIN) || capable(CAP_SYS_ADMIN))

so we don't carry CAP_SYS_ADMIN as an alternative way for ima into user
namespaces as well? This way containers don't need to drop CAP_SYS_ADMIN
just to prevent mac policy from being altered. But that's more on the
LSM side of questions.