linux-kernel - Re: [PATCH v6 15/17] ima: Use mac_admin_ns_capable() to check corresponding capability

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Message-ID: <20211211152901.GA25170@mail.hallyn.com>
Date:   Sat, 11 Dec 2021 09:29:01 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Stefan Berger <stefanb@...ux.ibm.com>
Cc:     linux-integrity@...r.kernel.org, zohar@...ux.ibm.com,
        serge@...lyn.com, christian.brauner@...ntu.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 v6 15/17] ima: Use mac_admin_ns_capable() to check
 corresponding capability

On Fri, Dec 10, 2021 at 02:47:34PM -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 a136d14f29ec..090ee85bfa3a 100644
> --- a/security/integrity/ima/ima_fs.c
> +++ b/security/integrity/ima/ima_fs.c
> @@ -440,7 +440,7 @@ 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(user_ns))

Sorry if I'm missing something.  But I'm looking at your tree's
version of ima_update_policy() and failing to see where it adds
extra capability checks.  Note that any unprivileged user can
unshare a user namespace, map its hostuid to nsuid 0, and pass
ns_capable(CAP_MAC_ADMIN).

Likewise, a host uid 0 process which does not have CAP_MAC_ADMIN
can create a new user namespace, map hostuid 0 to nsuid 0, and
have CAP_MAC_ADMIN against the new userns.

Somewhere you need to be checking for privilege against either
the parent ns or the init_user_ns.  I'm not seeing where that's
being done.  Can you point me to it?


>  			return -EPERM;
>  		return ima_seq_open(filp, &ima_policy_seqops);
>  #endif
> -- 
> 2.31.1