Re: [PATCH v10 12/27] ima: Define mac_admin_ns_capable() as a wrapper for ns_capable()

Serge E. Hallyn
2022-02-05T06:04:30Z

On Tue, Feb 01, 2022 at 03:37:20PM -0500, Stefan Berger wrote:
> Define mac_admin_ns_capable() as a wrapper for the combined ns_capable()
> checks on CAP_MAC_ADMIN and CAP_SYS_ADMIN in a user namespace. Return
> true on the check if either capability or both are available.
>
> Use mac_admin_ns_capable() in place of capable(SYS_ADMIN). This will allow
> an IMA namespace to read the policy with only CAP_MAC_ADMIN, which has
> less privileges than CAP_SYS_ADMIN.
>
> Signed-off-by: Denis Semakin <denis.semakin@huawei.com>
> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> ---
> include/linux/capability.h | 6 ++++++
> security/integrity/ima/ima.h | 6 ++++++
> security/integrity/ima/ima_fs.c | 5 ++++-
> 3 files changed, 16 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);

Do you care about audit warnings? If the task has CAP_SYS_ADMIN but
not CAP_MAC_ADMIN, is it desirable that selinux_capable() will audit the
CAP_MAC_ADMIN failure?

> +}
> +
> /* 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.h b/security/integrity/ima/ima.h
> index fb6bd054d899..0057b1fd6c18 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -487,4 +487,10 @@ static inline int ima_filter_rule_match(u32 secid, u32 field, u32 op,
> #define POLICY_FILE_FLAGS S_IWUSR
> #endif /* CONFIG_IMA_READ_POLICY */
>
> +static inline
> +struct user_namespace *ima_user_ns_from_file(const struct file *filp)
> +{
> + return file_inode(filp)->i_sb->s_user_ns;
> +}
> +
> #endif /* __LINUX_IMA_H */
> diff --git a/security/integrity/ima/ima_fs.c b/security/integrity/ima/ima_fs.c
> index 89d3113ceda1..c41aa61b7393 100644
> --- a/security/integrity/ima/ima_fs.c
> +++ b/security/integrity/ima/ima_fs.c
> @@ -377,6 +377,9 @@ static const struct seq_operations ima_policy_seqops = {
> */
> static int ima_open_policy(struct inode *inode, struct file *filp)
> {
> +#ifdef CONFIG_IMA_READ_POLICY
> + struct user_namespace *user_ns = ima_user_ns_from_file(filp);
> +#endif
> struct ima_namespace *ns = &init_ima_ns;
>
> if (!(filp->f_flags & O_WRONLY)) {
> @@ -385,7 +388,7 @@ 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))
> return -EPERM;
> return seq_open(filp, &ima_policy_seqops);
> #endif
> --
> 2.31.1