Re: [RFC PATCH 1/2] security, capabilities: Add CAP_SYS_MOUNT

[Stephen Smalley]

On Sat, 2017-10-21 at 15:43 +0200, Nicolas Belouin wrote:
> With CAP_SYS_ADMIN being bloated and inapropriate for actions such
> as mounting/unmounting filesystems, the creation of a new capability
> is needed.
> CAP_SYS_MOUNT is meant to give a process the ability to call for
> mount,
> umount and umount2 syscalls.

If adding a new capability isn't deemed acceptable, then another option
would be to introduce LSM hooks where there isn't already coverage and
implement finer-grained permission checks there.  In some cases, that
already occurs for mount and umount*.  That also offers the possibility
of taking the object of the operation into account, unlike capabilities
which are only subject/process-based.


> 
> Signed-off-by: Nicolas Belouin <nicolas@xxxxxxxxxx>
> ---
> Âinclude/uapi/linux/capability.hÂÂÂÂÂ| 5 ++++-
> Âsecurity/selinux/include/classmap.h | 4 ++--
> Â2 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/include/uapi/linux/capability.h
> b/include/uapi/linux/capability.h
> index 230e05d35191..ce230aa6d928 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -365,8 +365,11 @@ struct vfs_ns_cap_data {
> Â
> Â#define CAP_AUDIT_READ		37
> Â
> +/* Allow mounting, unmounting filesystems */
> Â
> -#define CAP_LAST_CAPÂÂÂÂÂÂÂÂÂCAP_AUDIT_READ
> +#define CAP_SYS_MOUNT		38
> +
> +#define CAP_LAST_CAPÂÂÂÂÂÂÂÂÂCAP_SYS_MOUNT
> Â
> Â#define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
> Â
> diff --git a/security/selinux/include/classmap.h
> b/security/selinux/include/classmap.h
> index 35ffb29a69cb..a873dce97fd5 100644
> --- a/security/selinux/include/classmap.h
> +++ b/security/selinux/include/classmap.h
> @@ -24,9 +24,9 @@
> Â	ÂÂÂÂ"audit_control", "setfcap"
> Â
> Â#define COMMON_CAP2_PERMSÂÂ"mac_override", "mac_admin", "syslog", \
> -		"wake_alarm", "block_suspend", "audit_read"
> +		"wake_alarm", "block_suspend", "audit_read",
> "sys_mount"
> Â
> -#if CAP_LAST_CAP > CAP_AUDIT_READ
> +#if CAP_LAST_CAP > CAP_SYS_MOUNT
> Â#error New capability defined, please update COMMON_CAP2_PERMS.
> Â#endif
> Â