Re: [PATCH 2/2] sysctl: allow CLONE_NEWUSER to be disabled

["Robert Święcki" <robert@swiecki.net>]

Seems that Debian and some older Ubuntu versions are already using

$ sysctl -a | grep usern
kernel.unprivileged_userns_clone = 0

Shall we be consistent wit it?

2016-01-22 23:39 GMT+01:00 Kees Cook <keescook@chromium.org>:
> There continues to be many CONFIG_USER_NS related security exposures.
> For admins running distro kernels with CONFIG_USER_NS, there is no way
> to disable CLONE_NEWUSER. As many systems do not need CLONE_NEWUSER,
> this provides a way for sysadmins to disable the feature.
>
> This is inspired by a similar restriction in Grsecurity, but adds
> a sysctl.
>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  Documentation/sysctl/kernel.txt | 17 +++++++++++++++++
>  kernel/sysctl.c                 | 14 ++++++++++++++
>  kernel/user_namespace.c         |  7 +++++++
>  3 files changed, 38 insertions(+)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index bbfc5e339a3d..e9e8a4f949f5 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -85,6 +85,7 @@ show up in /proc/sys/kernel:
>  - tainted
>  - threads-max
>  - unknown_nmi_panic
> +- userns_restrict
>  - watchdog
>  - watchdog_thresh
>  - version
> @@ -933,6 +934,22 @@ example.  If a system hangs up, try pressing the NMI switch.
>
>  ==============================================================
>
> +userns_restrict:
> +
> +This toggle indicates whether CLONE_NEWUSER is available. As CLONE_NEWUSER
> +has many unexpected side-effects and security exposures, this allows the
> +sysadmin to disable the feature without needing to rebuild the kernel.
> +
> +When userns_restrict is set to (0), the default, there are no restrictions.
> +
> +When userns_restrict is set to (1), CLONE_NEWUSER is only available to
> +processes that have CAP_SYS_ADMIN, CAP_SETUID, and CAP_SETGID.
> +
> +When userns_restrict is set to (2), CLONE_NEWUSER is not available at all,
> +and the value is locked to "2" for the duration of the boot.
> +
> +==============================================================
> +
>  watchdog:
>
>  This parameter can be used to disable or enable the soft lockup detector
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index fc8899dd636d..ceb8b107fe28 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -112,6 +112,9 @@ extern int sysctl_nr_open_min, sysctl_nr_open_max;
>  #ifndef CONFIG_MMU
>  extern int sysctl_nr_trim_pages;
>  #endif
> +#ifdef CONFIG_USER_NS
> +extern int sysctl_userns_restrict;
> +#endif
>
>  /* Constants used for minimum and  maximum */
>  #ifdef CONFIG_LOCKUP_DETECTOR
> @@ -812,6 +815,17 @@ static struct ctl_table kern_table[] = {
>                 .extra2         = &two,
>         },
>  #endif
> +#ifdef CONFIG_USER_NS
> +       {
> +               .procname       = "userns_restrict",
> +               .data           = &sysctl_userns_restrict,
> +               .maxlen         = sizeof(int),
> +               .mode           = 0644,
> +               .proc_handler   = proc_dointvec_minmax_cap_sysadmin,
> +               .extra1         = &zero,
> +               .extra2         = &two,
> +       },
> +#endif
>         {
>                 .procname       = "ngroups_max",
>                 .data           = &ngroups_max,
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 9bafc211930c..38395f9625ff 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -25,6 +25,7 @@
>
>  static struct kmem_cache *user_ns_cachep __read_mostly;
>  static DEFINE_MUTEX(userns_state_mutex);
> +int sysctl_userns_restrict __read_mostly;
>
>  static bool new_idmap_permitted(const struct file *file,
>                                 struct user_namespace *ns, int cap_setid,
> @@ -84,6 +85,12 @@ int create_user_ns(struct cred *new)
>             !kgid_has_mapping(parent_ns, group))
>                 return -EPERM;
>
> +       if (sysctl_userns_restrict == 2 ||
> +           (sysctl_userns_restrict == 1 && (!capable(CAP_SYS_ADMIN) ||
> +                                            !capable(CAP_SETUID) ||
> +                                            !capable(CAP_SETGID))))
> +               return -EPERM;
> +
>         ns = kmem_cache_zalloc(user_ns_cachep, GFP_KERNEL);
>         if (!ns)
>                 return -ENOMEM;
> --
> 2.6.3
>



-- 
Robert Święcki