On Thu, Jan 28, 2016 at 06:38:25AM -0800, Kees Cook wrote:
> There continue to be unexpected security exposures when users have access
> to CLONE_NEWUSER. For admins of systems that do not use user namespaces
> and are running distro kernels with CONFIG_USER_NS enabled, there is
> no way to disable CLONE_NEWUSER. This provides a way for sysadmins to
> disable the feature to reduce their attack surface without needing to
> rebuild their kernels.
>
> This is inspired by a similar restriction in Grsecurity, but adds
> a sysctl.
>
> Signed-off-by: Kees Cook <keesc...@chromium.org>
Acked-by: Serge Hallyn <serge.hal...@canonical.com>
> ---
> This is the simplified version of the sysctl.
> ---
> Documentation/sysctl/kernel.txt | 14 ++++++++++++++
> kernel/sysctl.c | 14 ++++++++++++++
> kernel/user_namespace.c | 6 ++++++
> 3 files changed, 34 insertions(+)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index a93b414672a7..dcbd3f99efb3 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
> @@ -930,6 +931,19 @@ 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.
> +
> +==============================================================
> +
> 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 97715fd9e790..9f99c8d9e968 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
> @@ -817,6 +820,17 @@ 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,
> + .extra1 = &zero,
> + .extra2 = &one,
> + },
> +#endif
> {
> .procname = "ngroups_max",
> .data = &ngroups_max,
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 9bafc211930c..3cace8637144 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 +85,11 @@ int create_user_ns(struct cred *new)
> !kgid_has_mapping(parent_ns, group))
> return -EPERM;
>
> + if (sysctl_userns_restrict && !(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
>
>
> --
> Kees Cook
> Chrome OS & Brillo Security
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