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Date: Fri, 22 Jan 2016 23:47:45 +0100
From: Robert Święcki <robert@...ecki.net>
To: Kees Cook <keescook@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Al Viro <viro@...iv.linux.org.uk>,
Richard Weinberger <richard@....at>, "Eric W. Biederman" <ebiederm@...ssion.com>,
Andy Lutomirski <luto@...capital.net>, Dmitry Vyukov <dvyukov@...gle.com>,
David Howells <dhowells@...hat.com>, Miklos Szeredi <mszeredi@...e.cz>,
Kostya Serebryany <kcc@...gle.com>, Alexander Potapenko <glider@...gle.com>, Eric Dumazet <edumazet@...gle.com>,
Sasha Levin <sasha.levin@...cle.com>, linux-doc@...r.kernel.org,
LKML <linux-kernel@...r.kernel.org>, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 2/2] sysctl: allow CLONE_NEWUSER to be disabled
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@...omium.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@...omium.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
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