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Message-ID: <20160128143825.GA17383@www.outflux.net>
Date: Thu, 28 Jan 2016 06:38:25 -0800
From: Kees Cook <keescook@...omium.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
Andy Lutomirski <luto@...nel.org>,
"Austin S. Hemmelgarn" <ahferroin7@...il.com>,
Richard Weinberger <richard@....at>,
Robert Święcki <robert@...ecki.net>,
Dmitry Vyukov <dvyukov@...gle.com>,
David Howells <dhowells@...hat.com>,
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,
linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: [PATCH v2] sysctl: allow CLONE_NEWUSER to be disabled
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 <keescook@...omium.org>
---
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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