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@xxxxxxxxxxxx>
---
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
--
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