[PATCH v2] capabilities: new kernel.ns_modules_allowed sysctl

[Vegard Nossum]

Creating a new user namespace grants you the ability to reach a lot of code
(including loading certain kernel modules) that would otherwise be out of
reach of an attacker. We can reduce the attack surface and block exploits
by ensuring that user namespaces cannot trigger module (auto-)loading.

A cursory search of exploits found online yields the following extremely
non-exhaustive list of vulnerabilities, and shows that the technique is
both old and still in use:

- CVE-2016-8655
- CVE-2017-1000112
- CVE-2021-32606
- CVE-2022-2588
- CVE-2022-27666
- CVE-2022-34918

This patch adds a new sysctl, kernel.ns_modules_allowed, which when set to
0 will block requests to load modules when the request originates in a
process running in a user namespace.

For backwards compatibility, the default value of the sysctl is set to
CONFIG_NS_MODULES_ALLOWED_DEFAULT_ON, which in turn defaults to 1, meaning
there should be absolutely no change in behaviour unless you opt in either
at compile time or at runtime.

This mitigation obviously offers no protection if the vulnerable module is
already loaded, but for many of these exploits the vast majority of users
will never actually load or use these modules on purpose; in other words,
for the vast majority of users, this would block exploits for the above
list of vulnerabilities.

Testing: Running the reproducer for CVE-2022-2588 fails and results in the
following message in the kernel log:

    [  130.208030] request_module: pid 4107 (a.out) requested kernel module rtnl-link-dummy; denied due to kernel.ns_modules_allowed sysctl

v2:
- fix build failure due to missing CONFIG_SYSCTL guard around register_sysctl_init()
- use .maxlen = sizeof(int) for proc_dobool()
- don't warn when sysctl_ns_modules_allowed == 1

Cc: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Cc: Serge Hallyn <serge@xxxxxxxxxx>
Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: linux-hardening@xxxxxxxxxxxxxxx
Cc: John Haxby <john.haxby@xxxxxxxxxx>
Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
---
 Documentation/admin-guide/sysctl/kernel.rst | 11 ++++++
 init/Kconfig                                | 17 +++++++++
 kernel/kmod.c                               | 39 +++++++++++++++++++++
 3 files changed, 67 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index ee6572b1edada..1e13f7f1a9550 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -604,6 +604,17 @@ A value of 0 means no change. The default value is 200 meaning the NMI
 watchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).
 
 
+ns_modules_allowed
+==================
+
+Control whether processes may trigger module loading inside a user namespace.
+
+= =================================
+0 Deny module loading requests.
+1 Accept module loading requests.
+= =================================
+
+
 numa_balancing
 ==============
 
diff --git a/init/Kconfig b/init/Kconfig
index 80fe60fa77fba..0b99268da5081 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1245,6 +1245,23 @@ config USER_NS
 
 	  If unsure, say N.
 
+config NS_MODULES_ALLOWED_DEFAULT_ON
+	bool "Allow user namespaces to auto-load kernel modules by default"
+	depends on MODULES
+	depends on USER_NS
+	default y
+	help
+	  This option makes it so that processes running inside user
+	  namespaces may auto-load kernel modules.
+
+	  Say N to mitigate some exploits that rely on being able to
+	  auto-load kernel modules; however, this may also cause some
+	  legitimate programs to fail unless kernel modules are loaded by
+	  hand.
+
+	  You can write 0 or 1 to /proc/sys/kernel/ns_modules_allowed to
+	  change behaviour at run-time.
+
 config PID_NS
 	bool "PID Namespaces"
 	default y
diff --git a/kernel/kmod.c b/kernel/kmod.c
index b717134ebe170..46f8c0ad6c921 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -25,6 +25,7 @@
 #include <linux/ptrace.h>
 #include <linux/async.h>
 #include <linux/uaccess.h>
+#include <linux/sysctl.h>
 
 #include <trace/events/module.h>
 
@@ -105,6 +106,12 @@ static int call_modprobe(char *module_name, int wait)
 	return -ENOMEM;
 }
 
+/*
+ * Allow processes running inside namespaces to trigger module loading?
+ */
+static bool sysctl_ns_modules_allowed __read_mostly =
+	IS_BUILTIN(CONFIG_NS_MODULES_ALLOWED_DEFAULT_ON);
+
 /**
  * __request_module - try to load a kernel module
  * @wait: wait (or not) for the operation to complete
@@ -148,6 +155,18 @@ int __request_module(bool wait, const char *fmt, ...)
 	if (ret)
 		return ret;
 
+	/*
+	 * Disallow if we're in a user namespace and we don't have
+	 * CAP_SYS_MODULE in the init namespace.
+	 */
+	if (current_user_ns() != &init_user_ns &&
+	    !capable(CAP_SYS_MODULE) &&
+	    !sysctl_ns_modules_allowed) {
+		pr_warn_ratelimited("request_module: pid %d (%s) in user namespace requested kernel module %s; denied due to kernel.ns_modules_allowed sysctl\n",
+			task_pid_nr(current), current->comm, module_name);
+		return -EPERM;
+	}
+
 	if (atomic_dec_if_positive(&kmod_concurrent_max) < 0) {
 		pr_warn_ratelimited("request_module: kmod_concurrent_max (%u) close to 0 (max_modprobes: %u), for module %s, throttling...",
 				    atomic_read(&kmod_concurrent_max),
@@ -175,3 +194,23 @@ int __request_module(bool wait, const char *fmt, ...)
 	return ret;
 }
 EXPORT_SYMBOL(__request_module);
+
+#ifdef CONFIG_SYSCTL
+static struct ctl_table kmod_sysctl_table[] = {
+	{
+		.procname       = "ns_modules_allowed",
+		.data           = &sysctl_ns_modules_allowed,
+		.maxlen         = sizeof(int),
+		.mode           = 0644,
+		.proc_handler   = proc_dobool,
+	},
+	{ }
+};
+
+static int __init kmod_sysctl_init(void)
+{
+	register_sysctl_init("kernel", kmod_sysctl_table);
+	return 0;
+}
+late_initcall(kmod_sysctl_init);
+#endif
-- 
2.35.1.46.g38062e73e0