Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 12 Jan 2023 12:00:45 -0600
From:   "Serge E. Hallyn" <serge@hallyn.com>
To:     Vegard Nossum <vegard.nossum@oracle.com>
Cc:     Luis Chamberlain <mcgrof@kernel.org>, linux-kernel@vger.kernel.org,
        Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
        Serge Hallyn <serge@hallyn.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Kees Cook <keescook@chromium.org>,
        linux-hardening@vger.kernel.org, linux-modules@vger.kernel.org,
        John Haxby <john.haxby@oracle.com>,
        Jann Horn <jannh@google.com>
Subject: Re: [PATCH v3] kmod: harden user namespaces with new
 kernel.ns_modules_allowed sysctl
Message-ID: <20230112180045.GA18314@mail.hallyn.com>
References: <20230112131911.7684-1-vegard.nossum@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230112131911.7684-1-vegard.nossum@oracle.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Precedence: bulk

On Thu, Jan 12, 2023 at 02:19:11PM +0100, Vegard Nossum wrote:
> 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
> 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
> - CVE-2023-0179

I think it would be worth pointing out how many of the above would
actually be aided by this patch.  The first two would not, but certainly
at least the can module one counts.  So I support this at least in
principle.  I'll take a closer look at the code hopefully tonight.

> A quick survey of common distros shows that Ubuntu, Fedora, RHEL, CentOS
> Stream, and Oracle Linux allow unprivileged user namespaces by default,
> probably to support sandboxing in browsers and containers. Major
> exceptions would be Debian and Arch Linux which carry an out-of-tree patch
> to disable user namespaces by default.
> 
> 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
> 
> v3:
> - drop capable(CAP_SYS_MODULE) check
> - add a new CVE to changelog :-)
> - add survey of distros that enable unpriv userns to changelog
> 
> Link: https://lore.kernel.org/lkml/20220809185229.28417-1-vegard.nossum@oracle.com/ # v1
> Link: https://lore.kernel.org/lkml/20220815082753.6088-1-vegard.nossum@oracle.com/ # v2
> Cc: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> Cc: Serge Hallyn <serge@hallyn.com>
> Cc: Eric Biederman <ebiederm@xmission.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: linux-hardening@vger.kernel.org
> Cc: linux-modules@vger.kernel.org
> Cc: John Haxby <john.haxby@oracle.com>
> Cc: Jann Horn <jannh@google.com>
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
> ---
>  Documentation/admin-guide/sysctl/kernel.rst | 11 ++++++
>  init/Kconfig                                | 17 ++++++++++
>  kernel/kmod.c                               | 37 +++++++++++++++++++++
>  3 files changed, 65 insertions(+)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 46e3d62c0eea..bd9b4e911a6a 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -612,6 +612,17 @@ 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 7e5c3ddc341d..5d9ab43a24b9 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1249,6 +1249,23 @@ 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 b717134ebe17..938c0a39381a 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 +155,16 @@ int __request_module(bool wait, const char *fmt, ...)
>  	if (ret)
>  		return ret;
>  
> +	/*
> +	 * Disallow module loading if we're in a user namespace.
> +	 */
> +	if (current_user_ns() != &init_user_ns &&
> +	    !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 +192,23 @@ 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