Re: [PATCH 1/3] capabilities: user namespace capabilities

[John Johansen <john.johansen@canonical.com>]

On 5/16/24 02:22, Jonathan Calmels wrote:
> Attackers often rely on user namespaces to get elevated (yet confined)
> privileges in order to target specific subsystems (e.g. [1]). Distributions
> have been pretty adamant that they need a way to configure these, most of
> them carry out-of-tree patches to do so, or plainly refuse to enable them.
> As a result, there have been multiple efforts over the years to introduce
> various knobs to control and/or disable user namespaces (e.g. [2][3][4]).
> 
> While we acknowledge that there are already ways to control the creation of
> such namespaces (the most recent being a LSM hook), there are inherent
> issues with these approaches. Preventing the user namespace creation is not
> fine-grained enough, and in some cases, incompatible with various userspace

agreed, though it really is application dependent. Some applications handle
the denial at userns creation better, than the capability after. Others
like anything based on QTWebEngine will crash on denial of userns creation
but handle denial of the capability within the userns just fine, and some
applications just crash regardless.

The userns cred from the LSM hook can be modified, yes it is currently
specified as const but is still under construction so it can be safely
modified the LSM hook just needs a small update.

The advantage of doing it under the LSM is an LSM can have a richer policy
around what can use them and tracking of what is allowed. That is to say the
LSM has the capability of being finer grained than doing it via capabilities.

I am not opposed to adding another mechanism to control user namespaces,
I am just not currently convinced that capabilities are the right
mechanism.

> expectations (e.g. container runtimes, browser sandboxing, service
> isolation)
> 
> This patch addresses these limitations by introducing an additional
> capability set used to restrict the permissions granted when creating user
> namespaces. This way, processes can apply the principle of least privilege
> by configuring only the capabilities they need for their namespaces.
> 
> For compatibility reasons, processes always start with a full userns
> capability set.
> 
> On namespace creation, the userns capability set (pU) is assigned to the
> new effective (pE), permitted (pP) and bounding set (X) of the task:
> 
>      pU = pE = pP = X
> 

this should be bounded by the creating task's bounding set, other wise
the capability model's bounding invariant will be broken, but having the
capabilities that the userns want to access in the task's bounding set is
a problem for all the unprivileged processes wanting access to user
namespaces.

Simply setting the userns fcap on the programs that want access to user
namespaces, does certainly reduce the attack surface, but really is
insufficient for utilities like unshare, bwrap, lxd etc. They can be
used to trivially by-pass the restriction.

> The userns capability set obeys the invariant that no bit can ever be set
> if it is not already part of the task’s bounding set. This ensures that no
> namespace can ever gain more privileges than its predecessors.
> Additionally, if a task is not privileged over CAP_SETPCAP, setting any bit
> in the userns set requires its corresponding bit to be set in the permitted
> set. This effectively mimics the inheritable set rules and means that, by
> default, only root in the initial user namespace can gain userns
> capabilities:
> 
>      p’U = (pE & CAP_SETPCAP) ? X : (X & pP)
> 
If I am reading this right for unprivileged processes the capabilities in
the userns are bounded by the processes permitted set before the userns is
created?

This is only being respected in PR_CTL, the user mode helper is straight
setting the caps.



> Note that since userns capabilities are strictly hierarchical, policies can
> be enforced at various levels (e.g. init, pam_cap) and inherited by every
> child namespace.
> 
> Here is a sample program that can be used to verify the functionality:
> 
> /*
>   * Test program that drops CAP_SYS_RAWIO from subsequent user namespaces.
>   *
>   * ./cap_userns_test unshare -r grep Cap /proc/self/status
>   * CapInh: 0000000000000000
>   * CapPrm: 000001fffffdffff
>   * CapEff: 000001fffffdffff
>   * CapBnd: 000001fffffdffff
>   * CapAmb: 0000000000000000
>   * CapUNs: 000001fffffdffff
>   */
> 
> int main(int argc, char *argv[])
> {
>          if (prctl(PR_CAP_USERNS, PR_CAP_USERNS_LOWER, CAP_SYS_RAWIO, 0, 0) < 0)
>                  err(1, "cannot drop userns cap");
> 
>          execvp(argv[1], argv + 1);
>          err(1, "cannot exec");
> }
> 
> Link: https://security.googleblog.com/2023/06/learnings-from-kctf-vrps-42-linux.html
> Link: https://lore.kernel.org/lkml/1453502345-30416-1-git-send-email-keescook@chromium.org
> Link: https://lore.kernel.org/lkml/20220815162028.926858-1-fred@cloudflare.com
> Link: https://lore.kernel.org/containers/168547265011.24337.4306067683997517082-0@git.sr.ht
> 
> Signed-off-by: Jonathan Calmels <jcalmels@3xx0.net>
> ---
>   fs/proc/array.c              |  9 ++++++
>   include/linux/cred.h         |  3 ++
>   include/uapi/linux/prctl.h   |  7 +++++
>   kernel/cred.c                |  3 ++
>   kernel/umh.c                 | 16 ++++++++++
>   kernel/user_namespace.c      | 12 +++-----
>   security/commoncap.c         | 59 ++++++++++++++++++++++++++++++++++++
>   security/keys/process_keys.c |  3 ++
>   8 files changed, 105 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/proc/array.c b/fs/proc/array.c
> index 34a47fb0c57f..364e8bb19f9d 100644
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -313,6 +313,9 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
>   	const struct cred *cred;
>   	kernel_cap_t cap_inheritable, cap_permitted, cap_effective,
>   			cap_bset, cap_ambient;
> +#ifdef CONFIG_USER_NS
> +	kernel_cap_t cap_userns;
> +#endif
>   
>   	rcu_read_lock();
>   	cred = __task_cred(p);
> @@ -321,6 +324,9 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
>   	cap_effective	= cred->cap_effective;
>   	cap_bset	= cred->cap_bset;
>   	cap_ambient	= cred->cap_ambient;
> +#ifdef CONFIG_USER_NS
> +	cap_userns	= cred->cap_userns;
> +#endif
>   	rcu_read_unlock();
>   
>   	render_cap_t(m, "CapInh:\t", &cap_inheritable);
> @@ -328,6 +334,9 @@ static inline void task_cap(struct seq_file *m, struct task_struct *p)
>   	render_cap_t(m, "CapEff:\t", &cap_effective);
>   	render_cap_t(m, "CapBnd:\t", &cap_bset);
>   	render_cap_t(m, "CapAmb:\t", &cap_ambient);
> +#ifdef CONFIG_USER_NS
> +	render_cap_t(m, "CapUNs:\t", &cap_userns);
> +#endif
>   }
>   
>   static inline void task_seccomp(struct seq_file *m, struct task_struct *p)
> diff --git a/include/linux/cred.h b/include/linux/cred.h
> index 2976f534a7a3..adab0031443e 100644
> --- a/include/linux/cred.h
> +++ b/include/linux/cred.h
> @@ -124,6 +124,9 @@ struct cred {
>   	kernel_cap_t	cap_effective;	/* caps we can actually use */
>   	kernel_cap_t	cap_bset;	/* capability bounding set */
>   	kernel_cap_t	cap_ambient;	/* Ambient capability set */
> +#ifdef CONFIG_USER_NS
> +	kernel_cap_t	cap_userns;	/* User namespace capability set */
> +#endif
>   #ifdef CONFIG_KEYS
>   	unsigned char	jit_keyring;	/* default keyring to attach requested
>   					 * keys to */
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 370ed14b1ae0..e09475171f62 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -198,6 +198,13 @@ struct prctl_mm_map {
>   # define PR_CAP_AMBIENT_LOWER		3
>   # define PR_CAP_AMBIENT_CLEAR_ALL	4
>   
> +/* Control the userns capability set */
> +#define PR_CAP_USERNS			48
> +# define PR_CAP_USERNS_IS_SET		1
> +# define PR_CAP_USERNS_RAISE		2
> +# define PR_CAP_USERNS_LOWER		3
> +# define PR_CAP_USERNS_CLEAR_ALL	4
> +
>   /* arm64 Scalable Vector Extension controls */
>   /* Flag values must be kept in sync with ptrace NT_ARM_SVE interface */
>   #define PR_SVE_SET_VL			50	/* set task vector length */
> diff --git a/kernel/cred.c b/kernel/cred.c
> index 075cfa7c896f..9912c6f3bc6b 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -56,6 +56,9 @@ struct cred init_cred = {
>   	.cap_permitted		= CAP_FULL_SET,
>   	.cap_effective		= CAP_FULL_SET,
>   	.cap_bset		= CAP_FULL_SET,
> +#ifdef CONFIG_USER_NS
> +	.cap_userns		= CAP_FULL_SET,
> +#endif
>   	.user			= INIT_USER,
>   	.user_ns		= &init_user_ns,
>   	.group_info		= &init_groups,
> diff --git a/kernel/umh.c b/kernel/umh.c
> index 1b13c5d34624..51f1e1d25d49 100644
> --- a/kernel/umh.c
> +++ b/kernel/umh.c
> @@ -32,6 +32,9 @@
>   
>   #include <trace/events/module.h>
>   
> +#ifdef CONFIG_USER_NS
> +static kernel_cap_t usermodehelper_userns = CAP_FULL_SET;
> +#endif
>   static kernel_cap_t usermodehelper_bset = CAP_FULL_SET;
>   static kernel_cap_t usermodehelper_inheritable = CAP_FULL_SET;
>   static DEFINE_SPINLOCK(umh_sysctl_lock);
> @@ -94,6 +97,10 @@ static int call_usermodehelper_exec_async(void *data)
>   	new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
>   	new->cap_inheritable = cap_intersect(usermodehelper_inheritable,
>   					     new->cap_inheritable);
> +#ifdef CONFIG_USER_NS
> +	new->cap_userns = cap_intersect(usermodehelper_userns,
> +					new->cap_userns);
> +#endif
>   	spin_unlock(&umh_sysctl_lock);
>   
>   	if (sub_info->init) {
> @@ -560,6 +567,15 @@ static struct ctl_table usermodehelper_table[] = {
>   		.mode		= 0600,
>   		.proc_handler	= proc_cap_handler,
>   	},
> +#ifdef CONFIG_USER_NS
> +	{
> +		.procname	= "userns",
> +		.data		= &usermodehelper_userns,
> +		.maxlen		= 2 * sizeof(unsigned long),
> +		.mode		= 0600,
> +		.proc_handler	= proc_cap_handler,
> +	},
> +#endif
>   	{ }
>   };
>   
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 0b0b95418b16..7e624607330b 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -42,15 +42,13 @@ static void dec_user_namespaces(struct ucounts *ucounts)
>   
>   static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
>   {
> -	/* Start with the same capabilities as init but useless for doing
> -	 * anything as the capabilities are bound to the new user namespace.
> -	 */
> -	cred->securebits = SECUREBITS_DEFAULT;
> +	/* Start with the capabilities defined in the userns set. */
> +	cred->cap_bset = cred->cap_userns;
> +	cred->cap_permitted = cred->cap_userns;
> +	cred->cap_effective = cred->cap_userns;
>   	cred->cap_inheritable = CAP_EMPTY_SET;
> -	cred->cap_permitted = CAP_FULL_SET;
> -	cred->cap_effective = CAP_FULL_SET;
>   	cred->cap_ambient = CAP_EMPTY_SET;
> -	cred->cap_bset = CAP_FULL_SET;
> +	cred->securebits = SECUREBITS_DEFAULT;
>   #ifdef CONFIG_KEYS
>   	key_put(cred->request_key_auth);
>   	cred->request_key_auth = NULL;
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 162d96b3a676..b3d3372bf910 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -228,6 +228,28 @@ static inline int cap_inh_is_capped(void)
>   	return 1;
>   }
>   
> +/*
> + * Determine whether a userns capability can be raised.
> + * Returns 1 if it can, 0 otherwise.
> + */
> +#ifdef CONFIG_USER_NS
> +static inline int cap_uns_is_raiseable(unsigned long cap)
> +{
> +	if (!!cap_raised(current_cred()->cap_userns, cap))
> +		return 1;
> +	/* a capability cannot be raised unless the current task has it in
> +	 * its bounding set and, without CAP_SETPCAP, its permitted set.
> +	 */
> +	if (!cap_raised(current_cred()->cap_bset, cap))
> +		return 0;
> +	if (cap_capable(current_cred(), current_cred()->user_ns,
> +			CAP_SETPCAP, CAP_OPT_NONE) != 0 &&
> +	    !cap_raised(current_cred()->cap_permitted, cap))
> +		return 0;
> +	return 1;
> +}
> +#endif
> +
>   /**
>    * cap_capset - Validate and apply proposed changes to current's capabilities
>    * @new: The proposed new credentials; alterations should be made here
> @@ -1382,6 +1404,43 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>   			return commit_creds(new);
>   		}
>   
> +#ifdef CONFIG_USER_NS
> +	case PR_CAP_USERNS:
> +		if (arg2 == PR_CAP_USERNS_CLEAR_ALL) {
> +			if (arg3 | arg4 | arg5)
> +				return -EINVAL;
> +
> +			new = prepare_creds();
> +			if (!new)
> +				return -ENOMEM;
> +			cap_clear(new->cap_userns);
> +			return commit_creds(new);
> +		}
> +
> +		if (((!cap_valid(arg3)) | arg4 | arg5))
> +			return -EINVAL;
> +
> +		if (arg2 == PR_CAP_USERNS_IS_SET) {
> +			return !!cap_raised(current_cred()->cap_userns, arg3);
> +		} else if (arg2 != PR_CAP_USERNS_RAISE &&
> +			   arg2 != PR_CAP_USERNS_LOWER) {
> +			return -EINVAL;
> +		} else {
> +			if (arg2 == PR_CAP_USERNS_RAISE &&
> +			    !cap_uns_is_raiseable(arg3))
> +				return -EPERM;
> +
> +			new = prepare_creds();
> +			if (!new)
> +				return -ENOMEM;
> +			if (arg2 == PR_CAP_USERNS_RAISE)
> +				cap_raise(new->cap_userns, arg3);
> +			else
> +				cap_lower(new->cap_userns, arg3);
> +			return commit_creds(new);
> +		}
> +#endif
> +
>   	default:
>   		/* No functionality available - continue with default */
>   		return -ENOSYS;
> diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
> index b5d5333ab330..e3670d815435 100644
> --- a/security/keys/process_keys.c
> +++ b/security/keys/process_keys.c
> @@ -944,6 +944,9 @@ void key_change_session_keyring(struct callback_head *twork)
>   	new->cap_effective	= old->cap_effective;
>   	new->cap_ambient	= old->cap_ambient;
>   	new->cap_bset		= old->cap_bset;
> +#ifdef CONFIG_USER_NS
> +	new->cap_userns		= old->cap_userns;
> +#endif
>   
>   	new->jit_keyring	= old->jit_keyring;
>   	new->thread_keyring	= key_get(old->thread_keyring);