[PATCH v2 1/4] capabilities: Add user namespace capabilities

[Serge E. Hallyn]

On Sun, Jun 09, 2024 at 03:43:34AM -0700, 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
> 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
> 
> 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 user namespace can regain userns capabilities
> previously dropped:
> 
>     p’U = (pE & CAP_SETPCAP) ? X : (X & pP)
> 
> 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
>  */

...

> +#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);
> +		if (arg2 != PR_CAP_USERNS_RAISE && arg2 != PR_CAP_USERNS_LOWER)
> +			return -EINVAL;
> +		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);

Now, one thing that does occur to me here is that there is a
very mild form of sendmail-capabilities vulnerability that
could happen here.  Unpriv user joe can drop CAP_SYS_ADMIN
from cap_userns, then run a setuid-root program which starts
a container which expects CAP_SYS_ADMIN.  This could be a
shared container, and so joe could be breaking expected
behavior there.

I *think* we want to say we don't care about this case, but
if we did, I suppose we could say that the normal cap raise
rules on setuid should apply to cap_userns?