linux-kernel - Re: [PATCH 0/3] Introduce user namespace capabilities

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Date: Thu, 16 May 2024 12:07:07 -0700
From: Casey Schaufler <casey@...aufler-ca.com>
To: Jonathan Calmels <jcalmels@...0.net>, brauner@...nel.org,
 ebiederm@...ssion.com, Luis Chamberlain <mcgrof@...nel.org>,
 Kees Cook <keescook@...omium.org>, Joel Granados <j.granados@...sung.com>,
 Serge Hallyn <serge@...lyn.com>, Paul Moore <paul@...l-moore.com>,
 James Morris <jmorris@...ei.org>, David Howells <dhowells@...hat.com>,
 Jarkko Sakkinen <jarkko@...nel.org>, Casey Schaufler <casey@...aufler-ca.com>
Cc: containers@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-security-module@...r.kernel.org,
 keyrings@...r.kernel.org
Subject: Re: [PATCH 0/3] Introduce user namespace capabilities

On 5/16/2024 2:22 AM, Jonathan Calmels wrote:
> It's that time of the year again where we debate security settings for user
> namespaces ;)
>
> I’ve been experimenting with different approaches to address the gripe
> around user namespaces being used as attack vectors.
> After invaluable feedback from Serge and Christian offline, this is what I
> came up with.
>
> There are obviously a lot of things we could do differently but I feel this
> is the right balance between functionality, simplicity and security. This
> also serves as a good foundation and could always be extended if the need
> arises in the future.
>
> Notes:
>
> - Adding a new capability set is far from ideal, but trying to reuse the
>   existing capability framework was deemed both impractical and
>   questionable security-wise, so here we are.

I suggest that adding a capability set for user namespaces is a bad idea:
	- It is in no way obvious what problem it solves
	- It is not obvious how it solves any problem
	- The capability mechanism has not been popular, and relying on a
	  community (e.g. container developers) to embrace it based on this
	  enhancement is a recipe for failure
	- Capabilities are already more complicated than modern developers
	  want to deal with. Adding another, special purpose set, is going
	  to make them even more difficult to use.

> - We might want to add new capabilities for some of the checks instead of
>   reusing CAP_SETPCAP every time. Serge mentioned something like
>   CAP_SYS_LIMIT?
>
> - In the last patch, we could decide to have stronger requirements and
>   perform checks inside cap_capable() in case we want to retroactively
>   prevent capabilities in old namespaces, this might be an overreach though
>   so I left it out.
>
>   I'm also not fond of the ulong logic for setting the sysctl parameter, on
>   the other hand, the usermodhelper code always uses two u32s which makes it
>   very confusing to set in userspace.
>
>
> Jonathan Calmels (3):
>   capabilities: user namespace capabilities
>   capabilities: add securebit for strict userns caps
>   capabilities: add cap userns sysctl mask
>
>  fs/proc/array.c                 |  9 ++++
>  include/linux/cred.h            |  3 ++
>  include/linux/securebits.h      |  1 +
>  include/linux/user_namespace.h  |  7 +++
>  include/uapi/linux/prctl.h      |  7 +++
>  include/uapi/linux/securebits.h | 11 ++++-
>  kernel/cred.c                   |  3 ++
>  kernel/sysctl.c                 | 10 ++++
>  kernel/umh.c                    | 16 +++++++
>  kernel/user_namespace.c         | 83 ++++++++++++++++++++++++++++++---
>  security/commoncap.c            | 59 +++++++++++++++++++++++
>  security/keys/process_keys.c    |  3 ++
>  12 files changed, 204 insertions(+), 8 deletions(-)
>