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

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Date: Thu, 16 May 2024 02:22:02 -0700
From: Jonathan Calmels <jcalmels@...0.net>
To: 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>
Cc: containers@...ts.linux.dev,
	Jonathan Calmels <jcalmels@...0.net>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	keyrings@...r.kernel.org
Subject: [PATCH 0/3] Introduce user namespace capabilities

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.

- 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(-)

-- 
2.45.0