[RFC 00/20] ima: Namespace IMA with audit support in IMA-ns

Stefan Berger
2021-11-30T16:07:53Z

The goal of this series of patches is to start with the namespacing of
IMA and support auditing within an IMA namespace (IMA-ns) as the first
step.

In this series the IMA namespace is piggy backing on the user namespace
and therefore an IMA namespace gets created when a user namespace is
created. The advantage of this is that the user namespace can provide
the keys infrastructure that IMA appraisal support will need later on.

We chose the goal of supporting auditing within an IMA namespace since it
requires the least changes to IMA. Following this series, auditing within
an IMA namespace can be activated by a user running the following lines
that rely on a statically linked busybox to be installed on the host for
execution within the minimal container environment:

mkdir -p rootfs/{bin,mnt,proc}
cp /sbin/busybox rootfs/bin
PATH=/bin unshare --user --map-root-user --mount-proc --pid --fork \
--root rootfs busybox sh -c \
"busybox mount -t securityfs_ns /mnt /mnt; \
busybox echo 'audit func=BPRM_CHECK mask=MAY_EXEC' > /mnt/ima/policy; \
busybox cat /mnt/ima/policy"

Following the audit log on the host the last line cat'ing the IMA policy
inside the namespace would have been audited. Unfortunately the auditing
line is not distinguishable from one stemming from actions on the host.
The hope here is that Richard Brigg's container id support for auditing
would help resolve the problem.

The following lines added to a suitable IMA policy on the host would
cause the execution of the commands inside the container (by uid 1000)
to be measured and audited as well on the host, thus leading to two
auditing messages for the 'busybox cat' above and log entries in IMA's
system log.

echo -e "measure func=BPRM_CHECK mask=MAY_EXEC uid=1000\n" \
"audit func=BPRM_CHECK mask=MAY_EXEC uid=1000\n" \
> /sys/kernel/security/ima/policy

The goal of supporting measurement and auditing by the host, of actions
occurring within IMA namespaces, is that users, particularly root,
should not be able to evade the host's IMA policy just by spawning
new IMA namespaces, running programs there, and discarding the namespaces
again. This is achieved through 'hierarchical processing' of file
accesses that are evaluated against the policy of the namespace where
the action occurred and against all namespaces' and their policies leading
back to the root IMA namespace (init_ima_ns).

The patch series adds support for virtualizing SecurityFS and introduces
a derivative of SecurityFS called 'securityfs_ns' whose goal is to
only display the data relevant to the IMA namespace rather than showing
files and directories of other security subsystems (TPM, evm, Tomoyo,
safesetid) that use the existing SecurityFS and that would show
if one was to bind-mount SecurityFS into a mount namespace.

Much of the code leading up to the virtualization of SecurityFS deals
with moving IMA's variables from various files into the IMA namespace
structure called 'ima_namespace'. When it comes to determining the
current IMA namespace I took the approach to get the current IMA
namespace (get_current_ns()) on the top level and pass the pointer all
the way down to those functions that now need access to the ima_namespace
to get to their variables. This later on comes in handy once hierarchical
processing is implemented in this series where we walk the list of
namespaces backwards and again need to pass the pointer into functions.

A side-effect of IMA being a child of the user namespace becomes apparent
when virtualizing SecurityFS which has the effect that the filesystem code
needs to take an additional reference to the user namespace (for the keyed
filesystem), which in turn leads to the problem that the one additional
reference doesn't allow the user namespace to be deleted. The work-around
for this is to introduce an early teardown reference counter and related
IMA function that gets invoked when the user namespace reference counter
has reached a certain value, '1'. Freeing the filesystem then finally also
leads to the deletion of the user namespace.

This patch also introduces CAP_INTEGRITY_ADMIN as a subset of
CAP_SYS_ADMIN's capabilities that allows access to the IMA policy via
reduced capabilities. We would again later on use this capability to allow
users to set file extended attributes for IMA appraisal support.

The basis for this series of patches is Linux v5.15.
My tree with these patches is here:
https://github.com/stefanberger/linux-ima-namespaces/tree/v5.15%2Bimans.20211119.v4

Regards,
Stefan

Denis Semakin (2):
capabilities: Introduce CAP_INTEGRITY_ADMIN
ima: Use integrity_admin_ns_capable() to check corresponding
capability

Mehmet Kayaalp (2):
ima: Define ns_status for storing namespaced iint data
ima: Namespace audit status flags

Stefan Berger (16):
ima: Add IMA namespace support
ima: Move delayed work queue and variables into ima_namespace
ima: Move IMA's keys queue related variables into ima_namespace
ima: Move policy related variables into ima_namespace
ima: Move ima_htable into ima_namespace
ima: Move measurement list related variables into ima_namespace
ima: Only accept AUDIT rules for IMA non-init_ima_ns namespaces for
now
ima: Implement hierarchical processing of file accesses
securityfs: Prefix global variables with securityfs_
securityfs: Pass static variables as parameters from top level
functions
securityfs: Build securityfs_ns for namespacing support
ima: Move some IMA policy and filesystem related variables into
ima_namespace
ima: Use ns_capable() for namespace policy access
userns: Introduce a refcount variable for calling early teardown
function
ima/userns: Define early teardown function for IMA namespace
ima: Setup securityfs_ns for IMA namespace

include/linux/capability.h | 6 +
include/linux/ima.h | 132 ++++++++-
include/linux/security.h | 18 ++
include/linux/user_namespace.h | 21 +-
include/uapi/linux/capability.h | 7 +-
include/uapi/linux/magic.h | 1 +
init/Kconfig | 12 +
kernel/user.c | 9 +-
kernel/user_namespace.c | 16 +
security/inode.c | 290 ++++++++++++++++---
security/integrity/ima/Makefile | 4 +-
security/integrity/ima/ima.h | 147 ++++++----
security/integrity/ima/ima_api.c | 33 ++-
security/integrity/ima/ima_appraise.c | 26 +-
security/integrity/ima/ima_asymmetric_keys.c | 8 +-
security/integrity/ima/ima_fs.c | 236 +++++++++++++--
security/integrity/ima/ima_init.c | 14 +-
security/integrity/ima/ima_init_ima_ns.c | 77 +++++
security/integrity/ima/ima_main.c | 128 +++++---
security/integrity/ima/ima_ns.c | 119 ++++++++
security/integrity/ima/ima_ns_status.c | 132 +++++++++
security/integrity/ima/ima_policy.c | 142 +++++----
security/integrity/ima/ima_queue.c | 75 +++--
security/integrity/ima/ima_queue_keys.c | 73 ++---
security/integrity/ima/ima_template.c | 4 +-
security/selinux/include/classmap.h | 4 +-
26 files changed, 1394 insertions(+), 340 deletions(-)
create mode 100644 security/integrity/ima/ima_init_ima_ns.c
create mode 100644 security/integrity/ima/ima_ns.c
create mode 100644 security/integrity/ima/ima_ns_status.c

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2.31.1