Re: [PATCH v9 00/23] ima: Namespace IMA with audit support in IMA-ns

[Stefan Berger <stefanb@linux.ibm.com>]

On 1/26/22 08:19, Christian Brauner wrote:
> On Tue, Jan 25, 2022 at 05:46:22PM -0500, Stefan Berger wrote:
>> From: Stefan Berger <stefanb@linux.ibm.com>
>>
>> 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 is created when a user namespace is
>> created, although this is done late when SecurityFS is mounted inside
>> a user namespace. The advantage of piggy backing on the user namespace
>> 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
>> cp /sbin/busybox rootfs/bin/busybox2
>> echo >> rootfs/bin/busybox2
>> PATH=/bin unshare --user --map-root-user --mount-proc --pid --fork \
>>    --root rootfs busybox sh -c \
>>   "busybox mount -t securityfs /mnt /mnt; \
>>    busybox echo 1 > /mnt/ima/active; \
>>    busybox echo 'audit func=BPRM_CHECK mask=MAY_EXEC' > /mnt/ima/policy; \
> I think we need to limit the number of rules that can be added to an ima
> namespace to prevent DOS attacks. The current implementation allows
> users to write as many ima rules as they want.
>
> My suggestion would be that you look at real-world data to figure out
> what a fairly common number of rules is that people write. Then use this
> as the hard-coded limit for a first implementation. If the use-case


I would now go with a hard-coded (generous) limit of 1024 rules for 
non-init_ima_ns, and leave init_ima_ns unbounded.


> arises you can later make this limit configurable by introducing a
> ucount for ima rules via /proc/sys/user/max_ima_rules.

Ok, let's defer this.


>
> Additionally, you should probably switch a lot of ima allocations from
> GFP_KERNEL to GFP_KERNEL_ACCOUNT as allocations triggerable from userns
> should be treated as untrusted.
Ok, done.