Re: [RFC] [PATCH bpf-next 1/1] bpf: Add a BPF helper for getting the cgroup path of current task

On Fri, May 14, 2021 at 6:06 AM xufeng zhang
[off-list ref] wrote:

在 2021/5/13 上午6:55, Alexei Starovoitov 写道:

quoted

On Wed, May 12, 2021 at 05:58:23PM +0800, Xufeng Zhang wrote:

quoted

To implement security rules for application containers by utilizing
bpf LSM, the container to which the current running task belongs need
to be known in bpf context. Think about this scenario: kubernetes
schedules a pod into one host, before the application container can run,
the security rules for this application need to be loaded into bpf
maps firstly, so that LSM bpf programs can make decisions based on
this rule maps.

However, there is no effective bpf helper to achieve this goal,
especially for cgroup v1. In the above case, the only available information
from user side is container-id, and the cgroup path for this container
is certain based on container-id, so in order to make a bridge between
user side and bpf programs, bpf programs also need to know the current
cgroup path of running task.

...

quoted

+#ifdef CONFIG_CGROUPS
+BPF_CALL_2(bpf_get_current_cpuset_cgroup_path, char *, buf, u32, buf_len)
+{
+    struct cgroup_subsys_state *css;
+    int retval;
+
+    css = task_get_css(current, cpuset_cgrp_id);
+    retval = cgroup_path_ns(css->cgroup, buf, buf_len, &init_cgroup_ns);
+    css_put(css);
+    if (retval >= buf_len)
+            retval = -ENAMETOOLONG;

Manipulating string path to check the hierarchy will be difficult to do
inside bpf prog. It seems to me this helper will be useful only for
simplest cgroup setups where there is no additional cgroup nesting
within containers.
Have you looked at *ancestor_cgroup_id and *cgroup_id helpers?
They're a bit more flexible when dealing with hierarchy and
can be used to achieve the same correlation between kernel and user cgroup ids.

KP,

do you have any suggestion?

I haven't really tried this yet, but have you considered using task local
storage to identify the container?

- Add a task local storage with container ID somewhere in the container
  manager
- Propagate this ID to all the tasks within a container using task security
  blob management hooks (like task_alloc and task_free) etc.

what I am thinking is the internal kernel object(cgroup id or ns.inum)
is not so user friendly, we can get the container-context from them for
tracing scenario, but not for LSM blocking cases, I'm not sure how
Google internally resolve similar issue.


Thanks!

Xufeng