On Tue, Sep 03, 2024 at 06:28:50PM -0700, Connor O'Brien wrote:
From: Daniel Borkmann daniel@iogearbox.net
commit 8520e224f547cd070c7c8f97b1fc6d58cff7ccaa upstream.
Fix cgroup v1 interference when non-root cgroup v2 BPF programs are used. Back in the days, commit bd1060a1d671 ("sock, cgroup: add sock->sk_cgroup") embedded per-socket cgroup information into sock->sk_cgrp_data and in order to save 8 bytes in struct sock made both mutually exclusive, that is, when cgroup v1 socket tagging (e.g. net_cls/net_prio) is used, then cgroup v2 falls back to the root cgroup in sock_cgroup_ptr() (&cgrp_dfl_root.cgrp).
The assumption made was "there is no reason to mix the two and this is in line with how legacy and v2 compatibility is handled" as stated in bd1060a1d671. However, with Kubernetes more widely supporting cgroups v2 as well nowadays, this assumption no longer holds, and the possibility of the v1/v2 mixed mode with the v2 root fallback being hit becomes a real security issue.
Many of the cgroup v2 BPF programs are also used for policy enforcement, just to pick _one_ example, that is, to programmatically deny socket related system calls like connect(2) or bind(2). A v2 root fallback would implicitly cause a policy bypass for the affected Pods.
In production environments, we have recently seen this case due to various circumstances: i) a different 3rd party agent and/or ii) a container runtime such as [0] in the user's environment configuring legacy cgroup v1 net_cls tags, which triggered implicitly mentioned root fallback. Another case is Kubernetes projects like kind [1] which create Kubernetes nodes in a container and also add cgroup namespaces to the mix, meaning programs which are attached to the cgroup v2 root of the cgroup namespace get attached to a non-root cgroup v2 path from init namespace point of view. And the latter's root is out of reach for agents on a kind Kubernetes node to configure. Meaning, any entity on the node setting cgroup v1 net_cls tag will trigger the bypass despite cgroup v2 BPF programs attached to the namespace root.
Generally, this mutual exclusiveness does not hold anymore in today's user environments and makes cgroup v2 usage from BPF side fragile and unreliable. This fix adds proper struct cgroup pointer for the cgroup v2 case to struct sock_cgroup_data in order to address these issues; this implicitly also fixes the tradeoffs being made back then with regards to races and refcount leaks as stated in bd1060a1d671, and removes the fallback, so that cgroup v2 BPF programs always operate as expected.
[0] https://github.com/nestybox/sysbox/ [1] https://kind.sigs.k8s.io/
Fixes: bd1060a1d671 ("sock, cgroup: add sock->sk_cgroup") Signed-off-by: Daniel Borkmann daniel@iogearbox.net Signed-off-by: Alexei Starovoitov ast@kernel.org Acked-by: Stanislav Fomichev sdf@google.com Acked-by: Tejun Heo tj@kernel.org Link: https://lore.kernel.org/bpf/20210913230759.2313-1-daniel@iogearbox.net [resolve trivial conflicts] Signed-off-by: Connor O'Brien connor.obrien@crowdstrike.com
Hello,
Requesting that these patches be applied to 5.10-stable. Tested to confirm that the cgroup_v1v2 bpf selftest for this issue passes on 5.10 with the first patch applied and fails without it. The syzkaller crash referenced in the second patch reproduces on 5.10 after applying just the first patch, but not with both patches applied.
Conflicts were due to unrelated changes to the surrounding context; the actual code change remains the same as in the upstream patch.
Both now queued up, thanks.
greg k-h