332ea1f697be ("bpf: Add bpf_cgroup_from_id() kfunc") added
bpf_cgroup_from_id() which calls current_cgns_cgroup_dfl() through
cgroup_get_from_id(). However, BPF programs may be attached to a point where
current->nsproxy has already been cleared to NULL by exit_task_namespace()
and calling bpf_cgroup_from_id() would cause an oops.

Just return the system-wide root if nsproxy has been cleared. This allows
all cgroups to be looked up after the task passed through
exit_task_namespace(), which semantically makes sense. Given that the only
way to get this behavior is through BPF programs, it seems safe but let's
see what others think.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Fixes: 332ea1f697be ("bpf: Add bpf_cgroup_from_id() kfunc")
---
Hello,

Given that this is only exposed through a commit in bpf/for-next, I think
it'd be best to route it together. If this looks okay, please apply to
bpf/for-next.

Thanks.

 kernel/cgroup/cgroup.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 8bfb0e2a87ee..c0da5cb9f193 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1465,8 +1465,20 @@ static struct cgroup *current_cgns_cgroup_dfl(void)
 {
 	struct css_set *cset;
 
-	cset = current->nsproxy->cgroup_ns->root_cset;
-	return __cset_cgroup_from_root(cset, &cgrp_dfl_root);
+	if (current->nsproxy) {
+		cset = current->nsproxy->cgroup_ns->root_cset;
+		return __cset_cgroup_from_root(cset, &cgrp_dfl_root);
+	} else {
+		/*
+		 * NOTE: This function may be called from bpf_cgroup_from_id()
+		 * on a task which has already passed exit_task_namespace() and
+		 * NULL nsproxy. Fall back to cgrp_dfl_root which will make all
+		 * cgroups visible for lookups. Given that BPF progs are
+		 * privileged, this shouldn't create security concerns but there
+		 * may be a better way to handle this.
+		 */
+		return &cgrp_dfl_root.cgrp;
+	}
 }
 
 /* look up cgroup associated with given css_set on the specified hierarchy */
-- 
2.39.2

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Tue, 14 Mar 2023 11:59:49 -1000 you wrote:
> 332ea1f697be ("bpf: Add bpf_cgroup_from_id() kfunc") added
> bpf_cgroup_from_id() which calls current_cgns_cgroup_dfl() through
> cgroup_get_from_id(). However, BPF programs may be attached to a point where
> current->nsproxy has already been cleared to NULL by exit_task_namespace()
> and calling bpf_cgroup_from_id() would cause an oops.
> 
> Just return the system-wide root if nsproxy has been cleared. This allows
> all cgroups to be looked up after the task passed through
> exit_task_namespace(), which semantically makes sense. Given that the only
> way to get this behavior is through BPF programs, it seems safe but let's
> see what others think.
> 
> [...]

Here is the summary with links:
  - [bpf/for-next] cgroup: Make current_cgns_cgroup_dfl() safe to call after exit_task_namespace()
    https://git.kernel.org/bpf/bpf-next/c/b8a2e3f93d41

You are awesome, thank you!
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