cgroup: Make current_cgns_cgroup_dfl() safe to call after exit_task_namespace()

The commit 332ea1f6 ("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.

Fixes: 332ea1f6 ("bpf: Add bpf_cgroup_from_id() kfunc")
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/ZBDuVWiFj2jiz3i8@slm.duckdns.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/cgroup/cgroup.c

@@ -1465,8 +1465,18 @@ 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_namespaces() and
+		 * nsproxy == NULL. Fall back to cgrp_dfl_root which will make all
+		 * cgroups visible for lookups.
+		 */
+		return &cgrp_dfl_root.cgrp;
+	}
 }
 
 /* look up cgroup associated with given css_set on the specified hierarchy */