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

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The commit 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.

Fixes: 332ea1f697be ("bpf: Add bpf_cgroup_from_id() kfunc")
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/ZBDuVWiFj2jiz3i8@slm.duckdns.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Tejun Heo 2023-03-14 11:59:49 -10:00 committed by Alexei Starovoitov
parent 3c2611bac0
commit b8a2e3f93d
1 changed files with 12 additions and 2 deletions
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14
kernel/cgroup/cgroup.c
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@ -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 */