Re: [PATCH bpf-next v3 02/17] bpf: allow attaching struct_ops to cgroups

[bot+bpf-ci@kernel.org]

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> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 69988af44b37..7b1903be6f69 100644

[ ... ]

> @@ -307,12 +308,23 @@ static void cgroup_bpf_release(struct work_struct *work)
>  					       bpf.release_work);
>  	struct bpf_prog_array *old_array;
>  	struct list_head *storages = &cgrp->bpf.storages;
> +	struct bpf_struct_ops_link *st_link, *st_tmp;
>  	struct bpf_cgroup_storage *storage, *stmp;
> +	LIST_HEAD(st_links);
>
>  	unsigned int atype;
>
>  	cgroup_lock();
>
> +	list_splice_init(&cgrp->bpf.struct_ops_links, &st_links);
> +	list_for_each_entry_safe(st_link, st_tmp, &st_links, list) {
> +		st_link->cgroup = NULL;
> +		st_link->cgroup_removed = true;
> +		cgroup_put(cgrp);
> +		if (IS_ERR(bpf_link_inc_not_zero(&st_link->link)))
> +			list_del(&st_link->list);
> +	}

Can this code access st_link after it has been freed?

Once st_link->cgroup is set to NULL, if another CPU concurrently closes
the link FD, bpf_struct_ops_map_link_dealloc() will see st_link->cgroup
as NULL, skip the cgroup_bpf_detach_struct_ops() call, and proceed to
kfree(st_link). Then the bpf_link_inc_not_zero() call here would access
freed memory.

The race window is between setting st_link->cgroup = NULL and calling
bpf_link_inc_not_zero(). During this window:

  CPU 0 (cgroup_bpf_release)     CPU 1 (close link FD)
  ---------------------------    ---------------------
  st_link->cgroup = NULL
                                 bpf_link_put_direct()
                                   bpf_link_free()
                                     bpf_struct_ops_map_link_dealloc()
                                       if (st_link->cgroup) // false
                                       kfree(st_link) // freed!
  bpf_link_inc_not_zero() // use-after-free

Would it be safer to call bpf_link_inc_not_zero() before modifying
st_link fields? That way, if the link is already at refcount zero,
the loop would skip it; otherwise, the held reference would prevent
the concurrent free.


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21382620878

AI-authorship-score: low
AI-authorship-explanation: The commit is authored by a known kernel maintainer with a consistent, human-like technical writing style and appropriate kernel development patterns.
issues-found: 1
issue-severity-score: medium
issue-severity-explanation: Use-after-free race condition in cgroup_bpf_release() that can cause kernel crash when link FD is closed concurrently with cgroup deletion.