Re: [PATCH net-next 1/9] batman-adv: bla: avoid double free after failed backbone_hash alloc

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- [Medium] The commit message describes a double free that cannot happen.

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batman-adv: bla: avoid double free after failed backbone_hash alloc

This commit sets bat_priv->bla.claim_hash to NULL after
batadv_hash_destroy() in the batadv_bla_init() error path taken when the
backbone_hash allocation fails. The commit message says that the
destructor later frees claim_hash a second time via batadv_mesh_free()
-> batadv_bla_free().

When batadv_bla_init() fails to initialize the backbone_hash, it is freeing
the (previously) allocated claim_hash. The initialization function will
then return an error and the net_device initialization will stop. The
destructor will be called instead and (indirectly via batadv_mesh_free() ->
batadv_bla_free()) will try to free the bat_priv->bla.claim_hash again.

[Severity: Medium]

Can this sequence actually happen? batadv_bla_init() has only one
caller, batadv_mesh_init(), and when it fails that function jumps past
batadv_bla_free():

net/batman-adv/main.c:batadv_mesh_init() {
    ...
	ret = batadv_bla_init(bat_priv);
	if (ret < 0) {
		WRITE_ONCE(bat_priv->mesh_state, BATADV_MESH_DEACTIVATING);
		goto err_bla;
	}
    ...
err_dat:
	batadv_bla_free(bat_priv);
err_bla:
	batadv_v_mesh_free(bat_priv);
    ...
}

batadv_mesh_init() is only reached from batadv_meshif_init_late(), the
.ndo_init callback, which returns the error. In register_netdevice(), an
ndo_init failure skips the priv_destructor call:

net/core/dev.c:register_netdevice() {
    ...
	if (dev->netdev_ops->ndo_init) {
		ret = dev->netdev_ops->ndo_init(dev);
		if (ret) {
			if (ret > 0)
				ret = -EIO;
			goto err_free_name;
		}
	}
    ...
err_uninit:
	if (dev->netdev_ops->ndo_uninit)
		dev->netdev_ops->ndo_uninit(dev);
	if (dev->priv_destructor)
		dev->priv_destructor(dev);
err_free_name:
	netdev_name_node_free(dev->name_node);
	goto out;
}

After that, free_netdev() sees reg_state == NETREG_UNINITIALIZED and
only calls kvfree(dev). So batadv_meshif_free() -> batadv_mesh_free() ->
batadv_bla_free() does not seem to run on this path, and nothing reads
or frees the stale claim_hash pointer again.

Also, if the destructor did run here, wouldn't batadv_bla_free() call
disable_delayed_work_sync(&bat_priv->bla.work) on work that was never
set up? INIT_DELAYED_WORK() only runs after both hashes are allocated.
The patch doesn't handle that case, so the description and the fix don't
seem to match.

This makes the change look like harmless hardening rather than a
double-free fix. Could the commit message describe it that way, or show
the path where the second batadv_hash_destroy() actually happens? As
written, stable or CVE tooling may treat it as a memory-safety fix.

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Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260930094558.3723766-2-sw%40simonwunderlich.de