bpf: Use RCU-safe iteration in dev_map_redirect_multi() SKB path

The DEVMAP_HASH branch in dev_map_redirect_multi() uses hlist_for_each_entry_safe() to iterate hash buckets, but this function runs under RCU protection (called from xdp_do_generic_redirect_map() in softirq context). Concurrent writers (__dev_map_hash_update_elem, dev_map_hash_delete_elem) modify the list using RCU primitives (hlist_add_head_rcu, hlist_del_rcu). hlist_for_each_entry_safe() performs plain pointer dereferences without rcu_dereference(), missing the acquire barrier needed to pair with writers' rcu_assign_pointer(). On weakly-ordered architectures (ARM64, POWER), a reader can observe a partially-constructed node. It also defeats CONFIG_PROVE_RCU lockdep validation and KCSAN data-race detection. Replace with hlist_for_each_entry_rcu() using rcu_read_lock_bh_held() as the lockdep condition, consistent with the rcu_dereference_check() used in the DEVMAP (non-hash) branch of the same functions. Also fix the same incorrect lockdep_is_held(&dtab->index_lock) condition in dev_map_enqueue_multi(), where the lock is not held either.

David Carlier committed 7 months ago

Commit 8ed82f81 parent 7f5b0a61 file · +2 −3Browse files

Showing the changes to kernel (1 of 1 file). Show all

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kernel/bpf/devmap.c+2−3

 		for (i = 0; i < dtab->n_buckets; i++) {
 			head = dev_map_index_hash(dtab, i);
 			hlist_for_each_entry_rcu(dst, head, index_hlist,
-						 lockdep_is_held(&dtab->index_lock)) {
+						rcu_read_lock_bh_held()) {
 				if (!is_valid_dst(dst, xdpf))
 					continue;
 	struct bpf_dtab_netdev *dst, *last_dst = NULL;
 	int excluded_devices[1+MAX_NEST_DEV];
 	struct hlist_head *head;
-	struct hlist_node *next;
 	int num_excluded = 0;
 	unsigned int i;
 	int err;
 	} else { /* BPF_MAP_TYPE_DEVMAP_HASH */
 		for (i = 0; i < dtab->n_buckets; i++) {
 			head = dev_map_index_hash(dtab, i);
-			hlist_for_each_entry_safe(dst, next, head, index_hlist) {
+			hlist_for_each_entry_rcu(dst, head, index_hlist, rcu_read_lock_bh_held()) {
 				if (is_ifindex_excluded(excluded_devices, num_excluded,
 							dst->dev->ifindex))
 					continue;