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.

Fixes: e624d4ed4aa8 ("xdp: Extend xdp_redirect_map with broadcast support")
Signed-off-by: David Carlier <devnexen@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260320072645.16731-1-devnexen@gmail.com

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index 3d619d01088e3e5cc1829b9186e740a5aaad4578..cc0a43ebab6b994feb412b2c9915610c1eeb2957 100644 (file)
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -665,7 +665,7 @@ int dev_map_enqueue_multi(struct xdp_frame *xdpf, struct net_device *dev_rx,
                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;
 
@@ -747,7 +747,6 @@ int dev_map_redirect_multi(struct net_device *dev, struct sk_buff *skb,
        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;
@@ -787,7 +786,7 @@ int dev_map_redirect_multi(struct net_device *dev, struct sk_buff *skb,
        } 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;