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

Overview

In the Linux kernel, the following vulnerability has been resolved:

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.

Affected

• Linux >= e624d4ed4aa8cc3c69d1359b0aaea539203ed266 < 4a3d0fe30b907ff324b1b49756f7e713d67f3645
• Linux >= e624d4ed4aa8cc3c69d1359b0aaea539203ed266 < b089aa6e94d7a08e74d076a0fe274842dc9feccc
• Linux >= e624d4ed4aa8cc3c69d1359b0aaea539203ed266 < 571a05ea1baaccc0dc1e0d227b2cbc978b96d392
• Linux >= e624d4ed4aa8cc3c69d1359b0aaea539203ed266 < cb2c1f3cf65b855548e1b8d55a08bfbaa5a0901a
• Linux >= e624d4ed4aa8cc3c69d1359b0aaea539203ed266 < d4c4bd231ebad70e6f30db429e9640bf378b2f52
• Linux >= e624d4ed4aa8cc3c69d1359b0aaea539203ed266 < 7027e705062482a8cea43a1c13ede3c35653966f
• Linux >= e624d4ed4aa8cc3c69d1359b0aaea539203ed266 < 8ed82f807bb09d2c8455aaa665f2c6cb17bc6a19
• Linux 5.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.