netlink: avoid hashing the network namespace pointer

Overview

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

netlink: avoid hashing the network namespace pointer

The netlink rhashtable key includes a raw struct net pointer and a user-controlled port ID. Both /proc/net/netlink and socket diagnostics expose the table's bucket order. By binding and rebinding chosen NETLINK_USERSOCK port IDs, an unprivileged reader can distinguish equal buckets and recover the low bits of the Jenkins hash. Its 32-bit seed and the limited set of kernel-image slides can then be searched offline to recover the address of init_net.

Use the namespace's unique, non-address net_cookie in the comparison key instead. It is assigned before the per-net initializers run and remains unchanged for the namespace's lifetime. The lookup key and object hash are still built by netlink_compare_arg_init(), keeping lookup, insertion, removal and rehashing consistent while preserving namespace separation. Neither public table walker needs to change.

Reading the socket's namespace cookie in netlink_compare() is safe under RCU, including during namespace teardown. netlink_release() removes the socket from the hash table and defers its final put with call_rcu(). cleanup_net() runs the per-net exit methods and waits for outstanding RCU callbacks with rcu_barrier() before freeing namespace storage.

Unlike ns.ns_id, net_cookie is also available and initialized during setup_net() in older stable kernels, making the change straightforward to backport to v5.15 and later.

Affected

• Linux >= c428ecd1a21f1457ca3beb4df71b8a079c410e41 < bdd55b8a8acbf6b4840ab420eb6a63774bf4593b
• Linux >= c428ecd1a21f1457ca3beb4df71b8a079c410e41 < 078deff25f8a849ecd1c3fe17b1e473b1cf44f28
• Linux >= c428ecd1a21f1457ca3beb4df71b8a079c410e41 < f88bb3606e4bd11ee035b682cddc2999e604609a
• Linux 4.1

Remediation

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