net: udp_tunnel: fix memory leak in udp_tunnel_nic_unregister()

Overview

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

net: udp_tunnel: fix memory leak in udp_tunnel_nic_unregister()

syzbot reported a memory leak [1] in the UDP tunnel NIC offload code.

When device registration fails (e.g. in register_netdevice()), netdev core unwinds by sending a single NETDEV_UNREGISTER notification. If work was queued during NETDEV_REGISTER (utn->work_pending is set), udp_tunnel_nic_unregister() returns early:

if (utn->work_pending)
	return;

Because failed registrations do not enter netdev_wait_allrefs_any(), no subsequent NETDEV_UNREGISTER rebroadcast will ever occur. As a result, the struct udp_tunnel_nic allocated in udp_tunnel_nic_alloc() is leaked permanently.

Fix this by removing the early return. Instead, synchronously cancel any pending work with cancel_delayed_work_sync() before freeing @utn.

To be able to call cancel_delayed_work_sync() while holding RTNL (the work also needs RTNL), switch udp_tunnel_nic_device_sync_work() to rtnl_trylock(). If RTNL is contended, requeue the work with a 1 jiffy delay (via queue_delayed_work()) to prevent high CPU contention while waiting for RTNL lock.

The utn->work_pending bookkeeping is no longer needed and is removed, as the workqueue core already tracks the pending/running state of the work.

[1] BUG: memory leak unreferenced object 0xffff888127d5f840 (size 96): comm "syz-executor", pid 5806, jiffies 4294942188 backtrace (crc 99fdb6c8): __kmalloc_noprof+0x3bf/0x550 udp_tunnel_nic_alloc net/ipv4/udp_tunnel_nic.c:756 [inline] udp_tunnel_nic_register net/ipv4/udp_tunnel_nic.c:833 [inline] udp_tunnel_nic_netdevice_event+0x804/0xab0 net/ipv4/udp_tunnel_nic.c:931 notifier_call_chain+0x59/0x160 kernel/notifier.c:85 call_netdevice_notifiers_info+0x7d/0xb0 net/core/dev.c:2250 register_netdevice+0xc10/0xeb0 net/core/dev.c:11478

Affected

• Linux >= cc4e3835eff474aa274d6e1d18f69d9d296d3b76 < 52c753b46265243406690d980faa9514d3f53827
• Linux >= cc4e3835eff474aa274d6e1d18f69d9d296d3b76 < b712da45bd9b9835d132e9fbc0c3e43c164cb1fd
• Linux >= cc4e3835eff474aa274d6e1d18f69d9d296d3b76 < f82a2ded3d7a9098267c84f14eafdb98c4550ac5
• Linux >= cc4e3835eff474aa274d6e1d18f69d9d296d3b76 < b322532a4b774db1b480a2de84125910a70739a5
• Linux >= cc4e3835eff474aa274d6e1d18f69d9d296d3b76 < 080695e6f005e2396f1207fd69d24c442cb230c6
• Linux 5.9

Remediation

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