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

nexthop: Fix memory leaks in nexthop notification chain listeners

syzkaller discovered memory leaks [1] that can be reduced to the following commands:

ip nexthop add id 1 blackhole

devlink dev reload pci/0000:06:00.0

As part of the reload flow, mlxsw will unregister its netdevs and then unregister from the nexthop notification chain. Before unregistering from the notification chain, mlxsw will receive delete notifications for nexthop objects using netdevs registered by mlxsw or their uppers. mlxsw will not receive notifications for nexthops using netdevs that are not dismantled as part of the reload flow. For example, the blackhole nexthop above that internally uses the loopback netdev as its nexthop device.

One way to fix this problem is to have listeners flush their nexthop tables after unregistering from the notification chain. This is error-prone as evident by this patch and also not symmetric with the registration path where a listener receives a dump of all the existing nexthops.

Therefore, fix this problem by replaying delete notifications for the listener being unregistered. This is symmetric to the registration path and also consistent with the netdev notification chain.

The above means that unregister_nexthop_notifier(), like register_nexthop_notifier(), will have to take RTNL in order to iterate over the existing nexthops and that any callers of the function cannot hold RTNL. This is true for mlxsw and netdevsim, but not for the VXLAN driver. To avoid a deadlock, change the latter to unregister its nexthop listener without holding RTNL, making it symmetric to the registration path.

[1] unreferenced object 0xffff88806173d600 (size 512): comm syz-executor.0, pid 1290, jiffies 4295583142 (age 143.507s) hex dump (first 32 bytes): 41 9d 1e 60 80 88 ff ff 08 d6 73 61 80 88 ff ff A..`……sa…. 08 d6 73 61 80 88 ff ff 01 00 00 00 00 00 00 00 ..sa………… backtrace: [] kmemleak_alloc_recursive include/linux/kmemleak.h:43 [inline] [] slab_post_alloc_hook+0x96/0x490 mm/slab.h:522 [] slab_alloc_node mm/slub.c:3206 [inline] [] slab_alloc mm/slub.c:3214 [inline] [] kmem_cache_alloc_trace+0x163/0x370 mm/slub.c:3231 [] kmalloc include/linux/slab.h:591 [inline] [] kzalloc include/linux/slab.h:721 [inline] [] mlxsw_sp_nexthop_obj_group_create drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:4918 [inline] [] mlxsw_sp_nexthop_obj_new drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:5054 [inline] [] mlxsw_sp_nexthop_obj_event+0x59a/0x2910 drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:5239 [] notifier_call_chain+0xbd/0x210 kernel/notifier.c:83 [] blocking_notifier_call_chain kernel/notifier.c:318 [inline] [] blocking_notifier_call_chain+0x72/0xa0 kernel/notifier.c:306 [] call_nexthop_notifiers+0x156/0x310 net/ipv4/nexthop.c:244 [] insert_nexthop net/ipv4/nexthop.c:2336 [inline] [] nexthop_add net/ipv4/nexthop.c:2644 [inline] [] rtm_new_nexthop+0x14e8/0x4d10 net/ipv4/nexthop.c:2913 [] rtnetlink_rcv_msg+0x448/0xbf0 net/core/rtnetlink.c:5572 [] netlink_rcv_skb+0x173/0x480 net/netlink/af_netlink.c:2504 [] rtnetlink_rcv+0x22/0x30 net/core/rtnetlink.c:5590 [] netlink_unicast_kernel net/netlink/af_netlink.c:1314 [inline] [] netlink_unicast+0x5ae/0x7f0 net/netlink/af_netlink.c:1340 [] netlink_sendmsg+0x8e1/0xe30 net/netlink/af_netlink.c:1929 [] sock_sendmsg_nosec net/socket.c:704 [inline —truncated—

Weakness

The product does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources.

Affected Software

Extended Description

Limited resources include memory, file system storage, database connection pool entries, and CPU. If an attacker can trigger the allocation of these limited resources, but the number or size of the resources is not controlled, then the attacker could cause a denial of service that consumes all available resources. This would prevent valid users from accessing the product, and it could potentially have an impact on the surrounding environment. For example, a memory exhaustion attack against an application could slow down the application as well as its host operating system. There are at least three distinct scenarios which can commonly lead to resource exhaustion:

Resource exhaustion problems are often result due to an incorrect implementation of the following situations:

Potential Mitigations

• Mitigation of resource exhaustion attacks requires that the target system either:

• The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.

• The second solution is simply difficult to effectively institute – and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/147.html
• https://cwe.mitre.org/data/definitions/227.html
• https://cwe.mitre.org/data/definitions/492.html

References

• https://git.kernel.org/stable/c/3106a0847525befe3e22fc723909d1b21eb0d520
• https://git.kernel.org/stable/c/741760fa6252628a3d3afad439b72437d4b123d9