HIGH
macvlan Source Hash UAF
CVE-2026-23209
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
KernelScan AI7.8HIGH
01Description
In the Linux kernel, the following vulnerability has been resolved: macvlan: fix error recovery in macvlan_common_newlink() valis provided a nice repro to crash the kernel: ip link add p1 type veth peer p2 ip link set address 00:00:00:00:00:20 dev p1 ip link set up dev p1 ip link set up dev p2 ip link add mv0 link p2 type macvlan mode source ip link add invalid% link p2 type macvlan mode source macaddr add 00:00:00:00:00:20 ping -c1 -I p1 1.2.3.4 He also gave a very detailed analysis: <quote valis> The issue is triggered when a new macvlan link is created with MACVLAN_MODE_SOURCE mode and MACVLAN_MACADDR_ADD (or MACVLAN_MACADDR_SET) parameter, lower device already has a macvlan port and register_netdevice() called from macvlan_common_newlink() fails (e.g. because of the invalid link name). In this case macvlan_hash_add_source is called from macvlan_change_sources() / macvlan_common_newlink(): This adds a reference to vlan to the port's vlan_source_hash using macvlan_source_entry. vlan is a pointer to the priv data of the link that is being created. When register_netdevice() fails, the error is returned from macvlan_newlink() to rtnl_newlink_create(): if (ops->newlink) err = ops->newlink(dev, ¶ms, extack); else err = register_netdevice(dev); if (err < 0) { free_netdev(dev); goto out; } and free_netdev() is called, causing a kvfree() on the struct net_device that is still referenced in the source entry attached to the lower device's macvlan port. Now all packets sent on the macvlan port with a matching source mac address will trigger a use-after-free in macvlan_forward_source(). </quote valis> With all that, my fix is to make sure we call macvlan_flush_sources() regardless of @create value whenever "goto destroy_macvlan_port;" path is taken. Many thanks to valis for following up on this issue.
02KernelScan AI Analysis
Risk summary
An attacker with network administration privileges can create malformed macvlan interfaces to cause a use-after-free vulnerability. When packets are subsequently processed through the macvlan port, the kernel accesses freed memory, potentially leading to system crashes or privilege escalation.
Vulnerability analysis
Root Cause: In macvlan_common_newlink(), when creating a new macvlan device in SOURCE mode with MACVLAN_MACADDR_ADD/SET parameters, the code calls macvlan_hash_add_source() which adds a reference to the vlan structure in the port's vlan_source_hash. If register_netdevice() subsequently fails (e.g., due to invalid device name), the error path calls free_netdev() which frees the net_device structure, but the source hash entry still holds a dangling pointer to the freed vlan data. Later packet forwarding through macvlan_forward_source() accesses this freed memory.
Attack Surface: Local attack surface requiring the ability to create network interfaces via netlink (CAP_NET_ADMIN or equivalent). The vulnerability is triggered by creating macvlan devices with specific parameters and causing registration failures. Network packet processing on the affected interface triggers the use-after-free.
Fix Mechanism: The fix ensures macvlan_flush_sources() is called regardless of the @create flag value when taking the 'destroy_macvlan_port' error path. This removes all source hash entries that reference the about-to-be-freed vlan structure, preventing the dangling pointer. The macvlan_port_destroy() call is now conditional on @create to preserve the original port cleanup logic.
03Fix Versions
| Branch | Fixed in | Patch commit |
|---|---|---|
| 5.10 | 5.10.250 | da5c6b8ae47e |
| 5.15 | 5.15.200 | 5dae6b36a7cb |
| 6.1 | 6.1.163 | c43d0e787cbb |
| 6.12 | 6.12.70 | 986967a16214 |
| 6.18 | 6.18.10 | cdedcd5aa3f3 |
| 6.6 | 6.6.124 | 11ba9f0dc865 |
| mainline | 6.19 | f8db6475a836 |