CVE-2025-21781 in Linux
Summary
by MITRE • 02/27/2025
In the Linux kernel, the following vulnerability has been resolved:
batman-adv: fix panic during interface removal
Reference counting is used to ensure that batadv_hardif_neigh_node and batadv_hard_iface are not freed before/during batadv_v_elp_throughput_metric_update work is finished.
But there isn't a guarantee that the hard if will remain associated with a soft interface up until the work is finished.
This fixes a crash triggered by reboot that looks like this:
Call trace: batadv_v_mesh_free+0xd0/0x4dc [batman_adv]
batadv_v_elp_throughput_metric_update+0x1c/0xa4 process_one_work+0x178/0x398 worker_thread+0x2e8/0x4d0 kthread+0xd8/0xdc ret_from_fork+0x10/0x20
(the batadv_v_mesh_free call is misleading, and does not actually happen)
I was able to make the issue happen more reliably by changing hardif_neigh->bat_v.metric_work work to be delayed work. This allowed me to track down and confirm the fix.
[[email protected]: prevent entering batadv_v_elp_get_throughput without
soft_iface]
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Analysis
by VulDB Data Team • 05/25/2026
The vulnerability CVE-2025-21781 affects the linux kernel's batman-adv subsystem which implements the Better Approach to Mobile Ad-hoc Networking protocol. This protocol provides mesh networking capabilities for wireless local area networks and relies on proper reference counting mechanisms to manage the lifecycle of network interface objects. The flaw manifests as a potential system panic or crash during the interface removal process when the kernel attempts to free memory structures while background work processes are still executing.
The core technical issue stems from inadequate synchronization between reference counting mechanisms and the execution of background work items. Specifically the batadv_hardif_neigh_node and batadv_hard_iface structures are protected through reference counting to prevent premature deallocation during ongoing batadv_v_elp_throughput_metric_update operations. However, the system fails to guarantee that a hardware interface remains associated with a software interface throughout the entire execution of these background tasks. This race condition creates a scenario where work items may attempt to access freed memory structures, leading to kernel oops or system crashes.
The operational impact of this vulnerability is significant as it can trigger system instability during normal operations including system reboots. The crash trace reveals a call sequence where batadv_v_mesh_free appears to be the point of failure but is misleading since the actual issue occurs in batadv_v_elp_throughput_metric_update function. The vulnerability is particularly dangerous because it can be reliably triggered by modifying the work queue behavior to use delayed execution, making it reproducible in testing environments. During reboot operations, this condition can cause the kernel to panic and potentially result in data loss or system unavailability.
The fix addresses this by preventing entry into batadv_v_elp_get_throughput function when the soft interface is no longer available, ensuring proper synchronization between interface lifecycle management and background processing tasks. This solution aligns with CWE-691, which addresses inadequate protection of code from uncontrolled recursion or excessive resource consumption. The mitigation strategy follows ATT&CK technique T1490 which covers denial of service through resource exhaustion or system instability. Organizations should apply the kernel patch immediately as the vulnerability affects core networking functionality and can be exploited to cause system-wide disruptions. The fix specifically targets the reference counting logic and work queue management to ensure proper lifecycle handling of mesh network interface objects during removal operations.