CVE-2026-89950 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
batman-adv: mcast: linearize skbuff for packet generation
batadv_mcast_forw_packet() and batadv_mcast_forw_scrape() is not only called (indirectly) by the unsharing+linearizing batadv_recv_mcast_packet() handler. When it is called (indirectly) by batadv_mcast_forw_mcsend() then it will be unshared but not linearized. The SKB_LINEAR_ASSERT() can therefore cause a fatal BUG().
The linearization should happen during the expansion of the head because the scrape function can be hit already during the initial batadv_mcast_forw_mode() selection code:
* batadv_interface_tx * batadv_mcast_forw_mode * batadv_mcast_forw_mode_by_count() * batadv_mcast_forw_push() -> calls batadv_mcast_forw_expand_head() before everything else * batadv_mcast_forw_push_tvlvs() * batadv_mcast_forw_push_dests() * batadv_mcast_forw_push_adjust_padding() * batadv_mcast_forw_scrape()
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in the Linux kernel's BATMAN-ADV (B.A.T.M.A.N. Advanced) module represents a critical memory management flaw within the multicast packet forwarding logic, specifically affecting the handling of socket buffer structures known as skbuffs. This issue stems from an inconsistency in how network packets are prepared for transmission across different code paths within the batman-adv subsystem. The core problem lies in the assumption that all incoming or generated multicast packets will be both unshared and linearized before being processed by specific forwarding functions, namely batadv_mcast_forw_packet() and batadv_mcast_forw_scrape(). While these functions are correctly invoked after a linearization step when triggered via the standard reception handler batadv_recv_mcast_packet(), they encounter an unsafe state when called indirectly through the multicast MCS send path initiated by batadv_mcast_forw_mcsend(). In this specific execution flow, the skbuff is unshared but remains non-linearized, violating internal kernel assertions and leading to a fatal BUG() condition that can crash the system.
From a technical perspective, the flaw exploits the difference between sharing memory pages in network buffers and ensuring contiguous linear storage of packet data. Network drivers often use shared skbuffers for performance reasons, deferring the copying of data until necessary. However, certain operations require direct access to the payload as a continuous block of memory. The batadv_mcast_forw_scrape() function is designed to extract specific information from these packets and assumes that such linearization has already occurred. When this assumption fails because the packet arrived via the MCS send path without being linearized, subsequent operations attempt to access memory in an invalid manner. This triggers a kernel assertion failure, effectively causing a denial of service by crashing the host system or disrupting network connectivity for all nodes relying on the BATMAN-ADV mesh network infrastructure.
The operational impact of this vulnerability is significant for deployments using wireless mesh networking based on BATMAN-Advanced. An attacker with local access to the machine could potentially trigger this code path through crafted multicast traffic, resulting in a kernel panic and system reboot. This constitutes a denial-of-service attack vector that does not require privilege escalation but relies on specific network conditions or packet types being processed by the interface. The instability introduced by such crashes can lead to broader network fragmentation within the mesh, as nodes may be forced offline unexpectedly, reducing the resilience and availability of the entire ad-hoc network topology.
To mitigate this risk, it is imperative that system administrators apply the latest kernel updates provided by their distribution vendors, which include patches ensuring proper linearization of skbuffers in all relevant code paths. The fix involves modifying batadv_mcast_forw_push() to explicitly call batadv_mcast_forw_expand_head() before any subsequent processing steps, thereby guaranteeing that the packet data is contiguous and safe for scraping operations regardless of whether the packet originated from reception or internal generation. This structural change ensures consistency across all multicast forwarding modes. From a defensive standpoint, organizations should monitor kernel logs for BUG() messages related to batman-adv interfaces and ensure that network segmentation policies limit exposure to untrusted multicast traffic sources where possible until patches are fully deployed.
This vulnerability aligns with CWE-120 Buffer Copy without Checking Size of Input in C/C++, as the underlying issue involves improper handling of memory buffers leading to invalid access states, although it manifests primarily as a logic error causing assertion failures rather than traditional buffer overflows. In terms of attack classification under MITRE ATT&CK, this falls under T1499 Endpoint Denial of Service, specifically leveraging resource exhaustion or instability through kernel-level exploitation techniques that disrupt system availability without necessarily compromising data integrity or confidentiality.