CVE-2026-98239 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
net: lan743x: fix RX checksum use-after-free
lan743x_rx_process_buffer() adds each non-first receive buffer to the head skb's frag_list. On the last descriptor, lan743x_rx_trim_skb() linearizes the head and frees the fragment skb metadata.
The checksum-success path then writes ip_summed through the local skb pointer, which still points to the final fragment. This causes a use-after-free write when a packet spans more than one receive buffer.
Set ip_summed on the surviving head skb instead. Multi-buffer receive can occur after a live MTU increase because existing ring entries keep their old buffer size until they are replenished.
A KUnit test invoking lan743x_rx_process_buffer() with a two-buffer packet produced a one-byte KASAN use-after-free write before this change. The same test passed after the change. The driver object also builds with W=1. This was not tested on physical LAN743x hardware.
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel network subsystem contains a critical memory safety vulnerability within the lan743x Ethernet driver, specifically identified as a use-after-free error in receive checksum processing. This flaw arises from an incorrect handling of scattered packet buffers during the reception of multi-segment frames. The root cause lies in the interaction between buffer management and checksum offloading logic. When the driver processes incoming network traffic that spans multiple receive descriptors, it initially constructs a socket buffer by linking non-first fragments to the head skb via the frag_list structure. However, upon encountering the final descriptor for a packet, the function lan743x_rx_trim_skb is invoked to linearize the data and free the metadata of the fragment skbs to reclaim memory resources.
The critical failure occurs immediately after this deallocation step. The code attempts to set the ip_summed field on the local skb pointer to indicate that checksum verification was successful. Due to a logic error, this local pointer still references the final fragment buffer rather than the surviving head socket buffer. Since the fragment metadata has already been freed by the trimming operation, writing to its memory space constitutes a use-after-free write. This scenario is particularly likely to occur when packets exceed the current receive buffer size or during periods where the Maximum Transmission Unit has been dynamically increased but not yet fully propagated through all ring entries, leading to mixed buffer sizes in the reception queue.
From an operational perspective, this vulnerability poses significant risks to system stability and security integrity. A use-after-free condition allows for arbitrary memory writes under specific conditions controlled by network traffic patterns. An attacker capable of sending crafted packets that trigger multi-buffer receptions could potentially exploit this flaw to corrupt kernel data structures, leading to a denial of service through kernel panic or crash. In more sophisticated attack scenarios involving precise heap layout manipulation, such memory corruption vulnerabilities can be leveraged for arbitrary code execution with root privileges, effectively compromising the entire host system. The vulnerability is classified under CWE-416, which denotes use after free errors, and aligns with ATT&CK techniques related to privilege escalation via kernel exploitation.
The resolution involves correcting the pointer reference so that the ip_summed field is updated on the head socket buffer, which remains valid throughout the packet processing lifecycle, rather than on the fragment buffers that are subject to immediate deallocation. This fix ensures that checksum status updates do not target freed memory regions. Validation of this patch was performed using KUnit tests that simulated two-buffer packet reception, confirming the elimination of the one-byte write detected by Kernel Address Sanitizer prior to the change. While the driver builds cleanly with warning flags enabled, it is noted that physical hardware testing on LAN743x devices has not yet been conducted for this specific fix. To mitigate similar risks in related drivers and general kernel development practices, engineers should adhere strictly to memory lifecycle management protocols, ensuring that pointers are invalidated or updated correctly after deallocation events. Regular static analysis and fuzzing of network driver code paths involving scatter-gather I/O operations are recommended to detect such logical errors before they reach production environments.