CVE-2026-80841 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
net/packet: defer vmalloc TX_RING free until skbs finish
AF_PACKET TX_RING skbs keep a raw pointer to their ring frame. The skb page references preserve page-backed ring blocks after pg_vec is freed, but they do not preserve a vmalloc mapping.
tpacket_destruct_skb() currently drops the pending reference before writing the timestamp and TP_STATUS_AVAILABLE to the frame. Move the decrement after those stores. The smp_wmb() in __packet_set_status() orders the frame stores before the decrement.
Also recheck pending TX frames under pg_vec_lock before non-closing ring replacement, so a racing send cannot add a pending skb between the initial check and the ring swap.
Ring allocation can produce a mixture of page-backed and vmalloc-backed blocks. Allocate deferred-work storage during TX ring setup when the first vmalloc-backed block is encountered, and keep its pointer in the pg_vec allocation header. If allocation fails, return -ENOMEM from ring setup. On socket close, a non-NULL pointer identifies a vmalloc-backed vector without a scan. If TX skbs remain, defer the whole vector to system_long_wq.
After pg_vec is detached, a late destructor can skip the pending decrement. Use socket write-memory accounting as the deferred lifetime gate instead: an skb remains charged through its final sock_wfree(), after all ring-frame accesses. The delayed work retains a socket reference and reschedules itself until no TX skbs remain.
Move pending_refcnt release to packet_sock_destruct() so late skb destructors and deferred cleanup can safely use it after packet_release(). Page-backed teardown remains synchronous, and no lock is added to the TX completion hot path.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel vulnerability in the AF_PACKET subsystem involves a race condition related to memory management during transmission ring buffer operations. Specifically, the issue arises from how socket buffers (skbs) maintain references to their underlying ring frames when those frames are allocated using vmalloc rather than standard page-backed allocations. While page-backed ring blocks remain valid after the packet group vector is freed due to persistent page references, vmalloc mappings do not enjoy this same protection. Consequently, if a skb destructor accesses a frame that has been unmapped or freed via vmalloc, it can lead to use-after-free conditions or memory corruption. This flaw highlights a critical gap in lifecycle management where the kernel fails to synchronize the deallocation of virtual memory mappings with the actual completion of network packet processing tasks.
The technical root cause lies in the ordering of operations within the skb destruction path and ring replacement logic. Previously, the pending reference count was decremented before writing status updates such as timestamps or marking frames as available for reuse. This premature decrement allowed a racing send operation to potentially add new skbs between an initial check and a subsequent ring swap, leading to inconsistent state. Furthermore, when replacing non-closing rings, there was no mechanism to ensure that pending transmission frames were fully processed before the old vector was discarded. The lack of proper memory ordering barriers meant that stores to frame status could be reordered relative to reference count decrements, creating windows where stale pointers might still be accessed by deferred work or late destructors.
To resolve this, the implementation defers the freeing of vmalloc-backed TX_RING blocks until all associated skbs have finished processing. This is achieved by introducing a mechanism that tracks pending references and ensures they are only released after critical status updates like TP_STATUS_AVAILABLE have been committed to memory. The smp_wmb() barrier in __packet_set_status() now correctly orders these frame stores before the reference decrement, preventing reordering issues. Additionally, ring allocation logic has been enhanced to detect vmalloc-backed blocks during setup and allocate deferred-work storage accordingly. If such a block is detected, its pointer is stored in the packet group vector header for efficient identification later without requiring a full scan of the vector structure.
The operational impact of this vulnerability includes potential kernel crashes, data corruption, or privilege escalation if an attacker can trigger rapid ring replacements while transmission operations are ongoing. By ensuring that deferred work retains a socket reference and reschedules itself until no TX skbs remain, the fix guarantees that memory is not freed prematurely. The use of socket write-memory accounting as the deferred lifetime gate ensures that each skb remains charged through its final sock_wfree() call, effectively tying the memory release to the actual completion of network I/O operations rather than arbitrary timing windows. This approach maintains synchronous teardown for page-backed allocations while safely handling asynchronous cleanup for vmalloc-backed ones without adding locking overhead to the TX completion hot path.
From a security standards perspective, this vulnerability aligns with CWE-416: Use After Free, as it involves accessing memory after it has been potentially freed or unmapped due to improper synchronization of resource lifecycles. It also relates to CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization, given the race condition between ring replacement and pending frame processing. In terms of MITRE ATT&CK, this could be leveraged in techniques associated with T1059: Command and Scripting Interpreter or more broadly within exploitation chains targeting kernel memory corruption to achieve arbitrary code execution. Mitigations involve applying the upstream Linux kernel patch that restructures the reference counting logic and introduces deferred work queues for vmalloc-backed ring buffers, ensuring strict ordering of status updates and reference releases. System administrators should ensure their kernels are updated to versions containing these fixes to prevent potential exploitation scenarios involving high-throughput packet socket operations.