CVE-2026-98369 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
xfrm: add missing rcu_read_lock(), skb_dst_force() and dev_hold() for xfrm_trans_reinject()
syzbot reported a suspicious RCU usage warning in ip6_pkt_drop():
WARNING: suspicious RCU usage in ip6_pkt_drop include/net/addrconf.h:389 suspicious rcu_dereference_check() usage!
Call Trace: __in6_dev_get_safely include/net/addrconf.h:389 [inline]
ip6_pkt_drop+0x596/0x610 net/ipv6/route.c:4620 ip6_pkt_discard+0x1c/0x30 net/ipv6/route.c:4651 xfrm_trans_reinject+0x324/0x630 net/xfrm/xfrm_input.c:806 process_one_work kernel/workqueue.c:3322 [inline]
process_scheduled_works+0xa8e/0x14e0 kernel/workqueue.c:3405 worker_thread+0xa47/0xfb0 kernel/workqueue.c:3486
When commit 4f4920669d21 ("xfrm: Reinject transport-mode packets through workqueue") converted xfrm_trans_reinject from a tasklet to a workqueue, the reinjection loop ceased running in softirq context. Workqueue workers run in process context where local_bh_disable() does not enter an RCU read-side critical section under CONFIG_PREEMPT_RCU.
Because finish callbacks (such as ip6_rcv_finish) expect to run under an RCU read lock (performing route lookups, l3mdev lookups, and accessing RCU-protected data structures), invoking them in workqueue context without rcu_read_lock() triggers RCU lockdep warnings.
Furthermore, packets queued to the workqueue via xfrm_trans_queue_net() may carry non-refcounted (noref) dst entries (e.g. from ip_route_input_noref). Additionally, on netdevice unregistration, dst_dev_put() replaces dst->dev with blackhole_netdev, so dst entries do not keep skb->dev alive while queued in the workqueue.
Fix these issues by: 1. Calling skb_dst_force(skb) in xfrm_trans_queue_net() while still in the caller's RCU section to ensure dst is reference-counted before queuing. 2. Holding a reference on skb->dev via dev_hold()/dev_put() across workqueue deferral so skb->dev remains valid during finish() callback processing. 3. Acquiring rcu_read_lock() around the finish callback invocation loop in xfrm_trans_reinject().
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Analysis
by VulDB Data Team • 10/06/2026
The Linux kernel vulnerability identified involves a concurrency and reference counting flaw within the IPsec subsystem, specifically affecting the function xfrm_trans_reinject. This issue arose from commit 4f4920669d21 which refactored the reinjection of transport-mode packets by converting the processing mechanism from tasklets to workqueues. Tasklets execute in softirq context where local_bh_disable implicitly enters an RCU read-side critical section under PREEMPT_RCU configurations, thereby protecting access to RCU-protected data structures. However, workqueue workers operate in process context, which does not provide this implicit protection. Consequently, when the reinjection loop runs via a workqueue, it invokes finish callbacks such as ip6_rcv_finish without holding an explicit RCU read lock. This omission triggers suspicious RCU usage warnings detected by syzbot and exposes the system to potential use-after-free scenarios where route lookups or layer three device model lookups access freed memory because the underlying data structures are no longer guaranteed to be valid during execution.
Beyond the locking deficiency, there is a critical reference counting issue regarding network destination entries associated with sk_buffs queued for processing. Packets passed through xfrm_trans_queue_net may carry non-refcounted dst entries generated by functions like ip_route_input_noref. These noref entries do not maintain their own lifetime independently of the socket buffer context in which they were created. Furthermore, during netdevice unregistration, the kernel replaces the device pointer within dst structures with a blackhole interface via dst_dev_put. This means that while packets are queued and deferred to the workqueue, the associated network device reference is not held, allowing the underlying dev structure to be freed or replaced before the packet processing completes. If the finish callback attempts to access this now-invalid destination entry or its associated device pointer, it results in a kernel panic or undefined behavior due to dereferencing invalid memory addresses.
The resolution implements three specific technical corrections to restore stability and safety. First, skb_dst_force is invoked within xfrm_trans_queue_net while still inside the caller's RCU section. This ensures that any non-refcounted dst entries are properly reference-counted before being queued for deferred processing, preventing premature deallocation of routing information. Second, dev_hold and dev_put calls are added to manage references on skb->dev across the workqueue deferral period. By holding a reference to the network device during the interval between queuing and execution, the kernel guarantees that the device structure remains valid throughout the lifecycle of the packet processing task. Third, rcu_read_lock is acquired around the invocation loop in xfrm_trans_reinject when calling finish callbacks. This explicitly establishes an RCU read-side critical section, ensuring safe access to all RCU-protected data structures such as routing tables and address configuration data during execution.
From a classification perspective, this vulnerability aligns with CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, specifically regarding the lack of proper locking mechanisms for shared kernel data structures accessed across different execution contexts. It also relates to CWE-416 Use After Free, as the absence of reference counting on destination entries and network devices allows access to memory that may have been freed by other subsystems like netdevice unregistration routines. In terms of attack vectors, this flaw could potentially be leveraged for Denial of Service attacks against the kernel stability if an attacker can trigger high volumes of IPsec packet reinjection under specific timing conditions involving device changes or route updates. Mitigation strategies involve applying the upstream Linux kernel patch that introduces these synchronization primitives and reference counting adjustments. System administrators should ensure their kernels are updated to versions containing this fix, particularly in environments where heavy IPsec traffic is processed concurrently with network interface configuration changes.