CVE-2026-68441 in Linux
Summary
by MITRE • 08/12/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: Handle TC_ACT_REDIRECT from qdisc filter chains
When a TC filter attached to a qdisc filter chain returns TC_ACT_REDIRECT (ex: via an eBPF program calling bpf_redirect() or an act_bpf action), the redirect was silently lost i.e no qdisc classify function handled TC_ACT_REDIRECT, so the packet fell through the switch and was enqueued normally instead of being redirected.
This has been broken since bpf_redirect() was introduced for TC in commit 27b29f63058d ("bpf: add bpf_redirect() helper"). We got lucky for a long time because bpf_net_context was a per-CPU variable that was always available.
commit 401cb7dae813 ("net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.") turned bpf_net_context into a task_struct member that is only set up by explicit callers. Without a caller setting it up, bpf_redirect() itself crashes with a NULL pointer dereference in bpf_net_ctx_get_ri(). However, even with bpf_net_context available, TC_ACT_REDIRECT from qdisc filter chains cannot be honored without adding skb_do_redirect() calls to every qdisc classify function, which would require changes across net/sched/. Isolate it to ebpf core where it belongs.
Instead, add a tcf_classify_qdisc() inline helper in pkt_cls.h, as a wrapper around tcf_classify() for use by qdisc classify functions and tcf_qevent_handle(). When the classify verdict is TC_ACT_REDIRECT, the wrapper converts it to TC_ACT_SHOT, dropping the packet rather than letting it continue silently. Dropping is preferred over letting the packet through because the user immediately sees packet loss. Silently passing the packet through would hide the problem and leave the user wondering why their redirect is not working.
The clsact fast path, tc_run() continues to call tcf_classify() directly and is unaffected: TC_ACT_REDIRECT is returned as-is and handled by sch_handle_egress/ingress() calling skb_do_redirect() as before.
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Analysis
by VulDB Data Team • 08/12/2026
This vulnerability exists within the linux kernel's traffic control subsystem where qdisc filter chains fail to properly handle TC_ACT_REDIRECT verdicts generated by bpf programs or act_bpf actions. The issue stems from a fundamental design flaw in how packet classification functions process redirect verdicts, creating a silent failure mode that masks configuration problems and prevents proper network packet routing behavior. When a TC filter returns TC_ACT_REDIRECT through mechanisms like bpf_redirect() helper calls, the system silently drops these packets rather than executing the intended redirection, leaving administrators unaware of misconfigured network policies.
The root cause of this vulnerability can be traced back to changes in how bpf_net_context is managed within the kernel's PREEMPT_RT configuration. Prior to commit 401cb7dae813, bpf_net_context was implemented as a per-CPU variable that remained consistently available, allowing bpf_redirect() operations to function properly even without explicit caller setup. However, this change introduced a dependency on task_struct members that are only initialized by explicit callers, creating a scenario where bpf_redirect() itself would crash with NULL pointer dereferences when called without proper context initialization.
The technical implementation flaw manifests in the absence of proper TC_ACT_REDIRECT handling within qdisc classify functions throughout the net/sched subsystem. Without specific integration of skb_do_redirect() calls into every qdisc classify function, redirect operations cannot be properly executed across the entire network classification chain. This architectural gap requires extensive modifications across multiple kernel components and creates an inconsistent behavior pattern where some packet paths handle redirects correctly while others silently ignore them entirely.
The impact of this vulnerability extends beyond simple packet delivery failures to create potential security implications through network policy bypasses and operational confusion in network management. When administrators configure bpf programs or TC filters with redirect actions, they expect these operations to function as designed, yet the silent failure mode means that misconfigurations remain undetected until network behavior anomalies manifest in production environments.
The solution implemented addresses this issue by introducing a tcf_classify_qdisc() inline helper function in pkt_cls.h that serves as a wrapper around the existing tcf_classify() functionality specifically designed for qdisc classify functions and tcf_qevent_handle() contexts. This approach isolates the redirect handling logic within the eBPF core where it belongs, rather than requiring modifications across the entire net/sched subsystem. The helper function converts TC_ACT_REDIRECT verdicts to TC_ACT_SHOT, effectively dropping packets rather than allowing them to fall through the classification system silently.
This mitigation strategy follows established security principles by prioritizing explicit failure detection over silent operation. Rather than letting misconfigured redirects pass through the system unnoticed, the implementation ensures that packet loss becomes immediately apparent to administrators, making it easier to identify and correct network policy issues. The clsact fast path through tc_run() continues to function correctly since it directly calls tcf_classify() and maintains its existing behavior of returning TC_ACT_REDIRECT verdicts that are properly handled by sch_handle_egress/ingress() functions calling skb_do_redirect().
The vulnerability demonstrates a clear pattern that aligns with CWE-835, which describes infinite loops or excessive iterations in software systems where the intended control flow is bypassed due to improper handling of conditional states. The design decision to silently drop packets rather than properly handle TC_ACT_REDIRECT verdicts creates an operational failure mode that violates the principle of least surprise in network administration. Furthermore, this issue relates to ATT&CK technique T1070.004, which involves the use of valid accounts and system processes to hide malicious activities, as the silent packet loss could mask unauthorized network redirection attempts or misconfigured security policies.
The fix represents a targeted approach that maintains backward compatibility while ensuring proper handling of redirect operations throughout the kernel's traffic control subsystem. By centralizing the redirect handling logic within the eBPF core components where it logically belongs, the solution avoids the need for extensive kernel modifications across multiple subsystems while providing clear operational feedback when redirect operations are not properly configured or executed. This approach aligns with security best practices by ensuring that system behavior is predictable and that administrators receive immediate feedback when network policies fail to execute as intended.