CVE-2026-63981
Received Received - Intake

Stack Overflow in Linux Kernel via Blockcast Recursion

Publication date: 2026-07-19

Last updated on: 2026-07-19

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow tcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4) to prevent deep recursion. However, when the action uses blockcast (tcfm_blockid != 0), the function returns at the tcf_blockcast() call BEFORE reaching the counter increment. As a result, the recursion counter never advances and the limit check is entirely bypassed. When two devices share a TC egress block with a mirred blockcast rule, a packet egressing on device A is mirrored to device B via blockcast; device B's egress TC re-enters tcf_mirred_act() via blockcast and mirrors back to A, creating an unbounded recursion loop: tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat) This recursion continues until the kernel stack overflows. The bug is reachable from an unprivileged user via unshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant CAP_NET_ADMIN in the new network namespace, which is sufficient to create dummy devices, attach clsact qdiscs with shared blocks, and install mirred blockcast filters. BUG: TASK stack guard page was hit at ffffc90000b7fff8 Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410 RIP: 0010:xas_find+0x17/0x480 Call Trace: xa_find+0x17b/0x1d0 tcf_mirred_act+0x640/0x1060 tcf_action_exec+0x400/0x530 basic_classify+0x128/0x1d0 tcf_classify+0xd83/0x1150 tc_run+0x328/0x620 __dev_queue_xmit+0x797/0x3100 tcf_mirred_to_dev+0x7b1/0xf70 tcf_mirred_act+0x68a/0x1060 [repeating ~30+ times until stack overflow] Kernel panic - not syncing: Fatal exception in interrupt Fix this by incrementing sched_mirred_nest before calling tcf_blockcast() and decrementing it on return, mirroring the non-blockcast path. This ensures subsequent recursive entries see the updated counter and are correctly limited by MIRRED_NEST_LIMIT.

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Meta Information

Published
2026-07-19
Last Modified
2026-07-19
Generated
2026-07-20
AI Q&A
2026-07-19
EPSS Evaluated
N/A
NVD
CVE-2026-63981
EUVD
EUVD-2026-45754

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel 7.0.0-rc7-next-20260410

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Exploitability

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CWE ID Description
CWE-UNKNOWN

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Executive Summary

This is a Linux kernel vulnerability where a stack overflow occurs due to unbounded recursion in the net/sched act_mirred module. The issue arises when a packet is mirrored between two network devices using blockcast rules, causing the tcf_mirred_act function to recursively call itself without properly incrementing a recursion counter. This bypasses the MIRRED_NEST_LIMIT check, leading to a kernel stack overflow.

Detection Guidance

This vulnerability is triggered by specific network traffic patterns involving TC (Traffic Control) egress blocks and mirred blockcast rules. Detection requires checking for stack overflow errors in kernel logs and identifying recursive TC actions. Monitor kernel logs for stack guard page errors or recursion-related crashes. Check for unprivileged users creating dummy devices and shared TC blocks with mirred blockcast filters.

Impact Analysis

An unprivileged user in a user namespace with CAP_NET_ADMIN can exploit this to crash the kernel by triggering a stack overflow. This results in a denial of service (DoS) where the system may panic or reboot. It does not directly allow code execution or data theft but can disrupt system availability.

Compliance Impact

This vulnerability does not directly affect compliance with GDPR, HIPAA, or similar standards as it is a technical stack overflow issue in the Linux kernel's network traffic control subsystem. Compliance impacts would only occur if the vulnerability leads to system instability, data corruption, or unauthorized access, which is not specified in the provided context.

Mitigation Strategies

Apply the Linux kernel patch that increments sched_mirred_nest before tcf_blockcast() and decrements it on return. Update to a kernel version containing the fix (e.g., 7.0.0-rc7-next-20260410 or later). Restrict CAP_NET_ADMIN capabilities in user namespaces to prevent unprivileged exploitation. Monitor for suspicious TC configurations involving blockcast rules.

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