CVE-2026-63981

CVE Details

Release Date:	2026-07-19
Impact:	Important	What is this?

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.

See more information about CVE-2026-63981 from MITRE CVE dictionary and NIST NVD

NOTE: The following CVSS metrics and score provided are preliminary and subject to review.

CVSS v3 metrics

Base Score:	7.0
Vector String:	CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
Version:	3.1
Attack Vector:	Local
Attack Complexity:	High
Privileges Required:	Low
User Interaction:	None
Scope:	Unchanged
Confidentiality Impact:	High
Integrity Impact:	High
Availability Impact:	High

Errata information

Platform	Errata	Release Date
Oracle Linux version 10 (kernel-uek)	ELSA-2026-500248	2026-09-04
Oracle Linux version 9 (kernel-uek)	ELSA-2026-500248	2026-09-04