Linux Kernel up to 7.0.11 Act Mirred net/sched/act_mirred.c tcf_mirred_act stack-based overflow

Summaryinfo

A vulnerability classified as critical has been found in Linux Kernel up to 7.0.11. This affects the function tcf_mirred_act of the file net/sched/act_mirred.c of the component Act Mirred. The manipulation leads to stack-based overflow. This vulnerability is listed as CVE-2026-63981. The attack may be initiated remotely. There is no available exploit.

Detailsinfo

A vulnerability was found in Linux Kernel up to 7.0.11. It has been rated as critical. This issue affects the function tcf_mirred_act of the file net/sched/act_mirred.c of the component Act Mirred. The manipulation with an unknown input leads to a stack-based overflow vulnerability. Using CWE to declare the problem leads to CWE-121. A stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function). Impacted is availability. The summary by CVE is:

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.

The advisory is shared at git.kernel.org. The identification of this vulnerability is CVE-2026-63981 since 07/19/2026. The exploitation is known to be easy. The attack may be initiated remotely. It demands that the victim is doing some kind of user interaction. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 07/19/2026).

Upgrading to version 7.0.12 eliminates this vulnerability.

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Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 7.0.0
• 7.0.1
• 7.0.2
• 7.0.3
• 7.0.4
• 7.0.5
• 7.0.6
• 7.0.7
• 7.0.8
• 7.0.9
• 7.0.10
• 7.0.11

License

• open-source

Website

• Vendor: https://www.kernel.org/

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Entryinfo

Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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