net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow

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.