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

["Kito Xu (veritas501)" <hxzene@gmail.com>]

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.

Fixes: 42f39036cda8 ("net/sched: act_mirred: Allow mirred to block")
Signed-off-by: Kito Xu (veritas501) <hxzene@gmail.com>
---
 net/sched/act_mirred.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/sched/act_mirred.c b/net/sched/act_mirred.c
index 05e0b14b5773..5928fcf3e651 100644
--- a/net/sched/act_mirred.c
+++ b/net/sched/act_mirred.c
@@ -444,8 +444,12 @@ TC_INDIRECT_SCOPE int tcf_mirred_act(struct sk_buff *skb,
 	tcf_action_update_bstats(&m->common, skb);
 
 	blockid = READ_ONCE(m->tcfm_blockid);
-	if (blockid)
-		return tcf_blockcast(skb, m, blockid, res, retval);
+	if (blockid) {
+		xmit->sched_mirred_nest++;
+		retval = tcf_blockcast(skb, m, blockid, res, retval);
+		xmit->sched_mirred_nest--;
+		return retval;
+	}
 
 	dev = rcu_dereference_bh(m->tcfm_dev);
 	if (unlikely(!dev)) {
-- 
2.43.0