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

[ Upstream commit a005fa5 ]

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: fe946a7 ("net/sched: act_mirred: add loop detection")
Signed-off-by: Kito Xu (veritas501) <hxzene@gmail.com>
Link: https://patch.msgid.link/20260525122556.973584-7-jhs@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: veritas501

net/sched/act_mirred.c

@@ -396,14 +396,12 @@ static int tcf_blockcast_mirror(struct sk_buff *skb, struct tcf_mirred *m,
 
 static int tcf_blockcast(struct sk_buff *skb, struct tcf_mirred *m,
 			 const u32 blockid, struct tcf_result *res,
-			 int retval)
+			 int m_eaction, int retval)
 {
 	const u32 exception_ifindex = skb->dev->ifindex;
 	struct tcf_block *block;
 	bool is_redirect;
-	int m_eaction;
 
-	m_eaction = READ_ONCE(m->tcfm_eaction);
 	is_redirect = tcf_mirred_is_act_redirect(m_eaction);
 
 	/* we are already under rcu protection, so can call block lookup
@@ -453,8 +451,16 @@ 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);
+	m_eaction = READ_ONCE(m->tcfm_eaction);
+	want_ingress = tcf_mirred_act_wants_ingress(m_eaction);
+	if (blockid) {
+		if (!want_ingress)
+			xmit->sched_mirred_dev[xmit->sched_mirred_nest++] = NULL;
+		retval = tcf_blockcast(skb, m, blockid, res, m_eaction, retval);
+		if (!want_ingress)
+			xmit->sched_mirred_nest--;
+		return retval;
+	}
 
 	dev = rcu_dereference_bh(m->tcfm_dev);
 	if (unlikely(!dev)) {
@@ -463,8 +469,6 @@ TC_INDIRECT_SCOPE int tcf_mirred_act(struct sk_buff *skb,
 		return retval;
 	}
 
-	m_eaction = READ_ONCE(m->tcfm_eaction);
-	want_ingress = tcf_mirred_act_wants_ingress(m_eaction);
 	if (!want_ingress) {
 		for (i = 0; i < xmit->sched_mirred_nest; i++) {
 			if (xmit->sched_mirred_dev[i] != dev)