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Mon, 13 Apr 2026 01:20:35 -0700 (PDT) Received: from localhost.localdomain ([180.167.178.215]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2b2d4f0983asm116757705ad.51.2026.04.13.01.20.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Apr 2026 01:20:34 -0700 (PDT) From: "Kito Xu (veritas501)" To: jhs@mojatatu.com, jiri@resnulli.us, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com Cc: horms@kernel.org, victor@mojatatu.com, pctammela@mojatatu.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, "Kito Xu (veritas501)" Subject: [PATCH] net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow Date: Mon, 13 Apr 2026 16:20:27 +0800 Message-ID: <20260413082027.2244884-1-hxzene@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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) --- 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