From f19f2b308beb158d0e6f6eb806ced636e4ebf6ce Mon Sep 17 00:00:00 2001 From: Jamal Hadi Salim Date: Thu, 29 Oct 2026 16:36:31 -0400 Subject: [PATCH] net/sched: clamp skb->queue_mapping before indexing the tx queue This issue was caught by Sashiko (nipa). A producer-side path cap is already merged (ea4d4b5dddb5); this patch adds the matching consumer-side caps. The issue: The tx queue is picked and capped in netdev_core_pick_tx() before the __dev_xmit_skb() qdisc enqueue path. A tc BPF program running inside an egress qdisc (not clsact) can then change __sk_buff->queue_mapping; that store is never re-checked. bpf_convert_ctx_access() bounds it only by NO_QUEUE_MAPPING, not by the device's queue count. Two consumers read the stale value: - On dequeue, qdisc_restart() calls netdev_get_tx_queue(), which indexes dev->_tx[]. netdev_get_tx_queue() guards it with just a DEBUG_NET_WARN_ON_ONCE, so sch_direct_xmit() takes the out-of-bounds txq->_xmit_lock and writes txq->xmit_lock_owner past the end of the allocation on non-lltx devices such as ifb. Drivers that index their rings from skb_get_queue_mapping() (ex: ifb) go out of range the same way. - On enqueue, taprio_enqueue() indexes q->qdiscs[] with the same value; the array is sized dev->num_tx_queues. A duplicating netem on a child qdisc clones the skb and re-enqueues the clone at the taprio root (qdisc_root_bh()) through a direct rootq->enqueue() call, so the value reaches taprio_enqueue() before any dequeue cap runs. The Fix: Cap the mapping at the consumers and write the capped value back, so the qdisc's _tx[] lookup, the taprio q->qdiscs[] lookup and any later driver read stay in range. On the dequeue side the cap folds into the dequeue path instead of a separate walk over the returned list: dequeue_skb() caps the head, and each bulk helper caps the element it appends. For a qdisc that is not TCQ_F_ONETXQUEUE, netdev_cap_txqueue() logs a ratelimited notice and selects queue 0. A TCQ_F_ONETXQUEUE qdisc is bound to a single tx queue: dequeue_skb() uses q->dev_queue for the frozen check, the BQL limit and the trace point, while qdisc_restart() transmits on the skb's queue. Capping an out-of-range mapping to queue 0 would therefore send a per-queue child's skb through a different queue than the one it was dequeued for. On mq, mqprio, taprio and htb offload children, a stopped queue 0 could then park the list in q->gso_skb, and the later wake would schedule queue 0's own child instead of this qdisc. Pin a TCQ_F_ONETXQUEUE mapping to the qdisc's own queue to keep the documented invariant. The pin covers every mapping, not only an out-of-range one: an in-range mapping that a filter sets on one of those per-queue children is rewritten to the child's own queue, and this path does not call netdev_cap_txqueue(), so it logs no notice. On a single-queue device qdisc_create() sets TCQ_F_ONETXQUEUE, so the one-queue trigger described below also takes this branch and is corrected without a notice. That also keeps a bulk list on one queue by construction: every element of a TCQ_F_ONETXQUEUE qdisc, including a follower changed by a filter, is pinned to q->dev_queue before it is appended. The fast bulk helper therefore needs no head comparison and no skb_bad_txq requeue; the slow helper, used only by non-ONETXQUEUE qdiscs, still compares and requeues a mismatched follower, where a wake on any queue reschedules q. On the enqueue side taprio_enqueue() caps the mapping against real_num_tx_queues and writes it back before selecting q->qdiscs[queue], so a duplicating child cannot steer the clone out of the array. Conditions to recreate the bug: The dequeue path: load a SCHED_CLS or SCHED_ACT program that stores __sk_buff->queue_mapping (a tc BPF action is enough), attach it to a transmit qdisc (for example a matchall filter on a prio root), and send a packet. With a 1-queue device a store of 1 indexes one past dev->_tx[]. Under KASAN with panic_on_warn=1 the unfixed kernel faults with "BUG: KASAN: slab-out-of-bounds in sch_direct_xmit". Loading the program needs CAP_BPF/CAP_SYS_ADMIN, so the trigger is root. The enqueue path: on a multi-queue device, install a software taprio root, graft a prio child with a duplicating netem on every class, and store an out-of-range queue_mapping from a matchall+bpf filter on the prio. The duplicate is re-enqueued at the taprio root and indexes q->qdiscs[] past dev->num_tx_queues; the unfixed kernel faults in taprio_enqueue_one(). Same CAP_BPF/CAP_SYS_ADMIN requirement. Fixes: 74e31ca850c1 ("bpf: add skb->queue_mapping write access from tc clsact") Reported-by: Sashiko (nipa) Closes: https://lore.kernel.org/netdev/179033713973.2160803.4914570693994398206@kernel.org/ Link: https://lore.kernel.org/netdev/179329777513.3134.12135506558709584471@kernel.org/ Link: https://lore.kernel.org/netdev/179293123940.3134.1149128679923234677@kernel.org/ Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/QDISC-MD1X.v4.r0.20261021042722-1%40mojatatu.com Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/QDISC-MD1X.v3.r0.20261015165943-1%40mojatatu.com Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/QDISC-MD1X.v1.20261005193231%40mojatatu.com Link: https://lore.kernel.org/netdev/20261015125822.GH83879@horms.kernel.org/ Cc: stable@vger.kernel.org Signed-off-by: Jamal Hadi Salim Reviewed-by: Simon Horman Link: https://patch.msgid.link/QDISC-MD1X.v6.r0.20261029162633-1@mojatatu.com Signed-off-by: Paolo Abeni --- net/sched/sch_generic.c | 41 ++++++++++++++++++++++++++++++++++++----- net/sched/sch_taprio.c | 5 +++++ 2 files changed, 41 insertions(+), 5 deletions(-) diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c index 3e8ee4a691c3..d2e0d20f3ba1 100644 --- a/net/sched/sch_generic.c +++ b/net/sched/sch_generic.c @@ -96,9 +96,34 @@ static void qdisc_maybe_clear_missed(struct Qdisc *q, #define SKB_XOFF_MAGIC ((struct sk_buff *)1UL) +/* Re-cap a stale queue_mapping; an ONETXQUEUE qdisc pins q->dev_queue. */ +static u16 qdisc_cap_skb_tx_queue(struct Qdisc *q, struct sk_buff *skb) +{ + u16 queue = skb_get_queue_mapping(skb); + struct net_device *dev = qdisc_dev(q); + u16 capped; + + if (q->flags & TCQ_F_ONETXQUEUE) + capped = q->dev_queue - netdev_get_tx_queue(dev, 0); + else + capped = netdev_cap_txqueue(dev, queue); + + if (unlikely(capped != queue)) + skb_set_queue_mapping(skb, capped); + + return capped; +} + +static struct netdev_queue *skb_cap_tx_queue(struct Qdisc *q, + struct sk_buff *skb) +{ + return netdev_get_tx_queue(qdisc_dev(q), + qdisc_cap_skb_tx_queue(q, skb)); +} + static inline struct sk_buff *__skb_dequeue_bad_txq(struct Qdisc *q) { - const struct netdev_queue *txq = q->dev_queue; + const struct netdev_queue *txq; spinlock_t *lock = NULL; struct sk_buff *skb; @@ -110,7 +135,8 @@ static inline struct sk_buff *__skb_dequeue_bad_txq(struct Qdisc *q) skb = skb_peek(&q->skb_bad_txq); if (skb) { /* check the reason of requeuing without tx lock first */ - txq = skb_get_tx_queue(txq->dev, skb); + /* re-cap: real_num_tx_queues may have shrunk */ + txq = skb_cap_tx_queue(q, skb); if (!netif_xmit_frozen_or_stopped(txq)) { skb = __skb_dequeue(&q->skb_bad_txq); if (qdisc_is_percpu_stats(q)) { @@ -224,6 +250,8 @@ static void try_bulk_dequeue_skb(struct Qdisc *q, if (!nskb) break; + /* Every element of an ONETXQUEUE qdisc is pinned to q->dev_queue. */ + qdisc_cap_skb_tx_queue(q, nskb); bytelimit -= nskb->len; /* covers GSO len */ skb->next = nskb; skb = nskb; @@ -241,7 +269,6 @@ static void try_bulk_dequeue_skb_slow(struct Qdisc *q, struct sk_buff *skb, int *packets) { - int mapping = skb_get_queue_mapping(skb); struct sk_buff *nskb; int cnt = 0; @@ -249,7 +276,9 @@ static void try_bulk_dequeue_skb_slow(struct Qdisc *q, nskb = q->dequeue(q); if (!nskb) break; - if (unlikely(skb_get_queue_mapping(nskb) != mapping)) { + /* A tc BPF store may have changed the mapping; cap it. */ + if (unlikely(qdisc_cap_skb_tx_queue(q, nskb) != + skb_get_queue_mapping(skb))) { qdisc_enqueue_skb_bad_txq(q, nskb); break; } @@ -294,7 +323,7 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate, if (xfrm_offload(skb)) *validate = true; /* check the reason of requeuing without tx lock first */ - txq = skb_get_tx_queue(txq->dev, skb); + txq = skb_cap_tx_queue(q, skb); if (!netif_xmit_frozen_or_stopped(txq)) { skb = __skb_dequeue(&q->gso_skb); if (qdisc_is_percpu_stats(q)) { @@ -330,6 +359,8 @@ static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate, skb = q->dequeue(q); if (skb) { bulk: + /* cap the head; the bulk helpers cap every appended element */ + qdisc_cap_skb_tx_queue(q, skb); if (qdisc_may_bulk(q)) try_bulk_dequeue_skb(q, skb, txq, packets, budget); else diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c index 86740f4e82f5..4f69ef3d847a 100644 --- a/net/sched/sch_taprio.c +++ b/net/sched/sch_taprio.c @@ -639,6 +639,11 @@ static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch, int queue; queue = skb_get_queue_mapping(skb); + if (unlikely(queue >= qdisc_dev(sch)->real_num_tx_queues)) { + /* A tc BPF program may have stored an out-of-range mapping. */ + queue = netdev_cap_txqueue(qdisc_dev(sch), queue); + skb_set_queue_mapping(skb, queue); + } child = q->qdiscs[queue]; if (unlikely(child == &noop_qdisc))