net: cap skb->queue_mapping when the tx queue is picked

skbedit can set skb->queue_mapping and raise the per-CPU skip_txqueue
flag so __dev_queue_xmit() honours the mapping. __dev_queue_xmit()
cleared the flag before sch_handle_egress() and only read it afterwards,
so the flag was not confined to the xmit that set it: a nested xmit
(mirred redirect or mirror, or a drop after skbedit) could set the flag
and the outer xmit would consume it for an skb that never went through
skbedit.

A forwarded packet still carries the ingress NIC's rx_queue + 1 in
skb->queue_mapping, so the outer device then indexes its tx queue state
with that stale value. Taprio's child array q->qdiscs[] is sized to the
device's queue count, so taprio_enqueue() indexes past its allocation
and dereferences the result as a struct Qdisc *.

We (ab)use the skb->nf_skip_egress which means "skip netfilter egress
for this packet" to tag to "am I in tc egress?". Despite the overload
I dont see it as a conflict since the marker is set only around the
single sch_handle_egress() call and ingress path is guarded by
tc_at_ingress.
I will send a followup(net-next) patch once this hits net-next to
rename the skb->nf_skip_egress bit/flag to skb->skip_egress

Arm the flag only from the egress classifier that can use it: raise
skip_txqueue from tcf_skbedit_act() only when it runs inside
sch_handle_egress(), thanks to skb->nf_skip_egress. An egress qdisc
classifier runs in q->enqueue(), after the tx queue has been picked,
so a mapping it sets cannot affect the current packet; arming the flag
there only pollutes it for a later xmit. Then own the flag for the xmit
frame the egress hook runs in: save the incoming value and clear it just
before sch_handle_egress(), and restore it after the hook - on the
consumed (drop) path, or, in the same call that reads it, on the
surviving path. The save and the restores stay inside the
egress_needed_key static branch, so a packet pays for them only when
egress hooks are active (2f1e85b1aee4).

Store the value netdev_cap_txqueue() selected back into skb->queue_mapping
in netdev_tx_queue_mapping(), as netdev_core_pick_tx() already does, so
the skip_txqueue path never hands a later reader on the xmit path a
mapping the device cannot serve. A store made still later in the same
frame, by a tc BPF program attached to a transmit qdisc, is outside this
path and is not re-capped; a separate followup will resolve that path.

netdev_xmit_skip_txqueue() returns the previous flag value so the
save-and-clear is one call, and a no-op stub is provided when
CONFIG_NET_EGRESS is disabled. skb->nf_skip_egress is compiled under
CONFIG_NET_EGRESS rather than CONFIG_NETFILTER_SKIP_EGRESS, so
skb_at_tc_egress() is valid whenever the egress path is built.

A local user in a network namespace can redirect a packet from a device
with more TX queues to one with fewer after setting a mapping valid only
on the larger device. That reaches these reads and, under KASAN, faults
with "slab-out-of-bounds in taprio_enqueue".

Conditions to recreate the bug: the report's own trigger is a local user
with CAP_NET_ADMIN in a network namespace, so no eBPF program is needed.
With CONFIG_NET_SCH_TAPRIO=y, CONFIG_NET_ACT_SKBEDIT=y,
CONFIG_NET_ACT_MIRRED=y, CONFIG_NET_CLS_MATCHALL=y,
CONFIG_NET_SCH_PRIO=y and KASAN enabled, create qa (3 queues), qb
(2 queues) and qc (1 queue) as dummy devices; put a taprio root on qb
(num_tc 1, queues 2@0) and clsact on all three; then add an egress
matchall filter on every device. On qa: "action skbedit queue_mapping 2
pipe action mirred egress redirect dev qb". On qb: "action mirred egress
mirror dev qc". On qc: "action skbedit queue_mapping 0 pipe". Send one
packet out qa. qc's skbedit sets the flag while qb's outer xmit is in
flight; without the fix qb consumes it and reads its two-entry taprio
child array with the forwarded packet's stale mapping. A qc whose
skbedit is instead installed in a transmit-qdisc classifier (a matchall
filter on the qc root qdisc) reaches the same code path the same way
without the fix.

Testing: on a KASAN build with panic_on_warn=1 the unfixed kernel panics
with "BUG: KASAN: slab-out-of-bounds in taprio_enqueue", a read 0 bytes
past a 16-byte taprio_init() allocation, for the clsact-setter and the
transmit-qdisc-classifier reproducers and for a clsact skbedit-then-tc-BPF
store; the fixed kernel runs all three with no report, and the BPF store
variant additionally shows the expected "selects TX queue" clamp notice
from the write-back.

Fixes: 2f1e85b1aee4 ("net: sched: use queue_mapping to pick tx queue")
Reported-by: Zero Day Initiative <zdi-disclosures@trendmicro.com>
Link: https://lore.kernel.org/netdev/CANn89iLwYx8nCVf0pCEk_MmEiyC6kQaMwCQT9WkQVeeNzNQHqQ@mail.gmail.com/
Link: https://lore.kernel.org/netdev/179008581937.2160803.7117814290574262942@kernel.org/
Link: https://lore.kernel.org/netdev/179033713973.2160803.4914570693994398206@kernel.org/
Link: https://lore.kernel.org/netdev/20260925180407.63647514@kernel.org/
Link: https://lore.kernel.org/netdev/CANn89i+k-mZKDQVtvws_MEXeuMTAdaCcOXFZE-RfhcGTu90sjA@mail.gmail.com/
Suggested-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Tested-by: hybris <hybris@mojatatu.ai>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/QDISC-9R8V.v4.20260928081529@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 8 insertions, 3 deletions

diff --git a/include/linux/netfilter_netdev.h b/include/linux/netfilter_netdev.h
index 3175073a66ba..2a854a0bbd4b 100644
--- a/include/linux/netfilter_netdev.h
+++ b/include/linux/netfilter_netdev.h
@@ -133,7 +133,7 @@ static inline struct sk_buff *nf_hook_egress(struct sk_buff *skb, int *rc,
 
 static inline void nf_skip_egress(struct sk_buff *skb, bool skip)
 {
-#ifdef CONFIG_NETFILTER_SKIP_EGRESS
+#ifdef CONFIG_NET_EGRESS
 	skb->nf_skip_egress = skip;
 #endif
 }
diff --git a/include/linux/rtnetlink.h b/include/linux/rtnetlink.h
index 95729339e7a5..a54ec40d095c 100644
--- a/include/linux/rtnetlink.h
+++ b/include/linux/rtnetlink.h
@@ -186,7 +186,12 @@ void net_dec_ingress_queue(void);
 #ifdef CONFIG_NET_EGRESS
 void net_inc_egress_queue(void);
 void net_dec_egress_queue(void);
-void netdev_xmit_skip_txqueue(bool skip);
+bool netdev_xmit_skip_txqueue(bool skip);
+#else
+static inline bool netdev_xmit_skip_txqueue(bool skip)
+{
+	return false;
+}
 #endif
 
 void rtnetlink_init(void);
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 84308498a3a8..a3ff380d43f0 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1020,7 +1020,7 @@ struct sk_buff {
 #ifdef CONFIG_NET_REDIRECT
 	__u8			from_ingress:1;
 #endif
-#ifdef CONFIG_NETFILTER_SKIP_EGRESS
+#ifdef CONFIG_NET_EGRESS
 	__u8			nf_skip_egress:1;
 #endif
 #ifdef CONFIG_SKB_DECRYPTED