[PATCH net] net: ioam6: fix OOB and missing lock

[Justin Iurman <justin.iurman@gmail.com>]

When trace->type.bit6 is set:

    if (trace->type.bit6) {
        ...
        queue = skb_get_tx_queue(dev, skb);
        qdisc = rcu_dereference(queue->qdisc);

This code can lead to an out-of-bounds access of the dev->_tx[] array
when is_input is true. In such a case, the packet is on the RX path and
skb->queue_mapping contains the RX queue index of the ingress device. If
the ingress device has more RX queues than the egress device (dev) has
TX queues, skb_get_queue_mapping(skb) will exceed dev->num_tx_queues.
Add a check to avoid this situation since skb_get_tx_queue() does not
clamp the index. This issue has also revealed that per queue visibility
cannot be accurate and will be replaced later as a new feature.

While at it, add missing lock around qdisc_qstats_qlen_backlog(). The
function __ioam6_fill_trace_data() is called from both softirq and
process contexts, hence the use of spin_lock_bh() here.

Fixes: b63c5478e9cb ("ipv6: ioam: Support for Queue depth data field")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Closes: https://lore.kernel.org/netdev/20260403214418.2233266-2-kuba@kernel.org/
Signed-off-by: Justin Iurman <justin.iurman@gmail.com>
---
 net/ipv6/ioam6.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ioam6.c b/net/ipv6/ioam6.c
index 05a0b7d7e2aa..e963a71858a7 100644
--- a/net/ipv6/ioam6.c
+++ b/net/ipv6/ioam6.c
@@ -803,12 +803,16 @@ static void __ioam6_fill_trace_data(struct sk_buff *skb,
 		struct Qdisc *qdisc;
 		__u32 qlen, backlog;
 
-		if (dev->flags & IFF_LOOPBACK) {
+		if (dev->flags & IFF_LOOPBACK ||
+		    skb_get_queue_mapping(skb) >= dev->num_tx_queues) {
 			*(__be32 *)data = cpu_to_be32(IOAM6_U32_UNAVAILABLE);
 		} else {
 			queue = skb_get_tx_queue(dev, skb);
 			qdisc = rcu_dereference(queue->qdisc);
+
+			spin_lock_bh(qdisc_lock(qdisc));
 			qdisc_qstats_qlen_backlog(qdisc, &qlen, &backlog);
+			spin_unlock_bh(qdisc_lock(qdisc));
 
 			*(__be32 *)data = cpu_to_be32(backlog);
 		}
-- 
2.34.1