[SRU Focal 1/1] net/sched: sch_qfq: account for stab overhead in qfq_enqueue

Cengiz Can cengiz.can at canonical.com
Thu Jul 27 23:22:18 UTC 2023

From: Pedro Tammela <pctammela at mojatatu.com>

Lion says:
In the QFQ scheduler a similar issue to CVE-2023-31436

Consider the following code in net/sched/sch_qfq.c:

static int qfq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                struct sk_buff **to_free)
     unsigned int len = qdisc_pkt_len(skb), gso_segs;

    // ...

     if (unlikely(cl->agg->lmax < len)) {
         pr_debug("qfq: increasing maxpkt from %u to %u for class %u",
              cl->agg->lmax, len, cl->common.classid);
         err = qfq_change_agg(sch, cl, cl->agg->class_weight, len);
         if (err) {
             return qdisc_drop(skb, sch, to_free);

    // ...


Similarly to CVE-2023-31436, "lmax" is increased without any bounds
checks according to the packet length "len". Usually this would not
impose a problem because packet sizes are naturally limited.

This is however not the actual packet length, rather the
"qdisc_pkt_len(skb)" which might apply size transformations according to
"struct qdisc_size_table" as created by "qdisc_get_stab()" in
net/sched/sch_api.c if the TCA_STAB option was set when modifying the qdisc.

A user may choose virtually any size using such a table.

As a result the same issue as in CVE-2023-31436 can occur, allowing heap
out-of-bounds read / writes in the kmalloc-8192 cache.

We can create the issue with the following commands:

tc qdisc add dev $DEV root handle 1: stab mtu 2048 tsize 512 mpu 0 \
overhead 999999999 linklayer ethernet qfq
tc class add dev $DEV parent 1: classid 1:1 htb rate 6mbit burst 15k
tc filter add dev $DEV parent 1: matchall classid 1:1
ping -I $DEV

This is caused by incorrectly assuming that qdisc_pkt_len() returns a
length within the QFQ_MIN_LMAX < len < QFQ_MAX_LMAX.

Fixes: 462dbc9101ac ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
Reported-by: Lion <nnamrec at gmail.com>
Reviewed-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: Jamal Hadi Salim <jhs at mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela at mojatatu.com>
Reviewed-by: Simon Horman <simon.horman at corigine.com>
Signed-off-by: Paolo Abeni <pabeni at redhat.com>
(backported from commit 3e337087c3b5805fe0b8a46ba622a962880b5d64)
[cengizcan: commit 25369891fcef ("net/sched: sch_qfq: refactor parsing of
netlink parameters") does not exist in tree and it's very hard to
backport its prerequisite commit 8cb081746c03 ("netlink: make validation more
configurable for future strictness"). so simply add QFQ_MAX_LMAX into
this patch.]
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
 net/sched/sch_qfq.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
index 603bd3097bd8..d8248eb25ee4 100644
--- a/net/sched/sch_qfq.c
+++ b/net/sched/sch_qfq.c
@@ -113,6 +113,7 @@
 #define QFQ_MTU_SHIFT		16	/* to support TSO/GSO */
 #define QFQ_MIN_LMAX		512	/* see qfq_slot_insert */
 #define QFQ_MAX_AGG_CLASSES	8 /* max num classes per aggregate allowed */
@@ -375,8 +376,13 @@ static int qfq_change_agg(struct Qdisc *sch, struct qfq_class *cl, u32 weight,
 			   u32 lmax)
 	struct qfq_sched *q = qdisc_priv(sch);
-	struct qfq_aggregate *new_agg = qfq_find_agg(q, lmax, weight);
+	struct qfq_aggregate *new_agg;
+	/* 'lmax' can range from [QFQ_MIN_LMAX, pktlen + stab overhead] */
+	if (lmax > QFQ_MAX_LMAX)
+		return -EINVAL;
+	new_agg = qfq_find_agg(q, lmax, weight);
 	if (new_agg == NULL) { /* create new aggregate */
 		new_agg = kzalloc(sizeof(*new_agg), GFP_ATOMIC);
 		if (new_agg == NULL)

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