Re: [PATCH] net/sched: sch_qfq: prevent aggregate self-replacement

[Jamal Hadi Salim]

On Wed, Oct 21, 2026 at 10:56 AM David Lee <david.lee@xxxxxxxxxxxxxxx> wrote:
>
> Hi Jamal,
>
> Following up on this thread. Please let me know if you had a chance to review the attached reproducer or if you need any additional information.
>

Please resend to the list - I lost track.
And if you follow up after like 2 months(!) you will really have to
retest with the new kernel and likely look at the list to see if there
are other fixes that cover this issue.


cheers,
jamal

> Best regards,
>
>
>
> On Mon, Aug 10, 2026 at 11:22 AM David Lee <david.lee@xxxxxxxxxxxxxxx> wrote:
>>
>> Hi Jamal,
>>
>> Reproducer has been attached to this email.
>>
>> ========== Reproduction ==========
>>
>> 1. Build Linux at 82a47586c0b9266622657009aa30573dddf09f53
>>    with the configuration options listed above and KASAN enabled.
>>    Use a system with at least two CPUs. If QFQ is configured as a
>>    module, make sch_qfq available before running the reproducer.
>>
>> 2. Build the attached poc.c:
>>
>> ~
>> gcc -static -O2 -pthread -Wall -Wextra -o poc poc.c
>> ~
>>
>> 3. Run the resulting binary:
>>
>> ~
>> ./poc
>> ~
>>
>> The reproducer creates user and network namespaces, brings up the
>> loopback device, installs a QFQ qdisc and class, and races class
>> changes against packet enqueue. Race timing varies between systems.
>> On the validated KASAN kernel, the expected result is a
>> slab-use-after-free report in qfq_add_to_agg(), followed by a kernel
>> panic. The full observed output is attached as splash.txt.
>>
>> he issue was reproduced on Linux 7.2-rc3 at:
>>
>> 82a47586c0b9266622657009aa30573dddf09f53
>>
>> It is also present in Linux 7.2-rc5 at:
>>
>> f5098b6bae761e346ebcd9da7f95622c04733cff
>>
>> As of July 27, 2026, the vulnerable sequence remains in upstream
>> master at:
>>
>> 62cc90241548d5570ee68e01aaba6506964e9811
>> https://github.com/torvalds/linux/commit/62cc90241548d5570ee68e01aaba6506964e9811
>>
>> Required configuration:
>>
>> * CONFIG_NET_SCHED
>> * CONFIG_NET_SCH_QFQ
>>
>> The attached reproducer additionally requires:
>>
>> * CONFIG_USER_NS
>> * CONFIG_NET_NS
>> * CONFIG_INET
>>
>> KASAN is recommended to observe the first invalid access.
>>
>> Regarding the format, I can send v2 if the reproducer looks good on your end.
>>
>> Thank you.
>>
>> Best regards,
>> David
>>
>> On Fri, Jul 31, 2026 at 12:33 PM Jamal Hadi Salim <jhs@xxxxxxxxxxxx> wrote:
>>>
>>> On Fri, Jul 31, 2026 at 10:05 AM David Lee <david.lee@xxxxxxxxxxxxxxx> wrote:
>>> >
>>> > qfq_change_class() snapshots the current aggregate settings while
>>> > holding the qdisc tree lock, but drops the lock before selecting the
>>> > destination aggregate. During that gap, qfq_enqueue() can move the
>>> > class to an aggregate matching the requested settings.
>>> >
>>> > When qfq_change_class() resumes, qfq_find_agg() then returns cl->agg.
>>> > If it is a singleton, qfq_deact_rm_from_agg() frees the aggregate
>>> > before qfq_add_to_agg() immediately accesses the same pointer, causing
>>> > a use-after-free.
>>> >
>>> > While holding the tree lock, skip the replacement when the destination
>>> > is already the current aggregate. Any estimator replacement has already
>>> > completed, so the class change can finish normally.
>>> >
>>> > Fixes: 462dbc9101ac ("pkt_sched: QFQ Plus: fair-queueing service at DRR cost")
>>> > Bug found and triaged by OpenAI Security Research and
>>> > validated by Trail of Bits.
>>> >
>>>
>>> Please always send a reproducer - either as a tdc test case or if it
>>> is sensitive send it privately to me and Cc the other maintainers.
>>> As trivial as this looks I will not look at it without a repro.
>>>
>>> cheers,
>>> jamal
>>>
>>> > Assisted-by: Codex:gpt-5.6-sol gpt-5.5-cyber
>>> > Signed-off-by: Kyle Zeng <kylebot@xxxxxxxxxx>
>>> > ---
>>> > Trail of Bits has a reproducer for this bug that triggers a
>>> > KASAN use-after-free and can share if needed.
>>> >
>>> >  net/sched/sch_qfq.c | 3 +++
>>> >  1 file changed, 3 insertions(+)
>>> >
>>> > diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
>>> > index 6f3b7273c..e900890e9 100644
>>> > --- a/net/sched/sch_qfq.c
>>> > +++ b/net/sched/sch_qfq.c
>>> > @@ -517,11 +517,14 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>>> >                 sch_tree_lock(sch);
>>> >                 qfq_init_agg(q, new_agg, lmax, weight);
>>> >         }
>>> > +       if (existing && new_agg == cl->agg)
>>> > +               goto unlock;
>>> >         if (existing)
>>> >                 qfq_deact_rm_from_agg(q, cl);
>>> >         else
>>> >                 qdisc_class_hash_insert(&q->clhash, &cl->common);
>>> >         qfq_add_to_agg(q, new_agg, cl);
>>> > +unlock:
>>> >         sch_tree_unlock(sch);
>>> >         qdisc_class_hash_grow(sch, &q->clhash);
>>> >
>>> > --
>>> > 2.53.0
>
>
>
> --
> David Lee
> Security Engineer I, Application Security | Trail of Bits
> trailofbits.com