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CVE-2025-38001: net_sched: hfsc: Address reentrant enqueue adding class to eltree twice

Description

In the Linux kernel, the following vulnerability has been resolved:

net_sched: hfsc: Address reentrant enqueue adding class to eltree twice

Savino says:
"We are writing to report that this recent patch
(141d34391abbb315d68556b7c67ad97885407547) [1]
can be bypassed, and a UAF can still occur when HFSC is utilized with
NETEM.

The patch only checks the cl->cl_nactive field to determine whether
it is the first insertion or not [2], but this field is only
incremented by init_vf [3].

By using HFSC_RSC (which uses init_ed) [4], it is possible to bypass the
check and insert the class twice in the eltree.
Under normal conditions, this would lead to an infinite loop in
hfsc_dequeue for the reasons we already explained in this report [5].

However, if TBF is added as root qdisc and it is configured with a
very low rate,
it can be utilized to prevent packets from being dequeued.
This behavior can be exploited to perform subsequent insertions in the
HFSC eltree and cause a UAF."

To fix both the UAF and the infinite loop, with netem as an hfsc child,
check explicitly in hfsc_enqueue whether the class is already in the eltree
whenever the HFSC_RSC flag is set.

[1] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=141d34391abbb315d68556b7c67ad97885407547
[2] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L1572
[3] https://elixir.bootlin.com/linux/v6.15-rc5/source/n...

Classification

CVE ID: CVE-2025-38001

Affected Products

Vendor: Linux

Product: Linux

Exploit Prediction Scoring System (EPSS)

EPSS Score: 0.04% (probability of being exploited)

EPSS Percentile: 12.6% (scored less or equal to compared to others)

EPSS Date: 2025-06-27 (when was this score calculated)

References

Timeline

2025-06-06 14:40:22 UTC
CVE

net_sched: hfsc: Address reentrant enqueue adding class to eltree twice