CVE-2025-38001

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/net/sched/sch_hfsc.c#L677

[4] https://elixir.bootlin.com/linux/v6.15-rc5/source/net/sched/sch_hfsc.c#L1574

[5] https://lore.kernel.org/netdev/8DuRWwfqjoRDLDmBMlIfbrsZg9Gx50DHJc1ilxsEBNe2D6NMoigR_eIRIG0LOjMc3r10nUUZtArXx4oZBIdUfZQrwjcQhdinnMis_0G7VEk=@willsroot.io/T/#u