CVE-2025-37953 sch_htb: make htb_deactivate() idempotent

What is CVE-2025-37953?

CVE-2025-37953 is a medium-severity vulnerability in Debian Debian Linux, published on 20 May 2025. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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

sch_htb: make htb_deactivate() idempotent

Alan reported a NULL pointer dereference in htb_next_rb_node() after we made htb_qlen_notify() idempotent.

It turns out in the following case it introduced some regression:

htb_dequeue_tree(): |-> fq_codel_dequeue() |-> qdisc_tree_reduce_backlog() |-> htb_qlen_notify() |-> htb_deactivate() |-> htb_next_rb_node() |-> htb_deactivate()

For htb_next_rb_node(), after calling the 1st htb_deactivate(), the clprio[prio]->ptr could be already set to NULL, which means htb_next_rb_node() is vulnerable here.

For htb_deactivate(), although we checked qlen before calling it, in case of qlen==0 after qdisc_tree_reduce_backlog(), we may call it again which triggers the warning inside.

To fix the issues here, we need to:

1) Make htb_deactivate() idempotent, that is, simply return if we already call it before. 2) Make htb_next_rb_node() safe against ptr==NULL.

Many thanks to Alan for testing and for the reproducer.

Weakness: CWE-476

Which versions are affected by CVE-2025-37953?

It affects 2 products.

Products include Debian Debian Linux, Linux kernel.

How do I fix CVE-2025-37953?

Apply the vendor's patch.

Is CVE-2025-37953 being exploited?

There is no known exploitation of CVE-2025-37953 in the wild and no public exploit code.

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Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2025-37953, DEBIAN-CVE-2025-37953, GHSA-ccmh-pfgg-525g, ROOT-OS-DEBIAN-12-CVE-2025-37953, UBUNTU-CVE-2025-37953.