CVE-2025-37953 Information

May 21, 2025 cve

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.

Reference

https://git.kernel.org/stable/c/31ff70ad39485698cf779f2078132d80b57f6c07 https://git.kernel.org/stable/c/3769478610135e82b262640252d90f6efb05be71 https://git.kernel.org/stable/c/98cd7ed92753090a714f0802d4434314526fe61d https://git.kernel.org/stable/c/c4792b9e38d2f61b07eac72f10909fa76130314b https://git.kernel.org/stable/c/c928dd4f6bf0c25c72b11824a1e9ac9bd37296a0

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