linux-cve-announce - CVE-2025-38468: net/sched: Return NULL when htb_lookup

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Message-ID: <2025072834-CVE-2025-38468-4110@gregkh>
Date: Mon, 28 Jul 2025 13:12:35 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-38468: net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

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

net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree

htb_lookup_leaf has a BUG_ON that can trigger with the following:

tc qdisc del dev lo root
tc qdisc add dev lo root handle 1: htb default 1
tc class add dev lo parent 1: classid 1:1 htb rate 64bit
tc qdisc add dev lo parent 1:1 handle 2: netem
tc qdisc add dev lo parent 2:1 handle 3: blackhole
ping -I lo -c1 -W0.001 127.0.0.1

The root cause is the following:

1. htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on
   the selected leaf qdisc
2. netem_dequeue calls enqueue on the child qdisc
3. blackhole_enqueue drops the packet and returns a value that is not
   just NET_XMIT_SUCCESS
4. Because of this, netem_dequeue calls qdisc_tree_reduce_backlog, and
   since qlen is now 0, it calls htb_qlen_notify -> htb_deactivate ->
   htb_deactiviate_prios -> htb_remove_class_from_row -> htb_safe_rb_erase
5. As this is the only class in the selected hprio rbtree,
   __rb_change_child in __rb_erase_augmented sets the rb_root pointer to
   NULL
6. Because blackhole_dequeue returns NULL, netem_dequeue returns NULL,
   which causes htb_dequeue_tree to call htb_lookup_leaf with the same
   hprio rbtree, and fail the BUG_ON

The function graph for this scenario is shown here:
 0)               |  htb_enqueue() {
 0) + 13.635 us   |    netem_enqueue();
 0)   4.719 us    |    htb_activate_prios();
 0) # 2249.199 us |  }
 0)               |  htb_dequeue() {
 0)   2.355 us    |    htb_lookup_leaf();
 0)               |    netem_dequeue() {
 0) + 11.061 us   |      blackhole_enqueue();
 0)               |      qdisc_tree_reduce_backlog() {
 0)               |        qdisc_lookup_rcu() {
 0)   1.873 us    |          qdisc_match_from_root();
 0)   6.292 us    |        }
 0)   1.894 us    |        htb_search();
 0)               |        htb_qlen_notify() {
 0)   2.655 us    |          htb_deactivate_prios();
 0)   6.933 us    |        }
 0) + 25.227 us   |      }
 0)   1.983 us    |      blackhole_dequeue();
 0) + 86.553 us   |    }
 0) # 2932.761 us |    qdisc_warn_nonwc();
 0)               |    htb_lookup_leaf() {
 0)               |      BUG_ON();
 ------------------------------------------

The full original bug report can be seen here [1].

We can fix this just by returning NULL instead of the BUG_ON,
as htb_dequeue_tree returns NULL when htb_lookup_leaf returns
NULL.

[1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4wmGvNk2ceHjKb8ORnzw0cGwfmVu42g9dL7XyJLf1NEzaztboTWcm0Ogxuojoeo=@willsroot.io/

The Linux kernel CVE team has assigned CVE-2025-38468 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.29 with commit 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 and fixed in 6.1.147 with commit 890a5d423ef0a7bd13447ceaffad21189f557301
	Issue introduced in 2.6.29 with commit 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 and fixed in 6.6.100 with commit 7ff2d83ecf2619060f30ecf9fad4f2a700fca344
	Issue introduced in 2.6.29 with commit 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 and fixed in 6.12.40 with commit e5c480dc62a3025b8428d4818e722da30ad6804f
	Issue introduced in 2.6.29 with commit 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 and fixed in 6.15.8 with commit 3691f84269a23f7edd263e9b6edbc27b7ae332f4
	Issue introduced in 2.6.29 with commit 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 and fixed in 6.16 with commit 0e1d5d9b5c5966e2e42e298670808590db5ed628

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-38468
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/sched/sch_htb.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/890a5d423ef0a7bd13447ceaffad21189f557301
	https://git.kernel.org/stable/c/7ff2d83ecf2619060f30ecf9fad4f2a700fca344
	https://git.kernel.org/stable/c/e5c480dc62a3025b8428d4818e722da30ad6804f
	https://git.kernel.org/stable/c/3691f84269a23f7edd263e9b6edbc27b7ae332f4
	https://git.kernel.org/stable/c/0e1d5d9b5c5966e2e42e298670808590db5ed628