Subject: CVE-2021-47125: sch_htb: fix refcount leak in htb_parent_to_leaf_offload

From: Greg Kroah-Hartman <[email-masked]>

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

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

sch_htb: fix refcount leak in htb_parent_to_leaf_offload

The commit ae81feb7338c ("sch_htb: fix null pointer dereference
on a null new_q") fixes a NULL pointer dereference bug, but it
is not correct.

Because htb_graft_helper properly handles the case when new_q
is NULL, and after the previous patch by skipping this call
which creates an inconsistency : dev_queue->qdisc will still
point to the old qdisc, but cl->parent->leaf.q will point to
the new one (which will be noop_qdisc, because new_q was NULL).
The code is based on an assumption that these two pointers are
the same, so it can lead to refcount leaks.

The correct fix is to add a NULL pointer check to protect
qdisc_refcount_inc inside htb_parent_to_leaf_offload.

The Linux kernel CVE team has assigned CVE-2021-47125 to this issue.

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

	Issue introduced in 5.12 with commit ae81feb7338c and fixed in 5.12.10 with commit 2411c02d0389
	Issue introduced in 5.12 with commit ae81feb7338c and fixed in 5.13 with commit 944d671d5faa

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-2021-47125
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/2411c02d03892a5057499f8102d0cc1e0f852416
	https://git.kernel.org/stable/c/944d671d5faa0d78980a3da5c0f04960ef1ad893