Subject: CVE-2024-46828: sched: sch_cake: fix bulk flow accounting logic for host fairness

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

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

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

sched: sch_cake: fix bulk flow accounting logic for host fairness

In sch_cake, we keep track of the count of active bulk flows per host,
when running in dst/src host fairness mode, which is used as the
round-robin weight when iterating through flows. The count of active
bulk flows is updated whenever a flow changes state.

This has a peculiar interaction with the hash collision handling: when a
hash collision occurs (after the set-associative hashing), the state of
the hash bucket is simply updated to match the new packet that collided,
and if host fairness is enabled, that also means assigning new per-host
state to the flow. For this reason, the bulk flow counters of the
host(s) assigned to the flow are decremented, before new state is
assigned (and the counters, which may not belong to the same host
anymore, are incremented again).

Back when this code was introduced, the host fairness mode was always
enabled, so the decrement was unconditional. When the configuration
flags were introduced the *increment* was made conditional, but
the *decrement* was not. Which of course can lead to a spurious
decrement (and associated wrap-around to U16_MAX).

AFAICT, when host fairness is disabled, the decrement and wrap-around
happens as soon as a hash collision occurs (which is not that common in
itself, due to the set-associative hashing). However, in most cases this
is harmless, as the value is only used when host fairness mode is
enabled. So in order to trigger an array overflow, sch_cake has to first
be configured with host fairness disabled, and while running in this
mode, a hash collision has to occur to cause the overflow. Then, the
qdisc has to be reconfigured to enable host fairness, which leads to the
array out-of-bounds because the wrapped-around value is retained and
used as an array index. It seems that syzbot managed to trigger this,
which is quite impressive in its own right.

This patch fixes the issue by introducing the same conditional check on
decrement as is used on increment.

The original bug predates the upstreaming of cake, but the commit listed
in the Fixes tag touched that code, meaning that this patch won't apply
before that.

The Linux kernel CVE team has assigned CVE-2024-46828 to this issue.

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

	Issue introduced in 5.1 with commit 712639929912 and fixed in 5.4.284 with commit 4a4eeefa514d
	Issue introduced in 5.1 with commit 712639929912 and fixed in 5.10.226 with commit 7725152b54d2
	Issue introduced in 5.1 with commit 712639929912 and fixed in 5.15.167 with commit cde71a567797
	Issue introduced in 5.1 with commit 712639929912 and fixed in 6.1.110 with commit 549e407569e0
	Issue introduced in 5.1 with commit 712639929912 and fixed in 6.6.51 with commit d4a9039a7b3d
	Issue introduced in 5.1 with commit 712639929912 and fixed in 6.10.10 with commit d7c01c0714c0
	Issue introduced in 5.1 with commit 712639929912 and fixed in 6.11 with commit 546ea84d07e3

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-2024-46828
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_cake.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/4a4eeefa514db570be025ab46d779af180e2c9bb
	https://git.kernel.org/stable/c/7725152b54d295b7da5e34c2f419539b30d017bd
	https://git.kernel.org/stable/c/cde71a5677971f4f1b69b25e854891dbe78066a4
	https://git.kernel.org/stable/c/549e407569e08459d16122341d332cb508024094
	https://git.kernel.org/stable/c/d4a9039a7b3d8005b90c7b1a55a306444f0e5447
	https://git.kernel.org/stable/c/d7c01c0714c04431b5e18cf17a9ea68a553d1c3c
	https://git.kernel.org/stable/c/546ea84d07e3e324644025e2aae2d12ea4c5896e