CVE-2026-90109: net: sched: fix 32-bit backlog wrap in gred, bfifo and plug enqueue

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

net: sched: fix 32-bit backlog wrap in gred, bfifo and plug enqueue

gred_enqueue(), bfifo_enqueue() and plug_enqueue() admit a packet when the
current backlog plus the packet length fits within the queue limit:

  sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit (gred default VQ)
  gred_backlog+qdisc_pkt_len(skb) <= q->limit  (gred configured VQ)
  sch->qstats.backlog + qdisc_pkt_len(skb) <= sch->limit (bfifo)
  sch->qstats.backlog + skb->len <= q->limit             (plug)

sch->qstats.backlog and q->backlog are u32, and qdisc_pkt_len()/skb->len
are unsigned int, so all sums are computed in 32 bits and wrap at 2^32.
Once the true backlog exceeds 4 GiB the wrapped sum becomes small and
admission keeps succeeding, so the queue grows without bound and the kernel
can be driven to OOM.

Promote the sums to u64 so admission stops once the true backlog exceeds
the limit.  The limit is u32, so the bounded queue stays below 2^32 and
the stored u32 backlog never wraps.

The bug can only be reproduced as root (albeit with ridiculous setup):
 attach a gred (or bfifo/plug) qdisc with a limit near 4 GiB,
 leaving the default VQ unconfigured (for gred), and drive >4 GiB of
 queued traffic (e.g. via a size table / stab to inflate qdisc_pkt_len,
 or sustained high-rate traffic). The u32 backlog+len sum wraps at 2^32,
 admission keeps succeeding, and the queue grows unboundedly to OOM.

The Linux kernel CVE team has assigned CVE-2026-90109 to this issue.


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

	Issue introduced in 4.2 with commit a3eb95f891d6130b1fc03dd07a8b54cf0a5c8ab8 and fixed in 6.12.110 with commit 3fb51b3960342bbee563543f8d33710fd0fb5a96
	Issue introduced in 4.2 with commit a3eb95f891d6130b1fc03dd07a8b54cf0a5c8ab8 and fixed in 6.18.52 with commit 8b2d8c70b969bf84f9e248bfaf2a7135129f2eb6
	Issue introduced in 4.2 with commit a3eb95f891d6130b1fc03dd07a8b54cf0a5c8ab8 and fixed in 7.2.6 with commit d98b92f3d57976ae185238946ec5ea0be32195aa
	Issue introduced in 4.2 with commit a3eb95f891d6130b1fc03dd07a8b54cf0a5c8ab8 and fixed in 7.3-rc1 with commit 4c660ee8c809637909f4f7eb1017f7b9401c75c4

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-2026-90109
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_fifo.c
	net/sched/sch_gred.c
	net/sched/sch_plug.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/3fb51b3960342bbee563543f8d33710fd0fb5a96
	https://git.kernel.org/stable/c/8b2d8c70b969bf84f9e248bfaf2a7135129f2eb6
	https://git.kernel.org/stable/c/d98b92f3d57976ae185238946ec5ea0be32195aa
	https://git.kernel.org/stable/c/4c660ee8c809637909f4f7eb1017f7b9401c75c4