---
id: CVE-2026-31649
title: 'net: stmmac: fix integer underflow in chain mode'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net: stmmac: fix integer underflow in chain mode

  The jumbo_frm() chain-mode implementation unconditionally computes

      len = nopaged_len - bmax;

  where nopaged_len =…
severity: critical
cvss: 9.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'
cvssSource: cna
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    513e06735f5be575b409d195822195348b164e48
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    275bdf762e82082f064e60a92448fa2ac43cf95b
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    a2b68a9a476b9544ff31f1fbcd5d80867a8a5e2f
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    b7b8012193fd98236d7ae05d4b553f010a77b2ef
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    2c91b39912278d0878f9ba60ba04d2518b18a08d
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    6fca757c20396dc2e604dcc61922264e9e3dc803
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    10d12b9240ebf96c785f0e2e4228318cd5f3a3eb
  - >-
    Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 <
    51f4e090b9f87b40c21b6daadb5c06e6c0a07b67
  - Linux 3.2
published: '2026-04-24'
updated: '2026-09-08'
sourceUpdated: '2026-09-08T08:47:51.175Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-31649'
references:
  - url: 'https://git.kernel.org/stable/c/513e06735f5be575b409d195822195348b164e48'
  - url: 'https://git.kernel.org/stable/c/275bdf762e82082f064e60a92448fa2ac43cf95b'
  - url: 'https://git.kernel.org/stable/c/a2b68a9a476b9544ff31f1fbcd5d80867a8a5e2f'
  - url: 'https://git.kernel.org/stable/c/b7b8012193fd98236d7ae05d4b553f010a77b2ef'
  - url: 'https://git.kernel.org/stable/c/2c91b39912278d0878f9ba60ba04d2518b18a08d'
  - url: 'https://git.kernel.org/stable/c/6fca757c20396dc2e604dcc61922264e9e3dc803'
  - url: 'https://git.kernel.org/stable/c/10d12b9240ebf96c785f0e2e4228318cd5f3a3eb'
  - url: 'https://git.kernel.org/stable/c/51f4e090b9f87b40c21b6daadb5c06e6c0a07b67'
tags:
  - cve.org
epss: 0.00868
epssPercentile: 0.57021
ingestedAt: '2026-09-08T15:33:26.990Z'
---

## Overview

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

net: stmmac: fix integer underflow in chain mode

The jumbo_frm() chain-mode implementation unconditionally computes

    len = nopaged_len - bmax;

where nopaged_len = skb_headlen(skb) (linear bytes only) and bmax is
BUF_SIZE_8KiB or BUF_SIZE_2KiB.  However, the caller stmmac_xmit()
decides to invoke jumbo_frm() based on skb->len (total length including
page fragments):

    is_jumbo = stmmac_is_jumbo_frm(priv, skb->len, enh_desc);

When a packet has a small linear portion (nopaged_len <= bmax) but a
large total length due to page fragments (skb->len > bmax), the
subtraction wraps as an unsigned integer, producing a huge len value
(~0xFFFFxxxx).  This causes the while (len != 0) loop to execute
hundreds of thousands of iterations, passing skb->data + bmax * i
pointers far beyond the skb buffer to dma_map_single().  On IOMMU-less
SoCs (the typical deployment for stmmac), this maps arbitrary kernel
memory to the DMA engine, constituting a kernel memory disclosure and
potential memory corruption from hardware.

Fix this by introducing a buf_len local variable clamped to
min(nopaged_len, bmax).  Computing len = nopaged_len - buf_len is then
always safe: it is zero when the linear portion fits within a single
descriptor, causing the while (len != 0) loop to be skipped naturally,
and the fragment loop in stmmac_xmit() handles page fragments afterward.

## Affected

- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < 513e06735f5be575b409d195822195348b164e48`
- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < 275bdf762e82082f064e60a92448fa2ac43cf95b`
- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < a2b68a9a476b9544ff31f1fbcd5d80867a8a5e2f`
- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < b7b8012193fd98236d7ae05d4b553f010a77b2ef`
- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < 2c91b39912278d0878f9ba60ba04d2518b18a08d`
- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < 6fca757c20396dc2e604dcc61922264e9e3dc803`
- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < 10d12b9240ebf96c785f0e2e4228318cd5f3a3eb`
- `Linux >= 286a837217204b1ef105e3a554d0757e4fdfaac1 < 51f4e090b9f87b40c21b6daadb5c06e6c0a07b67`
- `Linux 3.2`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.