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Linux KernelOperating system · Linux

CVE-2026-31649

CRITICAL · 9.8 CVSS v3.1 Published 2026-04-24
Fix available
A fix is available. Upgrade to 5.10.253 / 5.15.203 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Integer underflow in stmmac driver's jumbo_frm() chain-mode implementation causes unsigned wrap when skb_headlen(skb) <= BUF_SIZE but skb->len > BUF_SIZE, producing a huge len value that iterates beyond skb buffer boundaries, mapping arbitrary kernel memory to DMA on IOMMU-less SoCs.

MitigationApply the kernel patch introducing buf_len clamped to min(nopaged_len, bmax) to prevent the underflow; ensure the fix is backported to affected kernel versions.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Linux KernelOperating system
Affected:>= 3.2.1, < 5.10.253>= 5.11, < 5.15.203>= 5.16, < 6.1.169>= 6.2, < 6.6.135>= 6.7, < 6.12.82>= 6.13, < 6.18.23>= 6.19, < 6.19.13= 3.2= 7.0

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check kernel version
    Run 'uname -r' or 'cat /proc/version' to get the running kernel version
    Affected if The kernel version falls within any of these ranges: >=3.2.1 and <5.10.253, >=5.11 and <5.15.203, >=5.16 and <6.1.169, >=6.2 and <6.6.135, >=6.7 and <6.12.82, >=6.13 and <6.18.23, >=6.19 and <6.19.13, or is exactly 3.2 or 7.0
  2. Identify stmmac network interfaces
    Run 'ls /sys/class/net/' and check each interface's driver via 'cat /sys/class/net/<iface>/device/driver' or 'ethtool -i <iface>'
    Affected if Any network interface uses the stmmac driver (also shown as stmmac-4-ww, stmmac-5-ww, etc.)
  3. Check if jumbo frames are enabled
    Run 'ip link show' or 'ifconfig' and look for 'mtu 9000' or similar large MTU values on stmmac interfaces
    Affected if An stmmac interface has MTU set above the standard 1500 bytes (e.g., 9000 for jumbo frames)
  4. Verify chain mode configuration in stmmac
    Check stmmac driver parameters or module options: 'cat /sys/module/stmmac/parameters/*' or check device tree for 'snps,tx-channels' or 'snps,rx-channels' with chain mode enabled
    Affected if Chain mode is enabled in the stmmac driver configuration (typically indicated by specific channel/queue settings)
  5. Check for IOMMU status
    Run 'dmesg | grep -i iommu' or check 'cat /proc/cmdline' for 'intel_iommu=on' or 'amd_iommu=on'
    Affected if IOMMU is disabled (no iommu= parameter or iommu=off in kernel cmdline) - this makes DMA memory exposure possible

The system is affected if it runs a vulnerable kernel version, uses stmmac driver with jumbo frames enabled, has chain mode active, and has IOMMU disabled - allowing the underflow to expose kernel memory to DMA.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.10.253 / 5.15.203 / 6.1.169 or later
Fixed in 5.10.2535.15.2036.1.169
Vendor patch git.kernel.org →
Interim mitigation

Apply the kernel patch introducing buf_len clamped to min(nopaged_len, bmax) to prevent the underflow; ensure the fix is backported to affected kernel versions.

Recommended fix High confidence

Linux Kernel 6.6.135 (or 6.1.169, 5.15.203, or 5.10.253 depending on current version branch - choose the next stable release in your branch that meets or exceeds the minimum fixed version)

  1. 1. Identify the currently running Linux kernel version using 'uname -r'
  2. 2. Determine which version range your current kernel falls into from the affected versions list
  3. 3. For kernels >= 3.2.1 and < 5.10.253: upgrade to kernel version 5.10.253 or later
  4. 4. For kernels >= 5.11 and < 5.15.203: upgrade to kernel version 5.15.203 or later
  5. 5. For kernels >= 5.16 and < 6.1.169: upgrade to kernel version 6.1.169 or later
  6. 6. For kernels >= 6.2 and < 6.6.135: upgrade to kernel version 6.6.135 or later
  7. 7. After upgrade, verify the fix is present by checking the stmmac driver source for the buf_len variable in the jumbo_frm() function
  8. 8. Reboot into the new kernel and verify system stability
Caveat Kernel upgrades may require matching hardware drivers and could introduce regression; ensure compatibility with installed hardware and software stack before production deployment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Linux Kernel Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing6.0 h
  • Review / QA3.0 h
13.0 hours of engineering $2,200
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-31649 in production — separate from our analysis above.

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What this is

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  • Version or environment caveats, and links to real fixes
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