CVE-2026-31649: Linux Kernel Information Disclosure Flaw

CVE-2026-31649 Overview

CVE-2026-31649 is an integer underflow vulnerability in the Linux kernel's stmmac network driver, specifically in the jumbo_frm() chain-mode implementation. The vulnerability occurs when processing network packets where the linear portion of a socket buffer (skb) is smaller than the maximum buffer size, but the total packet length including page fragments exceeds it. This mismatch causes an unsigned integer subtraction to wrap around, producing an extremely large length value that leads to out-of-bounds memory access.

Critical Impact

This vulnerability enables kernel memory disclosure and potential memory corruption through DMA operations on IOMMU-less SoCs, allowing attackers to read arbitrary kernel memory or corrupt system memory via network packets.

Affected Products

• Linux Kernel version 3.2 and later
• Linux Kernel 7.0 release candidates (rc1 through rc7)
• All Linux distributions using the stmmac network driver in chain mode

Discovery Timeline

• April 24, 2026 - CVE CVE-2026-31649 published to NVD
• April 27, 2026 - Last updated in NVD database

Technical Details for CVE-2026-31649

Vulnerability Analysis

The vulnerability exists in the jumbo_frm() function within the stmmac network driver's chain mode implementation. The function unconditionally computes len = nopaged_len - bmax, where nopaged_len represents the linear bytes from skb_headlen(skb) and bmax is either BUF_SIZE_8KiB or BUF_SIZE_2KiB. The critical flaw arises because the calling function stmmac_xmit() determines whether to invoke jumbo_frm() based on skb->len (total packet length including page fragments), not the linear portion alone.

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 massive value (approximately 0xFFFFxxxx). This causes the while (len != 0) loop to execute hundreds of thousands of iterations, passing pointers calculated as skb->data + bmax * i that extend far beyond the legitimate skb buffer boundaries to dma_map_single().

Root Cause

The root cause is a type confusion between total packet length (skb->len) and linear buffer length (skb_headlen(skb)) when determining jumbo frame handling. The stmmac_is_jumbo_frm() function uses the total length to decide if jumbo frame processing is needed, but jumbo_frm() operates on the linear portion only. This disconnect creates a scenario where arithmetic operations assume the linear portion is always larger than bmax when it may not be, leading to unsigned integer underflow (CWE-190).

Attack Vector

An attacker can exploit this vulnerability by sending specially crafted network packets to a system using the stmmac driver in chain mode. The attack is particularly effective on IOMMU-less SoCs, which are the typical deployment environment for stmmac-based systems such as embedded devices, IoT devices, and some ARM-based platforms.

The attack flow involves:

1. Crafting a packet with a small linear header portion but large page fragment payload
2. Triggering the jumbo_frm() code path through the stmmac driver
3. Exploiting the integer underflow to cause the driver to map arbitrary kernel memory regions to the DMA engine
4. Reading leaked kernel memory contents or corrupting memory through DMA write operations

The vulnerability can be triggered remotely over the network without requiring authentication or user interaction, making it particularly dangerous for exposed systems.

Detection Methods for CVE-2026-31649

Indicators of Compromise

• Unexpected kernel crashes or oops messages referencing the stmmac driver or jumbo_frm() function
• Unusual DMA mapping errors or IOMMU violations in system logs
• Network interface anomalies or hangs on stmmac-based devices
• Memory corruption signatures or unexplained system instability following network traffic

Detection Strategies

• Monitor kernel logs for stmmac driver errors, particularly those involving DMA operations or buffer handling
• Implement network traffic analysis to detect unusually fragmented packets targeting stmmac interfaces
• Deploy kernel integrity monitoring to detect memory corruption patterns
• Use SentinelOne's kernel-level monitoring capabilities to detect anomalous driver behavior and memory access patterns

Monitoring Recommendations

• Enable verbose logging for the stmmac network driver during incident investigation
• Configure alerts for kernel oops or panic events related to network subsystems
• Monitor DMA allocation patterns for unusual activity on embedded and IoT systems
• Implement network segmentation to limit exposure of vulnerable embedded devices

How to Mitigate CVE-2026-31649

Immediate Actions Required

• Apply the official kernel patches to all systems running affected Linux kernel versions with stmmac driver
• Prioritize patching for IOMMU-less SoCs and embedded systems that are most vulnerable
• Review network exposure of affected devices and implement additional network-level controls where immediate patching is not possible
• Consider temporarily disabling chain mode in the stmmac driver if patches cannot be immediately applied

Patch Information

The Linux kernel maintainers have released patches across multiple stable branches to address this vulnerability. The fix introduces a buf_len local variable clamped to min(nopaged_len, bmax), ensuring that the length calculation is always safe. When the linear portion fits within a single descriptor, the resulting length is zero, causing the loop to be skipped naturally, while the fragment loop in stmmac_xmit() handles page fragments afterward.

Patches are available through the following kernel git commits:

• Kernel Git Commit 10d12b9
• Kernel Git Commit 275bdf7
• Kernel Git Commit 2c91b39
• Kernel Git Commit 513e067
• Kernel Git Commit 51f4e09
• Kernel Git Commit 6fca757
• Kernel Git Commit a2b68a9
• Kernel Git Commit b7b8012

Workarounds

• If the stmmac driver supports ring mode as an alternative to chain mode, consider switching to ring mode until patches are applied
• Implement network-level filtering to restrict traffic to affected devices from untrusted sources
• Enable IOMMU where supported to provide an additional layer of protection against DMA-based attacks
• Deploy intrusion detection systems to monitor for exploitation attempts targeting this vulnerability

# Check if stmmac driver is loaded and in use
lsmod | grep stmmac

# View current kernel version to assess patch status
uname -r

# Check for stmmac-related kernel log entries
dmesg | grep -i stmmac