CVE-2026-31517: Linux Kernel Buffer Overflow Vulnerability

CVE-2026-31517 Overview

CVE-2026-31517 is a kernel vulnerability in the Linux kernel's XFRM IP-TFS (IP Traffic Flow Security) implementation that causes a kernel panic when processing non-linear socket buffers during packet reassembly. The vulnerability exists in the iptfs_reassem_cont() function, where improper handling of non-linear SKBs triggers an invalid opcode crash via the SKB_LINEAR_ASSERT check in skb_put().

When IP-TFS successfully uses a zero-copy approach to append data to a reassembled packet (newskb), the buffer becomes non-linear. If a subsequent fragment in the same datagram fails to meet fast-path conditions, the code falls back to a memory copy operation that calls skb_put() directly, which is incompatible with non-linear buffers and causes a kernel panic.

Critical Impact

This vulnerability can cause system crashes and denial of service conditions in Linux systems using IP-TFS for IPsec traffic flow security, particularly in VPN and encrypted network tunneling scenarios.

Affected Products

• Linux Kernel with XFRM IP-TFS module (xfrm_iptfs)
• Systems using IPsec with IP Traffic Flow Security extensions
• VPN gateways and network security appliances running affected kernel versions

Discovery Timeline

• 2026-04-22 - CVE CVE-2026-31517 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-31517

Vulnerability Analysis

This vulnerability is a Null Pointer Dereference DoS / Denial of Service issue in the Linux kernel's network subsystem. The flaw resides specifically in the iptfs_reassem_cont() function within the xfrm_iptfs module, which handles IP Traffic Flow Security packet reassembly.

The vulnerability occurs due to an incorrect assumption about SKB (socket buffer) linearity during the packet reassembly process. When the IP-TFS implementation attempts to reassemble fragmented inner packets, it first tries a zero-copy approach for performance optimization. If successful, this operation transforms the newskb buffer from linear to non-linear.

The problem manifests when processing subsequent fragments that don't qualify for the fast-path zero-copy operation. In this scenario, the code attempts a traditional memory copy using skb_put() to append data to the existing buffer. However, skb_put() contains a SKB_LINEAR_ASSERT check that fails when operating on non-linear buffers, resulting in an invalid opcode exception and kernel panic.

Root Cause

The root cause is a missing linearity check before calling skb_put() in the iptfs_reassem_cont() function. The code did not account for the scenario where an SKB could become non-linear after a successful zero-copy operation, then need to handle a subsequent fragment via the slower memory copy path.

The fix involves checking if the SKB is non-linear using appropriate kernel APIs and, if so, calling skb_linearize() to convert it back to a linear buffer before proceeding with the skb_put() operation. Since the initial allocation of newskb reserved sufficient tailroom for the entire reassembled packet, no additional buffer extension is required after linearization.

Attack Vector

The vulnerability can be triggered through specially crafted network packets that cause the IP-TFS reassembly path to encounter the vulnerable code sequence:

1. An attacker sends IPsec-protected packets that utilize IP-TFS tunneling
2. The packets are crafted such that initial fragments meet zero-copy fast-path conditions, making the SKB non-linear
3. Subsequent fragments are designed to fail fast-path conditions, forcing the memory copy fallback
4. When skb_put() is called on the non-linear SKB, the kernel panics

The attack can be executed remotely over the network against any system processing IP-TFS traffic, potentially causing denial of service conditions in VPN concentrators, security gateways, or any Linux system using IPsec with IP-TFS extensions.

The kernel crash trace shows the execution path through iptfs_reassem_cont(), iptfs_input_ordered(), iptfs_input(), and ultimately to xfrm_input() in the ESP receive processing chain, demonstrating the vulnerability is triggered during normal packet processing in the receive path.

Detection Methods for CVE-2026-31517

Indicators of Compromise

• Kernel panic messages containing skb_put+0x3c/0x40 in the stack trace
• System crashes with "invalid opcode: 0000" errors referencing xfrm_iptfs module
• Repeated system reboots on systems actively processing IP-TFS traffic
• Crash dumps showing iptfs_reassem_cont in the call trace

Detection Strategies

• Monitor kernel logs (dmesg, /var/log/kern.log) for invalid opcode exceptions related to skb_put()
• Implement crash dump analysis for stack traces containing xfrm_iptfs module references
• Deploy network intrusion detection signatures for anomalous IPsec/IP-TFS packet patterns
• Use kernel tracing tools (ftrace, eBPF) to monitor iptfs_reassem_cont() function calls and SKB linearity states

Monitoring Recommendations

• Configure kdump or similar crash collection mechanisms to capture kernel panics for analysis
• Set up automated alerting for systems running IP-TFS that experience unexpected reboots
• Monitor IPsec tunnel stability and packet processing error rates
• Implement network flow analysis to detect potential exploitation attempts targeting IP-TFS endpoints

How to Mitigate CVE-2026-31517

Immediate Actions Required

• Apply the kernel patches referenced in the git commits to affected systems
• Consider temporarily disabling IP-TFS functionality if not critical to operations
• Implement network filtering to restrict IP-TFS traffic to trusted sources
• Ensure crash dump collection is enabled to capture diagnostic information if exploitation occurs

Patch Information

The Linux kernel maintainers have released fixes for this vulnerability through multiple commits. The patches add a linearity check before calling skb_put() in the iptfs_reassem_cont() function, and call skb_linearize() when necessary to handle non-linear buffers safely.

Patches are available through the following kernel git commits:

• Kernel Git Commit 0b352f83
• Kernel Git Commit 33a7b362
• Kernel Git Commit 7fdfe8f6

Organizations should update to patched kernel versions through their distribution's package management system or by applying the upstream patches directly.

Workarounds

• Disable the xfrm_iptfs kernel module if IP-TFS functionality is not required: modprobe -r xfrm_iptfs
• Configure IPsec policies to use alternative traffic flow confidentiality mechanisms
• Implement network access controls to limit exposure of IP-TFS endpoints to trusted networks only
• Deploy network-level rate limiting on IPsec traffic to reduce potential exploitation impact

# Disable xfrm_iptfs module (temporary workaround)
modprobe -r xfrm_iptfs

# Blacklist module to prevent automatic loading
echo "blacklist xfrm_iptfs" >> /etc/modprobe.d/blacklist-iptfs.conf

# Verify module is not loaded
lsmod | grep xfrm_iptfs