CVE-2024-26882: Linux Kernel Use-After-Free Vulnerability

CVE-2024-26882 Overview

A vulnerability has been discovered in the Linux kernel's IP tunnel receive path (ip_tunnel_rcv()) that fails to properly pull inner headers before processing, leading to uninitialized memory use. This flaw was identified through kernel fuzzing (syzbot) and allows a local attacker to potentially exploit uninitialized memory access in the ECN decapsulation functions, which could result in information disclosure, privilege escalation, or system compromise.

The vulnerability is related to similar issues previously fixed in IPv6 tunnels (__ip6_tnl_rcv()) and GENEVE tunnels (geneve_rx()), indicating a recurring pattern in tunnel header handling across the kernel's networking subsystem.

Critical Impact

Local attackers with low privileges can exploit uninitialized memory in the kernel's IP tunnel processing, potentially leading to information disclosure or privilege escalation on affected Linux systems.

Affected Products

• Linux Kernel (multiple versions)
• Debian Linux (via kernel packages)
• NetApp products using affected Linux kernels

Discovery Timeline

• April 17, 2024 - CVE-2024-26882 published to NVD
• December 20, 2024 - Last updated in NVD database

Technical Details for CVE-2024-26882

Vulnerability Analysis

The vulnerability exists in the ip_tunnel_rcv() function within net/ipv4/ip_tunnel.c. When processing incoming tunneled packets, the function fails to ensure that the inner packet headers are properly available in the socket buffer's head area before attempting to access them. This occurs because pskb_inet_may_pull() is not called to guarantee header availability, leading to reads from uninitialized memory pages.

The syzbot kernel fuzzer detected this issue through KMSAN (Kernel Memory Sanitizer), which flagged uninitialized value usage in the ECN decapsulation call chain: __INET_ECN_decapsulate() → INET_ECN_decapsulate() → IP_ECN_decapsulate() → ip_tunnel_rcv().

The fix requires saving skb->network_header to a temporary variable before calling pskb_inet_may_pull(), then recomputing the network header pointer afterward, as the pull operation may relocate the buffer data.

Root Cause

The root cause is improper socket buffer handling in the IP tunnel receive path. When packets arrive via tunneling interfaces (such as GRE tunnels via the TUN driver), the inner packet headers may not be present in the linear portion of the socket buffer (skb->head). The ip_tunnel_rcv() function attempts to access these headers for ECN (Explicit Congestion Notification) decapsulation without first ensuring they are accessible, resulting in reads from uninitialized memory pages allocated during skb_page_frag_refill().

Attack Vector

The vulnerability requires local access and can be triggered through normal user-space operations involving TUN/TAP devices or other interfaces that feed into the IP tunnel receive path. An attacker with low privileges could craft malformed tunnel packets through the TUN driver write interface to trigger the uninitialized memory read condition.

The attack path observed by syzbot involves:

1. User-space write to TUN device via tun_chr_write_iter()
2. Packet processing through tun_get_user() → tun_build_skb()
3. Network receive path through netif_receive_skb() → ip_rcv()
4. GRE tunnel processing via gre_rcv() → ipgre_rcv() → ip_tunnel_rcv()
5. Uninitialized memory access in ECN decapsulation functions

Detection Methods for CVE-2024-26882

Indicators of Compromise

• Kernel log messages containing "KMSAN: uninit-value" related to ip_tunnel_rcv or ECN decapsulation functions
• Unusual system behavior when processing GRE or other IP tunnel traffic
• Kernel crashes or panics in the IP tunnel receive path with stack traces involving inet_ecn.h functions
• Unexpected behavior in TUN/TAP device operations

Detection Strategies

• Monitor kernel logs for KMSAN warnings referencing ip_tunnel_rcv, INET_ECN_decapsulate, or related functions in net/ipv4/ip_tunnel.c
• Deploy SentinelOne Singularity Platform to detect anomalous kernel behavior patterns associated with memory corruption exploitation
• Enable kernel address sanitizer (KASAN) or memory sanitizer (KMSAN) on development and testing systems to catch exploitation attempts
• Audit systems for unexpected TUN/TAP device creation or unusual tunnel interface activity

Monitoring Recommendations

• Configure centralized logging to capture kernel messages from systems running IP tunnel configurations (GRE, IPIP, etc.)
• Monitor for processes with elevated privileges that interact with TUN/TAP devices unexpectedly
• Implement network monitoring for anomalous tunnel traffic patterns that could indicate exploitation attempts
• Use SentinelOne's kernel-level visibility to track suspicious memory access patterns in the networking subsystem

How to Mitigate CVE-2024-26882

Immediate Actions Required

• Update Linux kernel to a patched version as soon as available for your distribution
• Review systems for active IP tunnel configurations (GRE, IPIP, SIT) and assess exposure
• Restrict access to TUN/TAP device creation to trusted users only
• Monitor kernel logs for any indicators of exploitation attempts
• Prioritize patching on systems with network-facing tunnel interfaces

Patch Information

The Linux kernel maintainers have released patches across multiple stable kernel branches. Apply the appropriate patch for your kernel version:

Additional vendor advisories:

• Debian LTS Announcement June 2024
• NetApp Security Advisory ntap-20241220-0002

Workarounds

• If kernel updates cannot be immediately applied, consider temporarily disabling unused IP tunnel interfaces (GRE, IPIP, SIT)
• Restrict access to /dev/net/tun through permissions or SELinux/AppArmor policies to limit local attack surface
• Apply network segmentation to isolate systems with required tunnel functionality from general user access
• Use container isolation or virtualization to limit kernel exposure on multi-tenant systems

# Check for active IP tunnel interfaces
ip tunnel show

# Disable a GRE tunnel interface temporarily (example)
ip link set gre0 down
ip tunnel del gre0

# Restrict TUN device access to root only
chmod 600 /dev/net/tun