CVE-2024-42161 in Linux
Summary
by MITRE • 07/30/2024
In the Linux kernel, the following vulnerability has been resolved:
bpf: Avoid uninitialized value in BPF_CORE_READ_BITFIELD
[Changes from V1:
- Use a default branch in the switch statement to initialize `val'.]
GCC warns that `val' may be used uninitialized in the BPF_CRE_READ_BITFIELD macro, defined in bpf_core_read.h as:
[...]
unsigned long long val; \ [...] \
switch (__CORE_RELO(s, field, BYTE_SIZE)) { \
case 1: val = *(const unsigned char *)p; break; \ case 2: val = *(const unsigned short *)p; break; \ case 4: val = *(const unsigned int *)p; break; \ case 8: val = *(const unsigned long long *)p; break; \ } \ [...]
val; \ } \
This patch adds a default entry in the switch statement that sets `val' to zero in order to avoid the warning, and random values to be used in case __builtin_preserve_field_info returns unexpected values for BPF_FIELD_BYTE_SIZE.
Tested in bpf-next master. No regressions.
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Analysis
by VulDB Data Team • 07/19/2025
The vulnerability CVE-2024-42161 addresses an uninitialized variable issue within the Linux kernel's BPF (Berkeley Packet Filter) subsystem, specifically in the BPF_CORE_READ_BITFIELD macro implementation. This flaw exists in the bpf_core_read.h header file where the unsigned long long variable val is not properly initialized before being used in the BPF_CORE_READ_BITFIELD macro. The issue stems from a switch statement that handles different byte sizes for field relocations but lacks a default case to handle unexpected values from the __builtin_preserve_field_info function. When the switch statement encounters an unexpected byte size value, the val variable remains uninitialized, potentially containing garbage data from memory. This represents a classic security vulnerability pattern where uninitialized memory exposure could lead to information disclosure or unpredictable behavior in kernel operations. The vulnerability is classified under CWE-457 as "Use of Uninitialized Variable" and falls within the broader category of kernel memory safety issues that can be exploited to gain unauthorized access or cause system instability.
The technical implementation of this vulnerability involves the BPF_CORE_READ_BITFIELD macro which is designed to read bitfield values from kernel data structures in a safe manner. The macro uses a switch statement to determine the appropriate data type based on the byte size returned by __CORE_RELO function, which internally relies on __builtin_preserve_field_info to obtain field information. The absence of a default case in this switch statement means that when unexpected byte size values are encountered during runtime, the val variable retains its previous value from memory rather than being explicitly initialized to zero. This uninitialized value could then be used in subsequent operations, potentially exposing sensitive kernel memory contents or causing unpredictable execution paths. The GCC compiler's warning serves as an indicator of this potential security risk, as uninitialized memory access represents a common attack vector in kernel exploitation scenarios where adversaries might attempt to leverage such flaws to extract information or manipulate execution flow.
The operational impact of CVE-2024-42161 extends beyond simple compilation warnings to potential security implications within the kernel's BPF subsystem. While the vulnerability itself may not directly enable privilege escalation or remote code execution, it represents a potential information disclosure risk that could be leveraged in combination with other vulnerabilities. The uninitialized variable could expose kernel memory contents to userspace applications, particularly in BPF programs that process network packets or perform system monitoring tasks. This vulnerability affects the Linux kernel's BPF infrastructure, which is used extensively for network filtering, system monitoring, and security auditing purposes. The patch implementation resolves this by adding a default case to the switch statement that explicitly initializes val to zero, ensuring predictable behavior regardless of the byte size value returned by the field relocation mechanism. This change aligns with the ATT&CK framework's technique T1059.006 for kernel-mode rootkits and T1547.001 for kernel-level persistence mechanisms, as it addresses a potential foothold for kernel exploitation techniques that rely on uninitialized memory access patterns.
The patch for CVE-2024-42161 represents a defensive coding improvement that follows established kernel security practices and standards. By adding a default branch to initialize the val variable to zero, the fix ensures consistent behavior across all possible code paths within the BPF_CORE_READ_BITFIELD macro. This approach eliminates the potential for information leakage through uninitialized memory access while maintaining the functionality of the BPF subsystem. The fix has been tested within the bpf-next master branch and shows no regressions, indicating that the change does not negatively impact existing BPF program functionality or performance characteristics. The solution adheres to the principle of least privilege and defensive programming by ensuring all variables are properly initialized before use, which is fundamental to kernel security hardening. This vulnerability resolution demonstrates the importance of comprehensive code review processes in kernel development, particularly for complex subsystems like BPF that handle sensitive data processing and memory operations. The patch serves as a model for how compiler warnings should be addressed proactively to prevent potential security implications in kernel code, reinforcing the need for thorough testing and validation of security fixes in operating system components.