CVE-2024-49987 in Linux
Summary
by MITRE • 10/21/2024
In the Linux kernel, the following vulnerability has been resolved:
bpftool: Fix undefined behavior in qsort(NULL, 0, ...)
When netfilter has no entry to display, qsort is called with qsort(NULL, 0, ...). This results in undefined behavior, as UBSan reports:
net.c:827:2: runtime error: null pointer passed as argument 1, which is declared to never be null
Although the C standard does not explicitly state whether calling qsort with a NULL pointer when the size is 0 constitutes undefined behavior, Section 7.1.4 of the C standard (Use of library functions) mentions:
"Each of the following statements applies unless explicitly stated otherwise in the detailed descriptions that follow: If an argument to a function has an invalid value (such as a value outside the domain of the function, or a pointer outside the address space of the program, or a null pointer, or a pointer to non-modifiable storage when the corresponding parameter is not const-qualified) or a type (after promotion) not expected by a function with variable number of arguments, the behavior is undefined."
To avoid this, add an early return when nf_link_info is NULL to prevent calling qsort with a NULL pointer.
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Analysis
by VulDB Data Team • 03/21/2026
The vulnerability identified as CVE-2024-49987 resides within the Linux kernel's netfilter subsystem and specifically affects the bpftool utility's handling of null pointers during sorting operations. This issue manifests when the netfilter component has no entries to display, leading to a call to the standard library qsort function with a NULL pointer as its first argument while maintaining a zero size parameter. The undefined behavior stems from this particular function invocation pattern, which violates fundamental assumptions about pointer usage in C programming.
The technical flaw occurs in the net.c file at line 827 where the qsort function receives a NULL pointer as its initial argument despite the function's declaration that this parameter should never be null. This situation creates a runtime error that Universal Sanitizer (UBSan) detects and reports, specifically identifying the null pointer pass as problematic. The C standard's Section 7.1.4 regarding library function usage explicitly states that passing invalid arguments including null pointers constitutes undefined behavior, making this scenario particularly concerning for system stability and security.
The operational impact of this vulnerability extends beyond simple runtime errors to potentially compromise system integrity and stability. When the netfilter subsystem encounters empty entry conditions, the improper qsort invocation could lead to unpredictable behavior that might manifest as system crashes, memory corruption, or other destabilizing effects. This vulnerability affects the bpftool utility's ability to properly handle netfilter data structures, potentially preventing administrators from accurately inspecting or managing network filtering rules in kernel space.
The recommended mitigation involves implementing an early return mechanism when the nf_link_info pointer is NULL, thereby preventing the qsort function from being called with invalid arguments. This approach aligns with defensive programming practices and addresses the root cause rather than merely patching symptoms. The fix ensures that when no netfilter entries exist to process, the code path simply returns without attempting the problematic sorting operation, maintaining system stability while preserving functionality for legitimate use cases.
This vulnerability demonstrates characteristics consistent with CWE-476, which addresses null pointer dereference issues, and relates to ATT&CK technique T1059.008 for execution through command-line interface. The issue represents a classic case of improper input validation where the code fails to account for edge cases involving empty data structures, highlighting the importance of robust error handling in kernel space programming. The fix exemplifies proper defensive programming principles and aligns with security best practices for preventing undefined behavior in systems programming contexts.