CVE-2021-41864 in Linux
Summary
by MITRE • 10/02/2021
prealloc_elems_and_freelist in kernel/bpf/stackmap.c in the Linux kernel through 5.14.9 allows unprivileged users to trigger an eBPF multiplication integer overflow with a resultant out-of-bounds write.
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Analysis
by VulDB Data Team • 05/13/2025
The vulnerability identified as CVE-2021-41864 resides within the Linux kernel's eBPF (extended Berkeley Packet Filter) subsystem, specifically in the prealloc_elems_and_freelist function located in kernel/bpf/stackmap.c. This flaw represents a critical security issue that affects Linux kernel versions up to and including 5.14.9, potentially enabling unprivileged users to execute arbitrary code with kernel-level privileges. The vulnerability manifests through a multiplication integer overflow that leads to an out-of-bounds write condition, fundamentally compromising the integrity and security of the affected systems.
The technical implementation of this vulnerability stems from improper input validation and arithmetic handling within the eBPF stack map preallocation mechanism. When unprivileged users interact with eBPF programs that utilize stack maps, the prealloc_elems_and_freelist function processes user-supplied parameters without adequate overflow checking during multiplication operations. This allows attackers to craft malicious eBPF programs that manipulate the multiplication calculation to produce values that exceed the maximum representable integer, thereby causing an integer overflow. The resulting overflow corrupts memory layout and enables an out-of-bounds write operation that can be exploited to overwrite critical kernel memory regions.
The operational impact of CVE-2021-41864 extends beyond simple privilege escalation as it provides attackers with a pathway to achieve complete system compromise. The vulnerability specifically targets the kernel's BPF subsystem, which is designed for safe, sandboxed execution of user-provided programs. However, this flaw undermines the sandboxing guarantees by allowing unprivileged users to execute code that can manipulate kernel memory structures. The out-of-bounds write condition can potentially overwrite function pointers, return addresses, or other critical kernel data structures, enabling attackers to execute arbitrary code with kernel privileges. This makes the vulnerability particularly dangerous in multi-tenant environments or systems where untrusted users might have access to eBPF capabilities.
From a cybersecurity perspective, this vulnerability aligns with CWE-191, which describes integer underflow or overflow conditions, and represents a classic example of how seemingly benign arithmetic operations can lead to critical security flaws. The ATT&CK framework categorizes this vulnerability under T1068, which describes "Exploitation for Privilege Escalation," as it provides a method for unprivileged users to gain kernel-level access. The vulnerability also relates to T1548.001, which covers "Abuse Elevation Control Mechanism," since it exploits the legitimate eBPF subsystem to bypass privilege restrictions. The fact that this vulnerability affects unprivileged users makes it particularly concerning for systems that rely on eBPF for network monitoring, security policy enforcement, or other kernel-level operations that may be accessible to non-root users.
Mitigation strategies for CVE-2021-41864 primarily focus on patching the affected kernel versions with the official security updates released by the Linux kernel development team. Organizations should prioritize immediate deployment of kernel updates that contain the fix for this integer overflow vulnerability. In environments where immediate patching is not feasible, administrators can implement additional controls such as restricting eBPF program loading capabilities, disabling unnecessary eBPF features, or implementing stricter access controls for users who might interact with the BPF subsystem. The vulnerability also highlights the importance of comprehensive code review processes for kernel subsystems, particularly those involving memory management and arithmetic operations that handle user-supplied data. Security monitoring should include detection of unusual eBPF program loading patterns or attempts to manipulate stack map parameters that might indicate exploitation attempts.