CVE-2024-56786: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2024-56786

In the Linux kernel, the following vulnerability has been resolved: bpf: put bpf_link's program when link is safe to be deallocated In general, BPF link's underlying BPF program should be considered to be reachable through attach hook -> link -> prog chain, and, pessimistically, we have to assume that as long as link's memory is not safe to free, attach hook's code might hold a pointer to BPF program and use it. As such, it's not (generally) correct to put link's program early before waiting for RCU GPs to go through. More eager bpf_prog_put() that we currently do is mostly correct due to BPF program's release code doing similar RCU GP waiting, but as will be shown in the following patches, BPF program can be non-sleepable (and, thus, reliant on only "classic" RCU GP), while BPF link's attach hook can have sleepable semantics and needs to be protected by RCU Tasks Trace, and for such cases BPF link has to go through RCU Tasks Trace + "classic" RCU GPs before being deallocated. And so, if we put BPF program early, we might free BPF program before we free BPF link, leading to use-after-free situation. So, this patch defers bpf_prog_put() until we are ready to perform bpf_link's deallocation. At worst, this delays BPF program freeing by one extra RCU GP, but that seems completely acceptable. Alternatively, we'd need more elaborate ways to determine BPF hook, BPF link, and BPF program lifetimes, and how they relate to each other, which seems like an unnecessary complication. Note, for most BPF links we still will perform eager bpf_prog_put() and link dealloc, so for those BPF links there are no observable changes whatsoever. Only BPF links that use deferred dealloc might notice slightly delayed freeing of BPF programs. Also, to reduce code and logic duplication, extract program put + link dealloc logic into bpf_link_dealloc() helper.

AI Analysis

Technical Summary

CVE-2024-56786 is a high-severity vulnerability in the Linux kernel related to the handling of eBPF (extended Berkeley Packet Filter) links and their associated programs. The issue arises from improper management of the lifecycle and deallocation timing of BPF links and BPF programs. Specifically, the vulnerability concerns the premature freeing of a BPF program before the associated BPF link is safely deallocated, which can lead to a use-after-free condition (CWE-416). This occurs because the Linux kernel's reference counting and Read-Copy-Update (RCU) synchronization mechanisms were not correctly aligned for certain BPF links that require both classic RCU grace periods and RCU tasks trace to ensure safe memory reclamation. The kernel previously performed an eager bpf_prog_put() call that could free the BPF program too early, while the BPF link might still be referenced by attach hooks that have sleepable semantics. The patch defers the bpf_prog_put() call until the BPF link is ready for deallocation, ensuring that the BPF program remains valid throughout the link's lifetime. This fix introduces a slight delay in freeing BPF programs for some links but prevents potential use-after-free exploitation. The vulnerability affects specific Linux kernel versions identified by commit hashes and has a CVSS 3.1 score of 7.8, indicating high severity with local attack vector, low attack complexity, requiring low privileges but no user interaction, and impacting confidentiality, integrity, and availability. No known exploits are currently reported in the wild. The vulnerability is technical and subtle, involving kernel memory management and concurrency primitives related to eBPF, a widely used Linux kernel feature for networking, tracing, and security.

Potential Impact

For European organizations, this vulnerability poses a significant risk, particularly for those relying on Linux-based infrastructure that utilizes eBPF for advanced networking, security monitoring, or performance tracing. Exploitation could allow a local attacker with limited privileges to trigger a use-after-free condition, potentially leading to arbitrary code execution within the kernel context, privilege escalation, or system crashes (denial of service). This could compromise the confidentiality, integrity, and availability of critical systems. Given the widespread adoption of Linux servers, cloud environments, and embedded devices across Europe, the vulnerability could impact sectors such as finance, telecommunications, government, and critical infrastructure. The complexity of exploitation is low, but local access is required, which means attackers need some foothold on the system. However, once exploited, the consequences could be severe, including full system compromise. The lack of known exploits in the wild suggests that immediate exploitation is not widespread, but the high severity score warrants prompt attention. Organizations running kernels with affected versions should prioritize patching to mitigate risks. Additionally, the subtlety of the bug means that detection of exploitation attempts may be difficult without specialized monitoring of kernel behavior and eBPF usage.

Mitigation Recommendations

1. Apply the official Linux kernel patches that address CVE-2024-56786 as soon as they become available from trusted sources or distribution vendors. 2. For organizations using custom or embedded Linux kernels, ensure that kernel maintainers backport the fix appropriately. 3. Restrict local access to Linux systems to trusted users only, minimizing the risk of local attackers exploiting the vulnerability. 4. Monitor kernel logs and eBPF subsystem activity for unusual behavior that might indicate exploitation attempts, using advanced kernel tracing tools. 5. Employ mandatory access controls (e.g., SELinux, AppArmor) to limit the capabilities of processes that can load or manipulate eBPF programs and links. 6. Regularly update and audit eBPF programs and their usage policies to ensure they do not inadvertently expose attack surfaces. 7. Consider deploying kernel runtime security tools that can detect use-after-free or memory corruption attempts in kernel space. 8. Educate system administrators and security teams about the risks associated with eBPF vulnerabilities and the importance of timely patching.