CVE-2016-4794 in Linux
Summary
by MITRE
Use-after-free vulnerability in mm/percpu.c in the Linux kernel through 4.6 allows local users to cause a denial of service (BUG) or possibly have unspecified other impact via crafted use of the mmap and bpf system calls.
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Analysis
by VulDB Data Team • 08/22/2022
The CVE-2016-4794 vulnerability represents a critical use-after-free flaw located in the Linux kernel's memory management subsystem, specifically within the mm/percpu.c file. This vulnerability affects kernel versions through 4.6 and presents a significant security risk to systems relying on Linux kernel operations. The flaw manifests when the kernel processes certain combinations of mmap and bpf system calls, creating conditions where freed memory locations are accessed after their intended lifecycle has ended. This particular vulnerability falls under the CWE-416 category, which specifically addresses use-after-free conditions that occur when memory is accessed after it has been freed, making it a prime target for exploitation by malicious actors.
The technical implementation of this vulnerability exploits the interaction between the per-CPU memory allocation mechanisms and the Berkeley Packet Filter (bpf) system calls within the Linux kernel. When a user-space process invokes mmap followed by bpf operations, the kernel's memory management routines can inadvertently free memory structures that are subsequently accessed, creating a use-after-free condition. The memory allocation patterns in percpu.c are particularly susceptible to this race condition because they manage memory on a per-CPU basis, and the interaction with bpf system calls creates opportunities for memory corruption. This flaw demonstrates how complex kernel memory management systems can introduce subtle but dangerous vulnerabilities when multiple subsystems interact in unexpected ways, with the bpf subsystem being particularly vulnerable due to its extensive use of memory mapping operations.
The operational impact of CVE-2016-4794 extends beyond simple denial of service conditions to potentially enable more sophisticated attacks. Local attackers with access to the system can leverage this vulnerability to cause kernel BUG messages, leading to system crashes and denial of service conditions that can disrupt critical services. However, the potential for more severe consequences exists, as the use-after-free condition could theoretically be exploited to execute arbitrary code within kernel space, potentially allowing privilege escalation or complete system compromise. The vulnerability's local nature means that exploitation does not require network access, making it particularly dangerous in environments where local user access is possible, and it aligns with ATT&CK technique T1068 which covers local privilege escalation through kernel vulnerabilities. The memory corruption resulting from this vulnerability can manifest in unpredictable ways, making detection and prevention challenging for system administrators and security teams.
Mitigation strategies for CVE-2016-4794 primarily focus on kernel updates and system hardening measures. The most effective immediate solution involves upgrading to a patched kernel version beyond 4.6, where the vulnerability has been addressed through proper memory management controls and additional bounds checking. System administrators should also implement monitoring for suspicious mmap and bpf system call patterns, as these operations are the primary vectors for exploitation. Additional defensive measures include restricting local user access where possible, implementing proper privilege separation, and ensuring that system services do not run with unnecessary elevated privileges. The vulnerability highlights the importance of proper memory management practices in kernel code and the necessity of thorough testing for race conditions and memory lifecycle management in complex subsystem interactions. Organizations should also consider implementing kernel hardening techniques such as stack canaries, address space layout randomization, and kernel module signing to provide additional layers of protection against similar vulnerabilities.