CVE-2016-4557 in Linux
Summary
by MITRE • 01/25/2023
The replace_map_fd_with_map_ptr function in kernel/bpf/verifier.c in the Linux kernel before 4.5.5 does not properly maintain an fd data structure, which allows local users to gain privileges or cause a denial of service (use-after-free) via crafted BPF instructions that reference an incorrect file descriptor.
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Analysis
by VulDB Data Team • 11/08/2024
The vulnerability described in CVE-2016-4557 resides within the Linux kernel's Berkeley Packet Filter (BPF) subsystem, specifically in the replace_map_fd_with_map_ptr function located in kernel/bpf/verifier.c. This flaw represents a critical security issue that affects Linux kernel versions prior to 4.5.5, where the kernel fails to properly manage file descriptor data structures during BPF program verification. The vulnerability manifests when crafted BPF instructions reference incorrect file descriptors, creating a scenario where the kernel's verifier component does not adequately track the lifecycle of file descriptor references. This improper management creates a use-after-free condition that can be exploited by local attackers to escalate privileges or cause system instability through denial of service attacks.
The technical implementation of this vulnerability stems from inadequate memory management within the BPF verifier's handling of file descriptor mappings. When BPF programs are loaded into the kernel, the verifier must translate file descriptor references into actual map pointers to ensure proper program execution. However, the replace_map_fd_with_map_ptr function fails to maintain proper reference counting and validation of these file descriptor mappings. This flaw creates a race condition where a file descriptor can be freed while still referenced by a BPF program, leading to memory corruption. The vulnerability is particularly dangerous because it allows local users to manipulate the kernel's memory management during program verification, effectively bypassing kernel security mechanisms that should prevent such operations. According to CWE-416, this represents a classic use-after-free vulnerability where memory is accessed after it has been freed, potentially leading to arbitrary code execution.
The operational impact of CVE-2016-4557 extends beyond simple privilege escalation to encompass significant system stability risks. Local attackers can leverage this vulnerability to execute arbitrary code with kernel-level privileges, effectively compromising the entire system. The use-after-free condition can be exploited to manipulate kernel data structures, potentially leading to privilege escalation from unprivileged user accounts to root access. Additionally, the vulnerability can be weaponized to cause denial of service conditions, where carefully crafted BPF instructions can trigger kernel panics or system crashes. This makes the vulnerability particularly dangerous in multi-user environments where local users might attempt to exploit it for unauthorized access or system disruption. The ATT&CK framework categorizes this vulnerability under privilege escalation techniques, specifically targeting kernel-level exploits that leverage memory corruption flaws to gain elevated system privileges.
Mitigation strategies for CVE-2016-4557 primarily focus on kernel version upgrades and system hardening measures. The most effective immediate solution is updating to Linux kernel version 4.5.5 or later, where the vulnerability has been patched through proper file descriptor management and reference counting implementations. System administrators should also implement strict BPF program loading policies, restricting the ability of unprivileged users to load BPF programs that could exploit this vulnerability. Additional mitigations include disabling BPF functionality entirely if not required, implementing proper access controls for BPF-related system calls, and monitoring for suspicious BPF program loading activities. The patch addresses the root cause by ensuring that file descriptor references are properly validated and maintained throughout the BPF program verification process, preventing the use-after-free condition that enables privilege escalation attacks. Organizations should also conduct thorough security assessments to identify any potential exploitation attempts and implement intrusion detection systems capable of monitoring for suspicious BPF-related activities that might indicate exploitation attempts.