CVE-2015-3291 in Linux
Summary
by MITRE
arch/x86/entry/entry_64.S in the Linux kernel before 4.1.6 on the x86_64 platform does not properly determine when nested NMI processing is occurring, which allows local users to cause a denial of service (skipped NMI) by modifying the rsp register, issuing a syscall instruction, and triggering an NMI.
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Analysis
by VulDB Data Team • 06/13/2022
The vulnerability identified as CVE-2015-3291 resides within the Linux kernel's x86_64 architecture implementation, specifically in the arch/x86/entry/entry_64.S file. This flaw represents a critical design oversight in the kernel's handling of nested Non-Maskable Interrupts, which are critical system events that must be processed immediately regardless of other interrupt states. The vulnerability affects Linux kernel versions prior to 4.1.6, making it a significant concern for systems running older kernel versions. The issue manifests when the kernel fails to properly track the state of nested NMI processing, creating a scenario where NMIs can be inadvertently skipped or lost during system operation.
The technical implementation flaw occurs in the kernel's entry point handling for x86_64 systems, where the code fails to correctly identify when nested NMI processing is occurring. This misidentification stems from improper state tracking mechanisms that rely on the rsp register value to determine interrupt nesting levels. When a local user manipulates the rsp register and subsequently executes a syscall instruction, the kernel's interrupt handling logic becomes confused about the current interrupt context. The vulnerability exploits the fact that the kernel's NMI handling code does not properly validate the register state before proceeding with interrupt processing, leading to a scenario where the NMI is not properly acknowledged or processed. This creates a race condition where the NMI handler incorrectly assumes it's in a different interrupt context than actually exists.
The operational impact of this vulnerability is severe as it allows local users to achieve a denial of service condition that can effectively disable critical system interrupt handling mechanisms. When an NMI is skipped due to this vulnerability, it can lead to system instability, data corruption, or complete system hangs, as NMIs are essential for handling critical system events such as machine check exceptions, hardware errors, or other urgent conditions that require immediate attention. The vulnerability is particularly dangerous because it can be exploited by unprivileged local users, making it a significant security concern for systems where local access is possible. The attack vector requires specific sequence of operations involving register manipulation and syscall execution, but once successful, it can compromise the entire system's interrupt handling integrity and potentially lead to system crashes or data loss.
Mitigation strategies for CVE-2015-3291 primarily involve upgrading to Linux kernel version 4.1.6 or later, where the vulnerability has been addressed through improved interrupt state tracking mechanisms. System administrators should prioritize patching affected systems, particularly those running older kernel versions where local users might have access. Additional mitigations include implementing proper access controls to limit local user privileges, monitoring for unusual syscall patterns, and ensuring that systems are regularly updated with the latest security patches. The vulnerability aligns with CWE-691, which addresses Insufficient Control Flow Management, and maps to ATT&CK technique T1059.003 for system service manipulation. Organizations should also consider implementing intrusion detection systems that can monitor for suspicious register manipulation patterns and syscall sequences that might indicate exploitation attempts. The fix implemented in the patched kernel versions involves enhancing the NMI state tracking logic to properly validate the rsp register value and interrupt nesting context before proceeding with NMI processing, thereby preventing the race condition that enabled the denial of service attack.