CVE-2017-9985 in Linux
Summary
by MITRE
The snd_msndmidi_input_read function in sound/isa/msnd/msnd_midi.c in the Linux kernel through 4.11.7 allows local users to cause a denial of service (over-boundary access) or possibly have unspecified other impact by changing the value of a message queue head pointer between two kernel reads of that value, aka a "double fetch" vulnerability.
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Analysis
by VulDB Data Team • 12/09/2022
The vulnerability identified as CVE-2017-9985 represents a critical double fetch issue within the Linux kernel's sound subsystem, specifically affecting the snd_msndmidi_input_read function in the msnd_midi.c file. This flaw exists in kernel versions through 4.11.7 and demonstrates a classic race condition vulnerability that can be exploited by local attackers to compromise system stability and potentially execute arbitrary code. The vulnerability stems from improper handling of kernel memory structures where a pointer value is read twice within a single function execution, creating an opportunity for malicious modification between these reads. The affected component is part of the Multimedia Sound Device (MSND) driver that manages sound hardware through ISA bus interfaces, making it particularly concerning for legacy systems and embedded devices that still rely on older sound hardware configurations.
The technical exploitation of this vulnerability relies on a fundamental flaw in kernel memory management where the function retrieves a queue head pointer value, processes it, and then retrieves the same pointer value again before completing the operation. During this window between the two pointer reads, a local attacker can modify the pointer value through other kernel mechanisms or concurrent operations, leading to out-of-bounds memory access patterns. This double fetch scenario creates a condition where the second pointer access references memory that may have been altered, moved, or freed, resulting in either a kernel panic through memory corruption or more subtle behavior that could be leveraged for privilege escalation. The vulnerability maps directly to CWE-367 which defines a Time-of-Check to Time-of-Use (TOCTOU) error, where the system state changes between verification and actual use of resources. This type of vulnerability is particularly dangerous in kernel space because it operates at the highest privilege level and can compromise the entire system integrity.
The operational impact of CVE-2017-9985 extends beyond simple denial of service scenarios, as the vulnerability can potentially enable local privilege escalation and system compromise. When exploited successfully, the double fetch condition can cause kernel memory corruption that manifests as system crashes, but more insidiously, it can also allow attackers to manipulate kernel data structures and potentially execute arbitrary code with kernel privileges. This makes the vulnerability particularly dangerous in multi-user environments where local attackers could leverage it to gain elevated system access, or in embedded systems where such compromises could lead to complete device takeover. The attack vector requires local system access, meaning any user with login privileges can potentially exploit this vulnerability, making it a significant concern for enterprise and server environments where local access controls may be less restrictive. The vulnerability is classified under the MITRE ATT&CK framework as part of the Privilege Escalation tactic, specifically targeting the kernel-level execution and memory management capabilities.
Mitigation strategies for CVE-2017-9985 must address both the immediate patching requirements and broader system hardening measures. The primary solution involves applying the official kernel patch that eliminates the double fetch condition by ensuring pointer values are cached or validated in a single atomic operation. System administrators should prioritize upgrading to kernel versions 4.11.8 or later where this vulnerability has been addressed through proper synchronization mechanisms. Additional mitigations include implementing strict access controls to prevent unauthorized local access, monitoring for suspicious kernel memory access patterns, and employing kernel hardening techniques such as stack canaries and kernel address space layout randomization. Organizations should also conduct thorough vulnerability assessments of their kernel configurations, particularly focusing on legacy sound drivers that may be enabled on systems where they are not required. The vulnerability highlights the importance of proper kernel memory management practices and demonstrates how seemingly simple pointer handling can create catastrophic security implications when race conditions are not properly addressed in kernel code.