CVE-2015-7550 in Linux
Summary
by MITRE
The keyctl_read_key function in security/keys/keyctl.c in the Linux kernel before 4.3.4 does not properly use a semaphore, which allows local users to cause a denial of service (NULL pointer dereference and system crash) or possibly have unspecified other impact via a crafted application that leverages a race condition between keyctl_revoke and keyctl_read calls.
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Analysis
by VulDB Data Team • 07/07/2022
The vulnerability described in CVE-2015-7550 represents a critical race condition flaw within the Linux kernel's key management subsystem that affects versions prior to 4.3.4. This issue resides in the keyctl_read_key function located in security/keys/keyctl.c, where the improper handling of semaphores creates a window of opportunity for malicious local users to exploit the system. The vulnerability specifically manifests when multiple threads or processes attempt to access key objects simultaneously, creating a scenario where the kernel's synchronization mechanisms fail to properly protect shared resources during concurrent operations. The race condition occurs between keyctl_revoke and keyctl_read calls, where the revoke operation may invalidate a key object while another thread attempts to read from it, leading to unpredictable system behavior.
The technical exploitation of this vulnerability leverages the fundamental flaw in kernel semaphore implementation within the key management framework. When keyctl_revoke is executed concurrently with keyctl_read_key, the lack of proper synchronization allows a thread to access a key object that has been freed or invalidated by the revoke operation. This results in a NULL pointer dereference that crashes the kernel and causes a system-wide denial of service. The underlying issue stems from CWE-362, which classifies race conditions as a significant security weakness where two or more threads access shared data concurrently, and the timing of their access creates a vulnerability. The improper use of semaphores in this context violates the principle of atomic operations and proper resource management that should prevent concurrent access to protected kernel data structures.
The operational impact of this vulnerability extends beyond simple denial of service, as it represents a potential vector for more sophisticated attacks that could be exploited to gain elevated privileges or cause system instability. Local users with minimal privileges can leverage this vulnerability to crash the system or potentially execute arbitrary code depending on the specific conditions and timing of the race. The vulnerability affects all Linux systems running kernel versions before 4.3.4, making it particularly dangerous as it impacts a wide range of devices and platforms. Attackers can craft malicious applications that repeatedly trigger the race condition, causing repeated system crashes and potentially exhausting system resources through continuous denial of service attacks.
Mitigation strategies for CVE-2015-7550 primarily focus on upgrading to kernel versions 4.3.4 or later where the race condition has been properly addressed through enhanced synchronization mechanisms. System administrators should prioritize patching affected systems as this vulnerability represents a significant risk to system stability and availability. Additional mitigations include implementing proper access controls to limit local user privileges, monitoring for unusual system crashes or denial of service patterns, and maintaining up-to-date security patches for all kernel components. The fix implemented in kernel version 4.3.4 addresses the core synchronization issue by ensuring proper semaphore usage during concurrent key operations and by implementing more robust protection mechanisms around key object lifetimes. This vulnerability also highlights the importance of proper concurrency control in kernel space operations and aligns with ATT&CK technique T1068 which involves the exploitation of legitimate credentials and system privileges to gain unauthorized access or cause system instability. Organizations should also consider implementing intrusion detection systems to monitor for patterns consistent with this vulnerability and ensure comprehensive testing of kernel updates before deployment to production environments.