CVE-2018-7757 in Linux
Summary
by MITRE
Memory leak in the sas_smp_get_phy_events function in drivers/scsi/libsas/sas_expander.c in the Linux kernel through 4.15.7 allows local users to cause a denial of service (memory consumption) via many read accesses to files in the /sys/class/sas_phy directory, as demonstrated by the /sys/class/sas_phy/phy-1:0:12/invalid_dword_count file.
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Analysis
by VulDB Data Team • 09/14/2025
The vulnerability identified as CVE-2018-7757 represents a critical memory management flaw within the Linux kernel's SCSI subsystem, specifically affecting the Serial Attached SCSI (SAS) expander implementation. This issue resides in the sas_smp_get_phy_events function located in drivers/scsi/libsas/sas_expander.c, where improper memory handling during read operations on SAS phy attributes creates a persistent memory leak condition. The vulnerability affects Linux kernel versions through 4.15.7, making it a widespread concern across numerous production systems that rely on SAS storage infrastructure. The flaw manifests when local users repeatedly access files within the /sys/class/sas_phy directory, particularly targeting the invalid_dword_count file associated with specific phy identifiers, leading to progressive memory exhaustion.
The technical root cause of this vulnerability stems from inadequate memory deallocation within the sas_smp_get_phy_events function, which processes SMP (SCSI Management Protocol) requests for SAS phy events. When multiple read operations are performed on phy attribute files, the function fails to properly release allocated memory structures, creating a memory leak that accumulates with each access. This pattern of memory consumption occurs because the function does not maintain proper reference counting or memory cleanup mechanisms for the allocated buffers used during phy event processing. The vulnerability is particularly insidious as it operates at the kernel level, where memory leaks can accumulate silently over time until system resources are completely exhausted, resulting in system instability and potential crashes. The flaw is classified under CWE-401 as a failure to release memory after allocation, a common pattern that directly enables denial of service conditions.
The operational impact of CVE-2018-7757 extends beyond simple resource exhaustion, creating significant reliability concerns for systems running affected kernel versions. Local users with access to the system can exploit this vulnerability to consume increasing amounts of system memory through repeated read operations on SAS phy attributes, eventually leading to system denial of service. The attack vector is particularly concerning because it requires minimal privileges and can be executed by any local user, making it an attractive target for both accidental and malicious exploitation. Systems utilizing SAS storage arrays, including enterprise servers, storage appliances, and data center infrastructure, face the highest risk of impact. The vulnerability can cause cascading failures where system performance degrades progressively until the system becomes unresponsive, potentially leading to data loss or service disruption in mission-critical environments. This type of vulnerability aligns with ATT&CK technique T1499.001, which involves resource exhaustion attacks that consume system resources to deny service to legitimate users.
Mitigation strategies for CVE-2018-7757 focus on both immediate patching and system hardening measures. The primary solution involves upgrading to a patched kernel version where the memory leak has been addressed through proper memory management implementation. Kernel maintainers released fixes in versions subsequent to 4.15.7 that include proper memory deallocation mechanisms within the sas_smp_get_phy_events function, ensuring that allocated resources are correctly freed after each operation. System administrators should implement monitoring solutions to detect unusual memory consumption patterns related to kernel memory allocation, particularly in systems with active SAS storage components. Additionally, access controls should be implemented to restrict unnecessary access to the /sys/class/sas_phy directory, reducing the attack surface for local privilege escalation scenarios. The vulnerability demonstrates the importance of thorough memory management practices in kernel code, particularly for frequently accessed system interfaces, and highlights the need for comprehensive testing of resource allocation patterns in device drivers. Organizations should also consider implementing automated patch management processes to ensure timely deployment of security fixes across their infrastructure, as this vulnerability could be exploited by attackers to gain persistent access through resource exhaustion attacks.