CVE-2019-5595 in FreeBSD
Summary
by MITRE
In FreeBSD before 11.2-STABLE(r343782), 11.2-RELEASE-p9, 12.0-STABLE(r343781), and 12.0-RELEASE-p3, kernel callee-save registers are not properly sanitized before return from system calls, potentially allowing some kernel data used in the system call to be exposed.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 07/09/2023
This vulnerability exists in FreeBSD kernel versions prior to specific stable and release patches, creating a critical information disclosure risk through improper handling of callee-save registers during system call execution. The flaw stems from inadequate sanitization of kernel data that remains in CPU registers when system calls return to user space, allowing potential leakage of sensitive kernel memory contents. The vulnerability affects both FreeBSD 11.x and 12.x release branches, with specific revision numbers indicating the exact patches that address the issue. This represents a fundamental breakdown in kernel security boundaries where data intended to remain within kernel space can be inadvertently exposed to user processes through register contents that should be cleared.
The technical implementation of this vulnerability occurs at the system call interface level where the kernel fails to properly clear or sanitize callee-save registers before returning control to user space. When system calls execute, certain CPU registers that are designated as callee-save (meaning they must be preserved by the called function) contain kernel data that should not be accessible to user processes. The improper sanitization means that when these registers are not explicitly cleared, their contents from previous kernel operations may persist and be readable by user-space applications. This creates a direct pathway for information disclosure attacks where malicious code can potentially extract kernel memory contents through register inspection or by analyzing patterns in the returned data.
The operational impact of this vulnerability is significant as it enables information disclosure attacks that could expose kernel memory contents to unprivileged users. Attackers could potentially extract sensitive kernel data including memory addresses, cryptographic keys, or other confidential information that should remain protected within kernel space. This vulnerability can be exploited across multiple FreeBSD versions simultaneously, making it particularly dangerous for systems running affected releases. The impact extends beyond simple information disclosure as the leaked data could potentially aid in more sophisticated attacks including privilege escalation or further exploitation of other vulnerabilities. The vulnerability aligns with CWE-248, which addresses exposure of exception information, and represents a form of information leak that could be leveraged in combination with other attack vectors.
Mitigation strategies for this vulnerability require immediate patching of affected FreeBSD systems to the specified stable and release versions that contain the necessary kernel fixes. System administrators should prioritize updating to FreeBSD 11.2-STABLE after r343782, 11.2-RELEASE-p9, 12.0-STABLE after r343781, or 12.0-RELEASE-p3. Additionally, organizations should implement monitoring for unusual system call patterns or register usage that might indicate exploitation attempts. The fix involves ensuring proper register sanitization during system call return paths, which aligns with ATT&CK technique T1059.001 for command and scripting interpreter usage, though in this case the focus is on preventing unauthorized data exposure rather than command execution. Organizations should also consider implementing kernel hardening measures including stack canaries and control flow integrity checks to further reduce the attack surface.