CVE-2018-8897 in macOS
Summary
by MITRE
A statement in the System Programming Guide of the Intel 64 and IA-32 Architectures Software Developer's Manual (SDM) was mishandled in the development of some or all operating-system kernels, resulting in unexpected behavior for #DB exceptions that are deferred by MOV SS or POP SS, as demonstrated by (for example) privilege escalation in Windows, macOS, some Xen configurations, or FreeBSD, or a Linux kernel crash. The MOV to SS and POP SS instructions inhibit interrupts (including NMIs), data breakpoints, and single step trap exceptions until the instruction boundary following the next instruction (SDM Vol. 3A; section 6.8.3). (The inhibited data breakpoints are those on memory accessed by the MOV to SS or POP to SS instruction itself.) Note that debug exceptions are not inhibited by the interrupt enable (EFLAGS.IF) system flag (SDM Vol. 3A; section 2.3). If the instruction following the MOV to SS or POP to SS instruction is an instruction like SYSCALL, SYSENTER, INT 3, etc. that transfers control to the operating system at CPL < 3, the debug exception is delivered after the transfer to CPL < 3 is complete. OS kernels may not expect this order of events and may therefore experience unexpected behavior when it occurs.
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Analysis
by VulDB Data Team • 02/01/2025
The vulnerability described in CVE-2018-8897 stems from a misinterpretation of the Intel Software Developer's Manual regarding exception handling behavior during specific processor instructions. This flaw affects the fundamental operation of processor exception delivery mechanisms when dealing with MOV SS and POP SS instructions, which are critical for privilege level transitions in operating system kernels. The issue manifests when these instructions defer #DB (debug) exceptions, creating unexpected timing and delivery behavior that operating system developers may not have anticipated during kernel implementation. The vulnerability specifically targets the interaction between processor interrupt inhibition mechanisms and operating system exception handling routines, particularly in systems that rely on precise control flow during privilege transitions.
The technical core of this vulnerability lies in the misapplication of Section 6.8.3 of the Intel Software Developer's Manual, which describes how MOV SS and POP SS instructions inhibit interrupts including NMIs, data breakpoints, and single step trap exceptions until the instruction boundary following the next instruction. However, the manual also specifies that debug exceptions are not inhibited by the interrupt enable flag (EFLAGS.IF), creating a complex interaction scenario. When an instruction following MOV SS or POP SS causes a privilege transition to CPL < 3, such as SYSCALL, SYSENTER, or INT 3, the debug exception delivery occurs after the privilege transition completes rather than during the instruction execution. This timing discrepancy creates a race condition where kernel code may not properly handle the delayed exception delivery, leading to potential privilege escalation or system crashes.
The operational impact of CVE-2018-8897 spans multiple operating systems and hypervisor configurations, including Windows, macOS, FreeBSD, Linux, and Xen. The vulnerability enables privilege escalation attacks because kernel code may not properly account for the delayed delivery of debug exceptions, potentially allowing malicious code to exploit the timing mismatch to gain elevated privileges. Additionally, the vulnerability can cause system crashes in Linux kernels due to improper handling of the unexpected exception delivery sequence. This affects systems that depend on precise control flow during privilege transitions, particularly those implementing security features that rely on debug exception handling for privilege enforcement. The vulnerability is particularly dangerous because it operates at the intersection of processor architecture and operating system kernel implementation, making it difficult to detect and remediate.
Mitigation strategies for CVE-2018-8897 require careful kernel code review and modification to properly handle the delayed debug exception delivery behavior. Operating system vendors should implement defensive programming practices that account for the timing differences between exception delivery and privilege transitions, particularly in code that handles MOV SS and POP SS instructions. The fix typically involves modifying kernel exception handling routines to properly account for the possibility that debug exceptions may be delivered after privilege transitions, rather than during the instruction execution. This vulnerability aligns with CWE-122, which describes improper restriction of operations within a recognized security boundary, and maps to ATT&CK technique T1068, which covers privilege escalation through exploitation of system vulnerabilities. Organizations should prioritize patching affected systems, as the vulnerability represents a fundamental flaw in how operating system kernels handle processor exception timing, making it a critical security concern for all systems running affected software versions.