CVE-2018-1087 in Linux
Summary
by MITRE
kernel KVM before versions kernel 4.16, kernel 4.16-rc7, kernel 4.17-rc1, kernel 4.17-rc2 and kernel 4.17-rc3 is vulnerable to a flaw in the way the Linux kernel's KVM hypervisor handled exceptions delivered after a stack switch operation via Mov SS or Pop SS instructions. During the stack switch operation, the processor did not deliver interrupts and exceptions, rather they are delivered once the first instruction after the stack switch is executed. An unprivileged KVM guest user could use this flaw to crash the guest or, potentially, escalate their privileges in the guest.
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Analysis
by VulDB Data Team • 03/13/2023
The vulnerability identified as CVE-2018-1087 resides within the Linux kernel's KVM (Kernel-based Virtual Machine) hypervisor implementation, affecting versions prior to kernel 4.16. This flaw specifically manifests during stack switch operations executed through Mov SS or Pop SS instructions, creating a critical window where interrupt and exception handling becomes inconsistent. The vulnerability operates at the intersection of processor architecture behavior and virtualization software implementation, making it particularly dangerous in virtualized environments where guest operating systems rely on proper exception delivery mechanisms.
The technical flaw stems from how KVM handles exception delivery in the context of stack switching operations. When a processor executes Mov SS or Pop SS instructions, it temporarily switches the stack pointer and enters a state where interrupts and exceptions are not immediately delivered. Instead, these exceptions are queued and delivered only after the first instruction following the stack switch is executed. This timing behavior creates a race condition within the KVM implementation where guest operating systems can manipulate this sequence to trigger unexpected behavior. The vulnerability is classified under CWE-121 as a stack-based buffer overflow condition, though the actual exploitation involves more complex hypervisor-level manipulation rather than traditional buffer overflows.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation within the guest environment. An unprivileged user within a KVM guest can exploit this flaw to cause the guest operating system to crash, effectively creating a denial of service condition. More critically, the vulnerability could be leveraged for privilege escalation attacks, allowing guest users to gain elevated privileges within their virtual machine. This represents a significant security risk in multi-tenant virtualized environments where guest isolation is paramount, as it could enable one user to compromise the security boundaries of another user's guest instance. The attack vector requires the guest user to execute specific instructions that trigger the problematic stack switch sequence, making it a targeted exploit rather than a widespread vulnerability.
Mitigation strategies for CVE-2018-1087 focus primarily on kernel version updates, with patching being the most effective solution. System administrators should upgrade to kernel versions 4.16 or later where the vulnerability has been addressed through proper exception handling mechanisms during stack switch operations. Additionally, implementing virtualization-specific security measures such as disabling unnecessary guest capabilities and monitoring for suspicious instruction sequences can provide additional defense layers. The vulnerability aligns with ATT&CK technique T1055 which covers process injection and privilege escalation techniques, making it relevant to broader threat hunting and incident response activities. Organizations should also consider implementing hypervisor-level monitoring to detect anomalous stack switch operations that might indicate exploitation attempts. The fix implemented in kernel versions 4.16 ensures proper handling of interrupt and exception delivery during stack switch operations, resolving the timing discrepancy that enabled exploitation.