Misinterpretation of Intel Docs Leads to Flaw in Hypervisors, OSs

The developers of several major operating systems and hypervisors misinterpreted documentation from Intel and introduced a potentially serious vulnerability to their products.

According to an advisory published on Tuesday by CERT/CC, the flaw impacts the Linux kernel and software made by Apple, the DragonFly BSD Project, Red Hat, the FreeBSD Project, Microsoft, SUSE Linux, Canonical, VMware, and the Xen Project. CERT/CC also provides a long list of other companies whose products may be affected.

The vulnerability, tracked as CVE-2018-8897, exists due to the way operating systems and hypervisors handle MOV/POP to SS instructions. Exploitation requires local access to the targeted system.

Impact varies depending on the affected software. In the case of Linux, it can lead to a crash of the kernel and a denial-of-service (DoS) condition. Microsoft says an attacker can exploit the security hole on Windows for privilege escalation. The Xen Project says a malicious PV guest can escalate privileges to the ones of the hypervisor, while CERT/CC warns that an attacker can “read sensitive data in memory or control low-level operating system functions.”

Patches have been released for the Linux kernel, Windows, Xen and various Linux distributions, but in most cases the issue has been classified only as “moderate” or “important.” Proof-of-concept (PoC) exploits have been created for both Windows and Linux.

The researchers who discovered the vulnerability, Nick Peterson of Everdox Tech and Nemanja Mulasmajic of triplefault.io, say it impacts both Intel and AMD hardware. A paper published by the experts provides technical details.

According to CERT/CC, the problem appears to exist due to developers misinterpreting existing documentation.

“The 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,” CERT/CC wrote in its advisory.

“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 Current Privilege Level (CPL) < 3, a debug exception is delivered after the transfer to CPL < 3 is complete. Such deferred #DB exceptions by MOV SS and POP SS may result in unexpected behavior.

“Therefore, in certain circumstances after the use of certain Intel x86-64 architecture instructions, a debug exception pointing to data in a lower ring (for most operating systems, the kernel Ring 0 level) is made available to operating system components running in Ring 3. This may allow an attacker to utilize operating system APIs to gain access to sensitive memory information or control low-level operating system functions,” CERT/CC added.

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