Intel has disclosed another speculative execution side-channel vulnerability affecting many of its modern microprocessors that gives attackers a way to steal data, including cryptographic secrets, from computers running the flawed chips.

The problem exists in the way the microprocessors handle a technique called Lazy FP state restore for saving and restoring an application's state. As Intel describes it, system software running on Intel Core-based microprocessors may use the Lazy FP state restore technique to delay the restoring of an application's state in memory until when it is actually needed. Microsoft Windows, for instance, enables Lazy restore by default and does not allow the feature to be disabled.

The Lazy restore vulnerability gives attackers a way to use a local process to infer data stored in floating-point registers about other processes via speculative execution. The information that an attacker might be able to snag by exploiting the Lazy restore vulnerability depends on the code executing on the system and could include encryption secrets.

"An attacker, via a local process, could cause information stored in FP (Floating Point), MMX, and SSE register state to be disclosed across security boundaries on Intel Core family CPUs through speculative execution," Microsoft said in its advisory on the flaw this week.

In order to exploit the vulnerability, an attacker would need to already be able to execute code locally on a system, just as with any side channel speculation execution flaws, Microsoft said.

Speculative execution is a technique many modern microprocessors use to optimize performance by executing instructions before they are actually needed so there is no delay in case the instructions are indeed required. According to Intel, "Lazy restored states are potentially vulnerable to exploits where one process may infer register values of other processes through a speculative execution side channel that infers their value."

The Lazy FP State Restore vulnerability (CVE-2018-3665) is similar to the Spectre and Meltdown vulnerabilities in Intel hardware in that it involves speculative execution side channels. However, it is considerably less severe in impact with Intel giving it a "moderate" severity rating and like Microsoft calling it "important."

"Red Hat Product Security rates this issue as moderate," says Jon Masters, computer architect at Red Hat. In other words, the flaw can be difficult to exploit but could result in the compromise of data confidentiality and integrity or resource availability in some circumstances. Administrators, however, need to still make sure the issue is addressed expeditiously as part of their normal patching and maintenance processes, he says. "It's relatively difficult to exploit but we do expect to see public proof-of-concepts soon."

The mitigation for Linux, especially Red Hat Enterprise Linux "is to force the operating system to default to [an] 'eager floating point register restore," Masters says. The mitigation works for all Intel Sandy Bridge and newer processors. "For older processors, the kernel can be booted with the 'eagerfpu=on' parameter to enable the same functionality," Masters says. "Neither of these mitigations should negatively affect performance."

The mitigations will likely vary by operating system, though, for most versions of Linux, the measures that Red Hat has adopted should work. "Other operating systems likely have similar commands and mitigations in place with the same type of process."

In its advisory, Microsoft noted that updates addressing the issues are forthcoming. But the company has not provided any schedule for when those updates might become available. Microsoft has not identified any workarounds for the issue or any mitigation for it, the company noted.

Other vendors including hypervisor vendors are working on fixes for the issue and security researchers are being asked to refrain from publishing weaponizable exploits till those fixes become available, Masters says.

Currently, there appears to be no exploits available in the wild for the lazy restore flaw, but it is only a matter of time before one becomes available.

"As with all security vulnerabilities, it's up to specific organizations to assess their risk level with any vulnerability. While we rate this as a moderate flaw, it's not something that should be ignored," Masters says.

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