Intel patches Downfall bug, but slows CPUs in the process

Intel has patched a potentially serious vulnerability that affects many of its CPUs, but the chip giant admits the fix can have a major impact on performance.

The vulnerability, disclosed this week, has been given the name 'Downfall'. Discovered by researcher Daniel Moghimi, the Downfall vulnerability is a side-channel attack related to the way Intel CPUs access system memory.

Moghimi found that dating back to the 2014 product line, Intel chips have been unintentionally allowing local applications to view the system's internal hardware registers when performing speculative execution.

Normally, this data would be locked off by things like secure enclaves. Moghimi says this vulnerability allows an attacker to end-run those productions and infer memory contents.

"This allows untrusted software to access data stored by other programs, which should not normally be accessible," Moghimi explains.

"I discovered that the Gather instruction, meant to speed up accessing scattered data in memory, leaks the content of the internal vector register file during speculative execution."

In practice, this would mean an attacker who had local access to a machine could potentially be able to view other users' encryption keys or passwords, or potentially even stored data such as email contents.

While this might not be a particularly massive risk for individual PCs or workstations, it becomes a colossal vulnerability for servers, including those hosting cloud applications. It would mean that anyone who is able to run code on that system would potentially be able to harvest credentials for dozens if not hundreds of other users.

Fortunately, Moghimi says he has been working with Intel since last year to get the bug properly fixed. Earlier this week, the chipmaker issued to microcode update to address the flaw so most of the vulnerable chips will be protected.

Intel has posted a list of all CPUs affected by the flaw and their associated microcode update.

Unfortunately, that fix will come at a cost. According to Intel's own documentation, getting the side-channel bug cleaned up will mean a performance that, in some cases, will be significant.

"When the mitigation is enabled, there is additional latency before results of the gather load can be consumed," Intel said in a technical document.

"Although the performance impact to most workloads is minimal, specific workloads may show performance impacts of up to 50 percent."

While the microcode update can be turned off, it is not recommended due to the seriousness of the vulnerability.

This isn't anything unusual when it comes to addressing these sort of speculative execution vulnerabilities. The microcode fixes for the Spectre and Meltdown bugs, for example, similarly caused performance hits.