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What happens inside Amazon when there's a Xen vulnerability

Brandon Butler | March 4, 2015
The company is notified of all the Xen vulnerabilities on a regular basis before they're made public, which allows the company to determine if the vulnerability is applicable to AWS and if so develop and install a patch.

Amazon Web Services last year was estimated by Gartner to be five times bigger than its next 14 competitors combined. That's a lot of virtual machines. And they all run on a customized version of the open source Xen hypervisor, so when the Xen code has a security vulnerability, that's a big deal for AWS.

In the past six months AWS has twice had to reboot some of its Elastic Compute Cloud (EC2) servers because of a Xen vulnerability. In September, 2014 about 10% of EC2 instances were rebooted and just this week AWS announced that about 0.1% of instances had to be rebooted to install a security patch. That may not sound like a lot, but at the scale AWS operates, it's still a large number.

What happens inside AWS when there's a Xen vulnerability discovered?

The answer is that Steve Schmidt gets busy (not that he isn't already). Schmidt is AWS's vice president of security engineering and chief information security officer (CISO) and he's a former FBI section chief. He's the man keeping AWS's cloud secure. In November, Network World sat down with Schmidt at the AWS re:Invent conference and asked him to walk us through what happened inside AWS's cloud operations during the big September reboot.

Verify the vulnerability

AWS is a big user of Xen code, so company officials are some of the first to hear about Xen vulnerabilities that are identified in the open source community. When that occurs the first job for Schmidt's team is to determine if it will impact AWS. The company is notified of all the Xen vulnerabilities on a regular basis before they're made public. This allows the company to determine if the vulnerability is applicable to AWS and if so develop and install a patch.

"Xen is a huge software package and there are many aspects that AWS does not use," Schmidt said.

Most of the Xen vulnerabilities do not apply to AWS because the company has developed its own custom version of Xen. AWS has stripped out all the features of Xen that it doesn't need, both in order to customize the performance of the open source code to the company's unique use case, and to limit its exposure to vulnerabilities.

But AWS does something else, too: It doesn't just use one flavor of Xen, it uses many.

"We intentionally build our fleet differently across (the service)," he said. "You don't want everything to be homogeneous because if it is then if a problem effects the fleet, it effects everything." AWS has different custom versions of Xen deployed across different services and regions, and none of them are the vanilla open source code.

 

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