Zack's Kernel News

Zack's Kernel News

Article from Issue 262/2022
Author(s):

Chronicler Zack Brown reports on the latest news, views, dilemmas, and developments within the Linux kernel community.

Random Number Sanity

Random numbers are important for security. Generally to make random numbers, you grab entropy from somewhere, like the frequency of fingers tapping a keyboard, and use that to generate as many unpredictable numbers as needed. But what if no one's typing on the keyboard? What if you run out of entropy? Should the system just sit and wait for more?

For a long time, the Linux kernel had to choose between locking up the system until it found enough entropy to make truly random numbers and providing numbers anyway, even if they weren't really random enough.

But in 2019, Linus Torvalds wrote a patch that addressed the problem "by actively generating entropy noise using the CPU cycle counter when waiting for the random number generator to initialize. This only works when you have a high-frequency time stamp counter available, but that's the case on all modern x86 CPUs, and on most other modern CPUs too."

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