Wandering Thoughts archives


A fun problem: monitoring randomness reduces it

The Linux kernel exports a /proc entry that reports on how many bits of entropy are in the kernel's randomness pool, /proc/sys/kernel/random/entropy_avail. Recently, Matt Simmons noticed his server monitoring looking at this and noticed that when he checked the /proc entry by hand the reported entropy went down. It turns out that this was not random fluctuations and the explanation for it is rather interesting, as pointed out by @rejectreality on Twitter.

As a security measure, modern versions of Linux randomize various parts of a process's memory space in various ways; this goes by the general name of ASLR (address space layout randomization) and especially affects the stack. In order for this randomness to be really effective it should be unpredictable, which means that it can't be based on any of the usual simple seeds for pseudo-random generators like the time of day the process started or its PID. Instead it turns out that the kernel exports some randomness to user space processes when it execs them.

You can see where this is going. This exported randomness comes from the kernel's general random number infrastructure and so it winds up depleting some of the entropy from the entropy pool. So if you run a program to read out the value of entropy_avail, the very act of doing so will deplete some of that entropy. If you repeatedly run the program you'll repeatedly lower the entropy (although repeatedly reading the entropy value from within one program won't do it). Of course this isn't unique to looking at entropy_avail; running pretty much any program will deplete some entropy no matter what it does.

(It turns out that this has apparently led to some problems. Indeed most of our active servers turn out to have very low entropy_avail values. Hopefully people aren't doing much that required strong randomness.)

linux/MonitoringRandomnessProblem written at 00:24:24; Add Comment

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