Sneak peek

In the past few days the tech community has gone into a panic over a discovery that computers have been vulnerable to a specific kind of attack for over 20 years. Despite being present for a very long time, it would seem that nobody has exploited the vulnerability. The details are complicated, but let’s consider a part of their discovery in more simple terms:

The problem is in the processor (CPU), the thing that does calculations using information in the computer’s main memory (RAM). Decades ago, CPU designers from companies like Intel, AMD and others, decided that they could speed up a computer if they could get it to do some calculations ahead of time, even if the results of those calculations were eventually ignored.

Imagine you are travelling along a road looking for a particular house and you are making note of the houses you have passed, when you come upon a fork in the road. You know that the house you are looking for is down one of these two choices but which do you pick? Suppose you go left and reach the end of the road without getting to your destination, then you know you made the wrong choice, have to backtrack and go to the right instead. The same could be true if you went right first. But if you could walk down both roads at the same time, you would find your destination in the fastest time and could pretend that you hadn’t walked down that other road at all.

The CPU does something similar when it gets to a decision point. Go left, or go right? Actually, it proceeds down both possibilities, and when it figures out which one was the correct path it just ignores anything it was doing in the other path.

Where’s this going? Well suppose the CPUs paths were Good and Evil. In the good path it doesn’t do anything it shouldn’t be doing, but in the evil path it attempts to perform a calculation using some data in a place in the RAM where the program is not allowed to see. We also arrange it so that even though there are two paths, only the good path will eventually be chosen. You could consider the activity of the CPU in the evil path to be like a ghost that should, in theory, have no impact on the real world.

Except it does have an impact. The CPU during its journey down the evil path was attempting to read memory from somewhere that it should not access, and during that activity it temporarily made a note of the supposedly inaccessible data that it found. The clever evil code then used that knowledge to read a value from one of two possible places in an accessible (permitted) location. We will call these places Hot and Cold. So, as the CPU was going down the evil path it used knowledge about some off-limits memory to decide whether to then look at Hot or Cold. And then, because the good path finally figured out that it was the one that should be chosen, the work that took place in the evil path is discarded.


The fact that either Hot or Cold was accessed by the now-dead evil path means that the CPU has now temporarily loaded either Hot or Cold into its cache (a small place where it keeps copies of information it thinks it might need in the immediate future). That means that if the good path proceeds to check how long it takes to read Hot and Cold, whichever one it can read fastest must be the one that had been selected by the evil path. In this way, the good path can get some details from the ghost of the evil path.

So now, even though the evil path is always discarded, we can learn something about what it saw in the off-limits memory. There’s a good reason why some memory is off-limits to ordinary programs: that’s where important and sensitive information is kept, such as the keys and passwords to all your most valuable digital assets.

The researchers at Google were able to craft some code with the Good/Evil paths that could be used to slurp inaccessible memory at the rate of 2000 bytes per second. It wouldn’t take long for such a program to discover everything it needed to compromise your computer. No memory is off-limits to such a program. Woe is us!

I have massively simplified the details of this problem. The research work is far more involved than the narrative above. Nevertheless, at the core (no pun intended) it’s quite a simple hack.

Which makes me sceptical about the claim that it has not been exploited in the two decades that CPUs have been doing “speculative look-ahead processing”.

We await work-arounds at the software level that will mitigate these problems, but probably at a cost of slowing down our computers. Unfortunately, unlike software updates, you can’t change how your CPU is hardwired. You need a new CPU. Wait until the next generation of chips are in the market before buying a new computer.

Meanwhile, be prepared to watch your computer slow down after the next security patch is installed.

Categorised as: Hardware, Security

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