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  #11  
Old 12-01-18, 12:22 AM
NeverBackDown NeverBackDown is offline
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Originally Posted by SPS View Post
Misprediction happens all the time.
They have an extremely high accuracy and if it does occur they have methods to make it so the already executed instruction isn't entirely wasted. Which is one of the issues with the malware in question.

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  #12  
Old 12-01-18, 09:43 AM
SPS SPS is offline
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Originally Posted by NeverBackDown View Post
They have an extremely high accuracy and if it does occur they have methods to make it so the already executed instruction isn't entirely wasted. Which is one of the issues with the malware in question.
All they can do is use the history from the previous execution of that code. When a program first boots the branch prediction will have no accuracy. And your statement isn't really correct. If the prediction is incorrect they must go back and take the other branch, simple as that. The spectre bug is about forcing a misprediction so that it speculatively executes malicious code reading outside of the program's own memory that would have normally crashed if the prediction was correct. When it does this the unauthorised memory read will pull that data into cache, however, when the CPU rolls back as it predicted wrong, it doesn't clear the cache. This means they can then read back a bit they wrote to determine if it is in the cache or not, if it is they can read a byte successfully from another program's address space. If you keep repeating this procedure you can effectively read another program's memory for passwords etc or a ROP style attack. Luckily this is quite hard to do out of a testable environment where all variables are known. I'd highly recommend reading the spectre paper.
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  #13  
Old 12-01-18, 01:42 PM
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