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Date: Mon, 25 Mar 2019 01:42:07 +0300
From: Aleksey Cherepanov <>
Subject: can other user access secrets in GPU's memory?

I was playing around with OpenCL and it looks like memory is not zeroed
by default on GPUs, i.e. I can read old values from gpu on another run
of host program. So I guess it means that other users can read these
values too, right? Does john clean up memory after work to avoid this?
Or is it unusual to run john on a shared machine/gpu?

To be certain, I used code from the following tutorial:

I tried the code. Then I modified WorkSize[0] to 1 and got same values.
Then I modified + to * in the kernel and got the first value as with *
and other values as with +. So they were old and survived between runs.

I tried that on vega card on Openwall's super:


Aleksey Cherepanov

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