Re: [PATCHv2 0/4] perf/core: Add support to exclude kernel mode PMU tracing

Andi Kleen
2021-03-09T14:45:06Z

> The disk encryption is just one example and there might be others which
> we might not be aware of yet and we are not suspecting there is something
> wrong with the crypto code that needs to be fixed.

Then you don't have any leaks relating to branch tracing.

> restrict an external(in the sense that its not related to crypto or any
> other security related component) entity such as hardware assisted tracing
> like ARM coresight and so on. I don't see why or how the crypto code needs
> to be fixed for something that is not related to it although it is affected.

It's just a general property that if some code that is handling secrets
is data dependent it already leaks.

> The analogy would be like of the victims and a perpetrator. Lets take coresight
> as an example for perpetrator and crypto as the victim here. Now we can try

There's no victim with branch tracing, unless it is already leaky.

> If we just know one victim (lets say crypto code here), what happens to the
> others which we haven't identified yet? Do we just wait for someone to write
> an exploit based on this and then scramble to fix it?

For a useful security mitigation you need a threat model first I would say.

So you need to have at least some idea how an attack with branch
tracing would work.

> Initial change was to restrict this only to HW assisted instruction tracing [1]

I don't think it's needed for instruction tracing.

-Andi