Re: [PATCH 30/35] x86/speculation: Use Indirect Branch Prediction Barrier in context switch

[Kevin Easton]

On Thu, Jan 18, 2018 at 04:38:32PM -0800, Tim Chen wrote:
> On 01/18/2018 05:48 AM, Peter Zijlstra wrote:
> >
> >+		/*
> >+		 * Avoid user/user BTB poisoning by flushing the branch predictor
> >+		 * when switching between processes. This stops one process from
> >+		 * doing spectre-v2 attacks on another process's data.
> >+		 */
> >+		indirect_branch_prediction_barrier();
> >+
> 
> Some optimizations can be done here to avoid overhead in barrier call.
> 
> For example, don't do the barrier if prev and next mm are
> the same.  If the two process trust each other, or the new process
> already have rights to look into the previous process,
> the barrier could be skipped.

Isn't it the other way around with the BTB poisoning? previous is
potentially attacking next, so the barrier can be avoided only if previous
is allowed to ptrace next?

    - Kevin