Re: [PATCH v2 2/3] x86/speculation: Apply IBPB more strictly to avoid cross-process spectre v2 leak

[Peter Zijlstra]

On Mon, Sep 03, 2018 at 02:45:25PM +0200, Jiri Kosina wrote:
>  		if (tsk && tsk->mm &&
>  		    tsk->mm->context.ctx_id != last_ctx_id &&
> -		    get_dumpable(tsk->mm) != SUID_DUMP_USER)
> +		    ___ptrace_may_access(tsk, current, PTRACE_MODE_IBPB))
>  			indirect_branch_prediction_barrier();

So the question posed here is: can @next ptrace @prev?

Which confuses me greatly; it is @prev doing the BTB poisoning for
@next, so we want to know if @prev can ptrace @next, no?