Re: [PATCH v3 2/3] x86/speculation: apply IBPB more strictly to avoid cross-process data leak

[Peter Zijlstra]

On Tue, Sep 04, 2018 at 06:18:55PM +0200, Thomas Gleixner wrote:
> On Tue, 4 Sep 2018, 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(current, tsk, PTRACE_MODE_IBPB))
> 
> Uurgh. If X86_FEATURE_USE_IBPB is not enabled, then the whole
> __ptrace_may_access() overhead is just done for nothing.
> 
> >  			indirect_branch_prediction_barrier();
> 
> This really wants to be runtime patched:
> 
> 		if (static_cpu_has(X86_FEATURE_USE_IBPB))
> 			stop_speculation(tsk, last_ctx_id);
> 
> and have an inline for that:
> 
> static inline void stop_speculation(struct task_struct *tsk, u64 last_ctx_id)
> {
> 	if (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id &&
> 		___ptrace_may_access(current, tsk, PTRACE_MODE_IBPB))
> 		indirect_branch_prediction_barrier();
> }
> 
> which also makes the whole mess readable.

How about something like:

	if (static_cpu_has(X86_FEATURE_USE_IBPB) && need_ibpb(tsk, last_ctx_id))
		indirect_branch_predictor_barrier();

where:

static inline bool need_ibpb(struct task_struct *next, u64 last_ctx_id)
{
	return next && next->mm && next->mm->context.ctx_id != last_ctx_id &&
		__ptrace_may_access(next, PTRACE_MODE_IBPB));
}

I don't much like "stop_speculation" for a name here.