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

Peter Zijlstra
2018-09-05T07:59:22Z

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.