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SubjectRe: [PATCH v3 2/3] x86/speculation: apply IBPB more strictly to avoid cross-process data leak
On Wed, 5 Sep 2018, Peter Zijlstra wrote:

> 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.

Yeah, I did more or less that earlier today; my series currently has

+static bool ibpb_needed(struct task_struct *tsk, u64 last_ctx_id)
+{
+ return (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id &&
+ __ptrace_may_access(tsk, PTRACE_MODE_IBPB));
+}
+
[ ... ]
- if (tsk && tsk->mm &&
- tsk->mm->context.ctx_id != last_ctx_id &&
- get_dumpable(tsk->mm) != SUID_DUMP_USER)
+ if (static_cpu_has(X86_FEATURE_USE_IBPB) &&
+ ibpb_needed(tsk, last_ctx_id))
indirect_branch_prediction_barrier();

Thanks,

--
Jiri Kosina
SUSE Labs

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