Messages in this thread | | | Date | Wed, 5 Sep 2018 10:02:41 +0200 (CEST) | From | Jiri Kosina <> | Subject | Re: [PATCH v3 2/3] x86/speculation: apply IBPB more strictly to avoid cross-process data leak |
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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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