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

Thomas Gleixner
2018-09-04T16:19:07Z

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.

Hmm?

Thanks,

tglx