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

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Date:   Tue, 4 Sep 2018 18:18:55 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jiri Kosina <jikos@...nel.org>
cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v3 2/3] x86/speculation: apply IBPB more strictly to
 avoid cross-process data leak

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