linux-kernel - Re: [PATCH v2 2/3] x86/speculation: Apply IBPB more strictly to avoid cross-process spectre v2 leak

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Message-ID: <20180903134751.GV24124@hirez.programming.kicks-ass.net>
Date:   Mon, 3 Sep 2018 15:47:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v2 2/3] x86/speculation: Apply IBPB more strictly to
 avoid cross-process spectre v2 leak

On Mon, Sep 03, 2018 at 02:45:25PM +0200, 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(tsk, current, PTRACE_MODE_IBPB))
>  			indirect_branch_prediction_barrier();

So the question posed here is: can @next ptrace @prev?

Which confuses me greatly; it is @prev doing the BTB poisoning for
@next, so we want to know if @prev can ptrace @next, no?