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

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Message-ID: <20180905094012.GT24082@hirez.programming.kicks-ass.net>
Date:   Wed, 5 Sep 2018 11:40:12 +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>,
        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 Wed, Sep 05, 2018 at 10:02:41AM +0200, Jiri Kosina wrote:
> Yeah, I did more or less that earlier today; my series currently has

Excellent, maybe add a wee comment like so?

> +static bool ibpb_needed(struct task_struct *tsk, u64 last_ctx_id)
> +{
	/*
	 * Check if the current (previous) task has access to the memory
	 * of the @tsk (next) task. If access is denied, make sure to
	 * issue a IBPB to stop user->user Spectre-v2 attacks.
	 *
	 * Note: __ptrace_may_access() returns 0 or -ERRNO.
	 */
> +       return (tsk && tsk->mm && tsk->mm->context.ctx_id != last_ctx_id &&
> +                       __ptrace_may_access(tsk, PTRACE_MODE_IBPB));
> +}