linux-kernel - Re: [RFC PATCH] Fix: membarrier: racy access to p->mm in membarrier_global

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Date:   Mon, 28 Jan 2019 22:33:47 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrea Parri <parri.andrea@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, David Sehr <sehr@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Maged Michael <maged.michael@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will.deacon@....com>, stable@...r.kernel.org
Subject: Re: [RFC PATCH] Fix: membarrier: racy access to p->mm in membarrier_global_expedited()

On Mon, Jan 28, 2019 at 7:27 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
> Jann Horn identified a racy access to p->mm in the global expedited
> command of the membarrier system call.
>
> The suggested fix is to hold the task_lock() around the accesses to
> p->mm and to the mm_struct membarrier_state field to guarantee the
> existence of the mm_struct.
>
> Link: https://lore.kernel.org/lkml/CAG48ez2G8ctF8dHS42TF37pThfr3y0RNOOYTmxvACm4u8Yu3cw@mail.gmail.com
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

The patch looks good to me, and to be sure, I've also given it a spin
- I can't trigger a splat anymore. You can add:

Tested-by: Jann Horn <jannh@...gle.com>