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

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Date:   Mon, 28 Jan 2019 12:46:11 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>,
        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>,
        Maged Michael <maged.michael@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        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 12:27:03PM -0800, Linus Torvalds wrote:
> On Mon, Jan 28, 2019 at 10:27 AM 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.
> 
> Hmm. I think this is right. You shouldn't access another threads mm
> pointer without proper locking.
> 
> That said, we *could* make the mm_cachep be SLAB_TYPESAFE_BY_RCU,
> which would allow speculatively reading data off the mm pointer under
> RCU. It might not be the *right* mm if somebody just did an exit, but
> for things like this it shouldn't matter.

That sounds much simpler and more effective than the contention-reduction
approach that I suggested.  ;-)

							Thanx, Paul

> But if this is the only case that might care, it sounds like just
> doing the proper locking is the right approach.
> 
>            Linus
>