linux-kernel - Re: [PATCH RFC v2] Convert struct pid count to refcount

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Message-ID: <20190626215041.GA234202@google.com>
Date:   Wed, 26 Jun 2019 17:50:41 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jann Horn <jannh@...gle.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Matthew Wilcox <willy@...radead.org>,
        Will Deacon <will.deacon@....com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Kees Cook <keescook@...omium.org>,
        kernel-team <kernel-team@...roid.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH RFC v2] Convert struct pid count to refcount_t

On Tue, Jun 25, 2019 at 09:34:07AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 24, 2019 at 09:10:15PM +0200, Jann Horn wrote:
> > That part of the documentation only talks about cases where you have a
> > control dependency on the return value of the refcount operation. But
> > refcount_inc() does not return a value, so this isn't relevant for
> > refcount_inc().
> > 
> > Also, AFAIU, the control dependency mentioned in the documentation has
> > to exist *in the caller* - it's just pointing out that if you write
> > code like the following, you have a control dependency between the
> > refcount operation and the write:
> > 
> >     if (refcount_inc_not_zero(&obj->refcount)) {
> >       WRITE_ONCE(obj->x, y);
> >     }
> > 
> > For more information on the details of this stuff, try reading the
> > section "CONTROL DEPENDENCIES" of Documentation/memory-barriers.txt.
> 
> IIRC the argument went as follows:
> 
>  - if you use refcount_inc(), you've already got a stable object and
>    have ACQUIRED it otherwise, typically through locking.
> 
>  - if you use refcount_inc_not_zero(), you have a semi stable object
>    (RCU), but you still need to ensure any changes to the object happen
>    after acquiring a reference, and this is where the control dependency
>    comes in as Jann already explained.
> 
> Specifically, it would be bad to allow STOREs to happen before we know
> the refcount isn't 0, as that would be a UaF.
> 
> Also see the comment in lib/refcount.c.
> 

Thanks a lot for the explanations and the pointers to the comment in
lib/refcount.c . It makes it really clearly.

Also, does this patch look good to you? If so and if ok with you, could you
Ack it? The patch is not really "RFC" but I still tagged it as such since I
wanted to have this discussion.

Thanks!

- Joel