linux-kernel - Re: [PATCH 13/17] prmem: linked list: disable layout randomization

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Date:   Tue, 30 Oct 2018 16:39:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Igor Stoppa <igor.stoppa@...il.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        kernel-hardening@...ts.openwall.com,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, igor.stoppa@...wei.com,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Pekka Enberg <penberg@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Lihao Liang <lianglihao@...wei.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/17] prmem: linked list: disable layout randomization

On Fri, Oct 26, 2018 at 03:17:07AM -0700, Matthew Wilcox wrote:
> On Fri, Oct 26, 2018 at 11:32:05AM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 24, 2018 at 12:35:00AM +0300, Igor Stoppa wrote:
> > > Some of the data structures used in list management are composed by two
> > > pointers. Since the kernel is now configured by default to randomize the
> > > layout of data structures soleley composed by pointers, this might
> > > prevent correct type punning between these structures and their write
> > > rare counterpart.
> > 
> > 'might' doesn't really work for me. Either it does or it does not.
> 
> He means "Depending on the random number generator, the two pointers
> might be AB or BA.  If they're of opposite polarity (50% of the time),
> it _will_ break, and 50% of the time it _won't_ break."

So don't do that then. If he were to include struct list_head inside his
prlist_head, then there is only the one randomization and things will
just work.

Also, I really don't see why he needs that second type and all that type
punning crap in the first place.