linux-kernel - Re: [RFC PATCH v16 0/6] mm: security: ro protection for dynamic data

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Message-ID: <20180221213629.GF3728@rh>
Date:   Thu, 22 Feb 2018 08:36:29 +1100
From:   Dave Chinner <dchinner@...hat.com>
To:     Igor Stoppa <igor.stoppa@...wei.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Jonathan Corbet <corbet@....net>,
        Michal Hocko <mhocko@...nel.org>,
        Laura Abbott <labbott@...hat.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christoph Lameter <cl@...ux.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC PATCH v16 0/6] mm: security: ro protection for dynamic data

On Wed, Feb 21, 2018 at 11:56:22AM +0200, Igor Stoppa wrote:
> On 21/02/18 03:36, Dave Chinner wrote:
> > On Tue, Feb 20, 2018 at 03:56:00PM -0800, Matthew Wilcox wrote:
> >> On Wed, Feb 21, 2018 at 08:36:04AM +1100, Dave Chinner wrote:
> >>> FWIW, I'm not wanting to use it to replace static variables. All the
> >>> structures are dynamically allocated right now, and get assigned to
> >>> other dynamically allocated pointers. I'd likely split the current
> >>> structures into a "ro after init" 
> 
> I would prefer to use a different terminology, because, if I have
> understood the use case, this is not exactly the same as __ro_after_init

I want a dynamically allocated "write once" structure.

A "write once" structure is, conceptually, is exactly the same as
"ro after init". Implementation wise, it may be different to
"__ro_after_init", especially when compared to static/global
variables.

It seems lots of people get confused when discussing concepts vs
implementation... :)

> >>> 	......
> >>
> >> No, you'd do:
> >>
> >> struct xfs_mount_ro {
> >> 	[...]
> >> };
> 
> is this something that is readonly from the beginning and then shared
> among mount points or is it specific to each mount point?

It's dynamically allocated for each mount point, made read-only
before the mount completes and lives for the length of the mount
point.

> >> struct xfs_mount {
> >> 	const struct xfs_mount_ro *ro;
> >> 	[...]
> >> };
> > 
> > .... so that's pretty much the same thing :P
> 
> The "const" modifier is a nice way to catch errors through the compiler,
> iff the ro data will not be initialized through this handle, when it's
> still writable.

That's kinda implied by the const, isn't it? If we don't do it that
way, then the compiler will throw errors....

Cheers,

Dave.
-- 
Dave Chinner
dchinner@...hat.com