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

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Message-ID: <20180220235600.GA3706@bombadil.infradead.org>
Date:   Tue, 20 Feb 2018 15:56:00 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <dchinner@...hat.com>
Cc:     Igor Stoppa <igor.stoppa@...wei.com>,
        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 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" structure and rw structure, so
> how does the "__ro_after_init" attribute work in that case? Is it
> something like this?
> 
> struct xfs_mount {
> 	struct xfs_mount_ro{
> 		.......
> 	} *ro __ro_after_init;
> 	......

No, you'd do:

struct xfs_mount_ro {
	[...]
};

struct xfs_mount {
	const struct xfs_mount_ro *ro;
	[...]
};

We can't do protection on less than a page boundary, so you can't embed
a ro struct inside a rw struct.

> Also, what compile time checks are in place to catch writes to
> ro structure members? Is sparse going to be able to check this sort
> of thing, like is does with endian-specific variables?

Just labelling the pointer const should be enough for the compiler to
catch unintended writes.

> > I'd be interested to have your review of the pmalloc API, if you think
> > something is missing, once I send out the next revision.
> 
> I'll look at it in more depth when it comes past again. :P

I think the key question is whether you want a slab-style interface
or whether you want a kmalloc-style interface.  I'd been assuming
the former, but Igor has implemented the latter already.