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Subject: Re: [RFC PATCH v16 0/6] mm: security: ro protection for dynamic data
To:     Dave Chinner <dchinner@redhat.com>,
        Kees Cook <keescook@chromium.org>
CC:     Matthew Wilcox <willy@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Michal Hocko <mhocko@kernel.org>,
        Laura Abbott <labbott@redhat.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Christoph Lameter" <cl@linux.com>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
References: <20180212165301.17933-1-igor.stoppa@huawei.com>
 <CAGXu5j+ZNFX17Vxd37rPnkahFepFn77Fi9zEy+OL8nNd_2bjqQ@mail.gmail.com>
 <20180220012111.GC3728@rh>
From:   Igor Stoppa <igor.stoppa@huawei.com>
Message-ID: <24e65dec-f452-a444-4382-d1f88fbb334c@huawei.com>
Date:   Tue, 20 Feb 2018 20:03:49 +0200
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On 20/02/18 03:21, Dave Chinner wrote:
> On Mon, Feb 12, 2018 at 03:32:36PM -0800, Kees Cook wrote:
>> On Mon, Feb 12, 2018 at 8:52 AM, Igor Stoppa <igor.stoppa@huawei.com> wrote:
>>> This patch-set introduces the possibility of protecting memory that has
>>> been allocated dynamically.
>>>
>>> The memory is managed in pools: when a memory pool is turned into R/O,
>>> all the memory that is part of it, will become R/O.
>>>
>>> A R/O pool can be destroyed, to recover its memory, but it cannot be
>>> turned back into R/W mode.
>>>
>>> This is intentional. This feature is meant for data that doesn't need
>>> further modifications after initialization.
>>
>> This series came up in discussions with Dave Chinner (and Matthew
>> Wilcox, already part of the discussion, and others) at LCA. I wonder
>> if XFS would make a good initial user of this, as it could allocate
>> all the function pointers and other const information about a
>> superblock in pmalloc(), keeping it separate from the R/W portions?
>> Could other filesystems do similar things?
> 
> I wasn't cc'd on this patchset, (please use david@fromorbit.com for
> future postings) 

Apologies, somehow I didn't realize that I should have put you too in
CC. It will be fixed at the next iteration.

> so I can't really say anything about it right
> now. My interest for XFS was that we have a fair amount of static
> data in XFS that we set up at mount time and it never gets modified
> after that.

This is the typical use case I had in mind, although it requires a
conversion.
Ex:

before:

static int a;


void set_a(void)
{
	a = 4;
}


after:

static int *a __ro_after_init;
struct gen_pool *pool;

void init_a(void)
{
	pool = pmalloc_create_pool("pool", 0);
	a = (int *)pmalloc(pool, sizeof(int), GFP_KERNEL);
}

void set_a(void)
{
	*a = 4;
	pmalloc_protect_pool(pool);
}

> I'm not so worried about VFS level objects (that's a
> much more complex issue) but there is a lot of low hanging fruit in
> the XFS structures we could convert to write-once structures.

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.

--
igor