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From:   Jann Horn <jannh@google.com>
Date:   Tue, 13 Feb 2018 04:39:38 +0100
Message-ID: <CAG48ez1utN_vwHUwk=BU6zM4Wa_53TPu8rm9JuTtY-vGP0Shqw@mail.gmail.com>
Subject: Re: [kernel-hardening] [PATCH 4/6] Protectable Memory
To:     Kees Cook <keescook@chromium.org>
Cc:     Laura Abbott <labbott@redhat.com>,
        Igor Stoppa <igor.stoppa@huawei.com>,
        Boris Lukashev <blukashev@sempervictus.com>,
        Christopher Lameter <cl@linux.com>,
        Matthew Wilcox <willy@infradead.org>,
        Jerome Glisse <jglisse@redhat.com>,
        Michal Hocko <mhocko@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
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On Tue, Feb 13, 2018 at 2:25 AM, Kees Cook <keescook@chromium.org> wrote:
> On Mon, Feb 12, 2018 at 4:40 PM, Laura Abbott <labbott@redhat.com> wrote:
>> On 02/12/2018 03:27 PM, Kees Cook wrote:
>>>
>>> On Sun, Feb 4, 2018 at 7:05 AM, Igor Stoppa <igor.stoppa@huawei.com>
>>> wrote:
>>>>
>>>> On 04/02/18 00:29, Boris Lukashev wrote:
>>>>>
>>>>> On Sat, Feb 3, 2018 at 3:32 PM, Igor Stoppa <igor.stoppa@huawei.com>
>>>>> wrote:
>>>>
>>>>
>>>> [...]
>>>>
>>>>>> What you are suggesting, if I have understood it correctly, is that,
>>>>>> when the pool is protected, the addresses already given out, will
>>>>>> become
>>>>>> traps that get resolved through a lookup table that is built based on
>>>>>> the content of each allocation.
>>>>>>
>>>>>> That seems to generate a lot of overhead, not to mention the fact that
>>>>>> it might not play very well with the MMU.
>>>>>
>>>>>
>>>>> That is effectively what i'm suggesting - as a form of protection for
>>>>> consumers against direct reads of data which may have been corrupted
>>>>> by some irrelevant means. In the context of pmalloc, it would probably
>>>>> be a separate type of ro+verified pool
>>>>
>>>> ok, that seems more like an extension though.
>>>>
>>>> ATM I am having problems gaining traction to get even the basic merged
>>>> :-)
>>>>
>>>> I would consider this as a possibility for future work, unless it is
>>>> said that it's necessary for pmalloc to be accepted ...
>>>
>>>
>>> I would agree: let's get basic functionality in first. Both
>>> verification and the physmap part can be done separately, IMO.
>>
>>
>> Skipping over physmap leaves a pretty big area of exposure that could
>> be difficult to solve later. I appreciate this might block basic
>> functionality but I don't think we should just gloss over it without
>> at least some idea of what we would do.
>
> What's our exposure on physmap for other regions? e.g. things that are
> executable, or made read-only later (like __ro_after_init)?

I just checked on a system with a 4.9 kernel, and there seems to be no
physical memory that is mapped as writable in the init PGD and
executable elsewhere.

Ah, I think I missed something. At least on X86, set_memory_ro,
set_memory_rw, set_memory_nx and set_memory_x all use
change_page_attr_clear/change_page_attr_set, which use
change_page_attr_set_clr, which calls __change_page_attr_set_clr()
with a second parameter "checkalias" that is set to 1 unless the bit
being changed is the NX bit, and that parameter causes the invocation
of cpa_process_alias(), which will, for mapped ranges, also change the
attributes of physmap ranges. set_memory_ro() and so on are also used
by the module loading code.

But in the ARM64 code, I don't see anything similar. Does anyone with
a better understanding of ARM64 want to check whether I missed
something? Or maybe, with a recent kernel, check whether executable
module pages show up with a second writable mapping in the
"kernel_page_tables" file in debugfs?