linux-kernel - Re: [PATCH v2 0/3] initramfs: add support for xattrs in the initial ram disk

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Date:   Tue, 14 May 2019 19:20:15 +0200
From:   Roberto Sassu <roberto.sassu@...wei.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     Andy Lutomirski <luto@...nel.org>, Rob Landley <rob@...dley.net>,
        "Arvind Sankar" <niveditas98@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Linux API" <linux-api@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        <initramfs@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] initramfs: add support for xattrs in the initial
 ram disk

On 5/14/2019 6:58 PM, Greg KH wrote:
> On Tue, May 14, 2019 at 06:33:29PM +0200, Roberto Sassu wrote:
>> On 5/14/2019 5:19 PM, Andy Lutomirski wrote:
>>> On Mon, May 13, 2019 at 5:47 AM Roberto Sassu <roberto.sassu@...wei.com> wrote:
>>>>
>>>> On 5/13/2019 11:07 AM, Rob Landley wrote:
>>>>>
>>>>>
>>>>> On 5/13/19 2:49 AM, Roberto Sassu wrote:
>>>>>> On 5/12/2019 9:43 PM, Arvind Sankar wrote:
>>>>>>> On Sun, May 12, 2019 at 05:05:48PM +0000, Rob Landley wrote:
>>>>>>>> On 5/12/19 7:52 AM, Mimi Zohar wrote:
>>>>>>>>> On Sun, 2019-05-12 at 11:17 +0200, Dominik Brodowski wrote:
>>>>>>>>>> On Thu, May 09, 2019 at 01:24:17PM +0200, Roberto Sassu wrote:
>>>>>>>>>>> This proposal consists in marshaling pathnames and xattrs in a file called
>>>>>>>>>>> .xattr-list. They are unmarshaled by the CPIO parser after all files have
>>>>>>>>>>> been extracted.
>>>>>>>>>>
>>>>>>>>>> Couldn't this parsing of the .xattr-list file and the setting of the xattrs
>>>>>>>>>> be done equivalently by the initramfs' /init? Why is kernel involvement
>>>>>>>>>> actually required here?
>>>>>>>>>
>>>>>>>>> It's too late.  The /init itself should be signed and verified.
>>>>>>>>
>>>>>>>> If the initramfs cpio.gz image was signed and verified by the extractor, how is
>>>>>>>> the init in it _not_ verified?
>>>>>>>>
>>>>>>>> Ro
>>>>>>>
>>>>>>> Wouldn't the below work even before enforcing signatures on external
>>>>>>> initramfs:
>>>>>>> 1. Create an embedded initramfs with an /init that does the xattr
>>>>>>> parsing/setting. This will be verified as part of the kernel image
>>>>>>> signature, so no new code required.
>>>>>>> 2. Add a config option/boot parameter to panic the kernel if an external
>>>>>>> initramfs attempts to overwrite anything in the embedded initramfs. This
>>>>>>> prevents overwriting the embedded /init even if the external initramfs
>>>>>>> is unverified.
>>>>>>
>>>>>> Unfortunately, it wouldn't work. IMA is already initialized and it would
>>>>>> verify /init in the embedded initial ram disk.
>>>>>
>>>>> So you made broken infrastructure that's causing you problems. Sounds unfortunate.
>>>>
>>>> The idea is to be able to verify anything that is accessed, as soon as
>>>> rootfs is available, without distinction between embedded or external
>>>> initial ram disk.
>>>>
>>>> Also, requiring an embedded initramfs for xattrs would be an issue for
>>>> systems that use it for other purposes.
>>>>
>>>>
>>>>>> The only reason why
>>>>>> opening .xattr-list works is that IMA is not yet initialized
>>>>>> (late_initcall vs rootfs_initcall).
>>>>>
>>>>> Launching init before enabling ima is bad because... you didn't think of it?
>>>>
>>>> No, because /init can potentially compromise the integrity of the
>>>> system.
>>>
>>> I think Rob is right here.  If /init was statically built into the
>>> kernel image, it has no more ability to compromise the kernel than
>>> anything else in the kernel.  What's the problem here?
>>
>> Right, the measurement/signature verification of the kernel image is
>> sufficient.
>>
>> Now, assuming that we defer the IMA initialization until /init in the
>> embedded initramfs has been executed, the problem is how to handle
>> processes launched with the user mode helper or files directly read by
>> the kernel (if it can happen before /init is executed). If IMA is not
>> yet enabled, these operations will be performed without measurement and
>> signature verification.
> 
> If you really care about this, don't launch any user mode helper
> programs (hint, you have the kernel option to control this and funnel
> everything into one, or no, binaries).  And don't allow the kernel to
> read any files either, again, you have control over this.
> 
> Or start IMA earlier if you need/want/care about this.

Yes, this is how it works now. It couldn't start earlier than
late_initcall, as it has to wait until the TPM driver is initialized.

Anyway, it is enabled at the time /init is executed. And this would be
an issue because launching /init and reading xattrs from /.xattr-list
would be denied (the signature is missing).

And /.xattr-list won't have a signature, if initramfs is generated
locally.

Roberto

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