linux-kernel - Re: [PATCH 1/4] KEYS: Insert incompressible bytes to reserve space in bzImage

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Date:   Fri, 25 Nov 2016 14:49:17 -0500
From:   Mehmet Kayaalp <mkayaalp@...ux.vnet.ibm.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        George Wilson <gcwilson@...ibm.com>,
        LSM <linux-security-module@...r.kernel.org>,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] KEYS: Insert incompressible bytes to reserve space in
 bzImage


> On Nov 25, 2016, at 4:06 AM, David Howells <dhowells@...hat.com> wrote:
> 
> Mehmet Kayaalp <mkayaalp@...ux.vnet.ibm.com> wrote:
> 
>> Include a random filled binary in vmlinux at the space reserved with
>> CONFIG_SYSTEM_EXTRA_CERTIFICATE. This results in an uncompressed reserved
>> area inside the bzImage as well, so that it can be replaced with an actual
>> certificate later (after the bzImage is distributed).
> 
> You haven't said *why* you need an incompressible buffer.  I presume it's
> something to do with the decompression wrapper.
> 

After you replace zeroes with a certificate and compress again, the image size
increases. Then, repackaging the bzImage becomes difficult.

>From the commit message of 3/4:

> This patch adds the capability of extracting the
> vmlinux, inserting the certificate, and repackaging the result into a
> bzImage.
> 
> It only works if the resulting compressed vmlinux is smaller than the
> original. Otherwise re-linking would be required. To make the reserved
> space allocate actual space in bzImage, incompressible bytes are
> inserted into the vmlinux as a placeholder for the extra certificate.

Mehmet