linux-kernel - Re: [PATCH] Check files' signatures before doing suid/sgid [2/4]

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Date:	Sat, 23 Jun 2007 01:06:58 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Alexander Wuerstlein" <arw@....name>
Cc:	linux-kernel@...r.kernel.org,
	"Johannes Schlumberger" <spjsschl@...d.informatik.uni-erlangen.de>,
	linux-crypto@...r.kernel.org
Subject: Re: [PATCH] Check files' signatures before doing suid/sgid [2/4]

Hi Alexander, Johannes,

[ Added linux-crypto to Cc: ]

Wow, this is _one_ *intrusive* patchset indeed :-)

But first: Have you checked the digsig project? It's been doing
(for some time) what your current patchset proposes -- and
it uses public key cryptosystems for the key management,
which is decidedly better than using secret-keyed hashes
(HMAC, XCBC). Also, digsig aims to protect executable
binaries in general, and not just suid / sgid ones.

Second: Can we have some discussion on the security model /
threat model / trust model / cryptographic key management
scheme of your signing mechanism? [I had read through the
[0/4] mail you had sent yesterday, but found no relevant
discussion on these aspects there.]

>From the patchset, it appears you use a *common* secret key
for _all_ signed binaries, and it is set at kernel build-time itself:

On 6/22/07, Alexander Wuerstlein <arw@....name> wrote:
> sns_secret_key.dat contains the 'secret key' which is used for HMAC.

Where:

> --- /dev/null
> +++ b/security/sns_secret_key.dat
> +#define SNS_SECRET_KEY_SIZE 8
> +static char sns_secret_key[SNS_SECRET_KEY_SIZE] =
> +       {
> +               'd', 'e', 'a', 'd', 'b', 'e', 'e', 'f'
> +       };

[ Ok, I won't nitpick as to why does this file look like a header,
is #include-d in the C source as a header, but still has a .dat
extension :-) ]

Anyway, this is *totally* insecure and broken. Do you realize anybody
who lays hands on the kernel image can now _trivially_ extract the
should-have-been-a-secret key from it and use it to sign his own
binaries?

Satyam
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