Re: [Linux-ima-devel] [PATCH 0/7] IMA: new parser for ima_restore_measurement_list()

From: Roberto Sassu <roberto.sassu@huawei.com>
To: Ken Goldman <kgold@linux.vnet.ibm.com>,
	<linux-ima-devel@lists.sourceforge.net>
Cc: <linux-security-module@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [Linux-ima-devel] [PATCH 0/7] IMA: new parser for ima_restore_measurement_list()
Date: Wed, 24 May 2017 10:18:46 +0200	[thread overview]
Message-ID: <e7386c0d-332b-1d61-a927-ad9b9dcd1449@huawei.com> (raw)
In-Reply-To: <08bd9fcb-d3ba-e76e-928e-f569e8a4be74@linux.vnet.ibm.com>

On 5/23/2017 10:48 PM, Ken Goldman wrote:
> On 5/18/2017 5:38 AM, Roberto Sassu wrote:
>> On 5/17/2017 6:28 PM, Ken Goldman wrote:
>>> On 5/17/2017 3:25 AM, Roberto Sassu wrote:
>>>>
>>>> The format of digestN is: <algo name>:\0<digest value>, the same used
>>>> for the file digest.
>>>
>>> Since the format is changing from the SHA-1 log format anyway ...
>>>
>>> How do people feel about the colon and null terminated string format for
>>> algorithm identifiers?
>>>
>>> The TCG standard enumerations are uint16_t, and there is a registry of
>>> hash algorithms.
>>>
>>> As a consuming parser, it feels nice to know it's always 2 bytes and not
>>> have to worry about a missing colon or a missing nul terminator risking
>>> a buffer overflow.
>>
>> There cannot be buffer overflow, because the length of each digest
>> field is known.
>>
>> Roberto
>>
>
> I was not referring to the digest, but the digest algorithm.
>
> I wanted opinions on the colon and null terminated string format for
> algorithm identifiers.
>
> The TCG standard log uses the TCG standard enumerations.  They're always
> exactly 2 bytes.  Parsing is trivial.

I have two concerns regarding this:

is there a standard way to convert TPM_ALG_ to strings, like a function
exposed by the TSS? If not, suppose that a parser uses openssl to verify
the integrity of event data, by calculating the digest. Then,
the parser should maintain an association table between TPM_ALG_
and a string (the string will be passed to EVP_get_digestbyname()).
When a new TPM algorithm is added to the TCG registry, all parsers
should be modified to update the association table. If IMA sends
a string, only the crypto subsystem has to be updated.

The format I'm proposing for event data digests would be the same
of that used for file digests. Should IMA provide a list with
two different formats?

Roberto

> If IMA uses strings, the attacker can send, e.g., sha1: and not null
> terminate it.  A careful parser can go a byte at a time until it reaches
> a maximum length - if you specify a maximum length.  But it is an attack
> surface.  Is there a corresponding advantage?
>
>
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