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

Roberto Sassu
2017-05-24T08:19:42Z

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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