Re: [PATCH 1/2] ima: split ima_add_digest_entry() function

From: Roberto Sassu
Date: Tue Dec 06 2011 - 09:51:34 EST

On 12/06/2011 03:24 PM, Mimi Zohar wrote:
On Tue, 2011-12-06 at 11:27 +0100, Roberto Sassu wrote:
On 12/05/2011 09:57 PM, Mimi Zohar wrote:
On Mon, 2011-12-05 at 14:56 +0100, Roberto Sassu wrote:
On 12/05/2011 02:03 PM, Mimi Zohar wrote:
On Mon, 2011-12-05 at 11:04 +0100, Roberto Sassu wrote:

Hi Mimi

i think moving this logic to the TPM driver (or in general, delaying
the action after the list mutex is unlocked) is not safe, because in
this way you are relying on the kernel trustworthiness to protect
itself and IMA against unmeasured potential attacks. So, the verifier
is unable to detect a kernel tampering that removed the limitation
on the TPM Quote operation.

What i'm proposing in the patch:

https://lkml.org/lkml/2011/11/21/202

is in fact a new extension, which is triggered by a new kernel
parameter, so that the behaviour of the base IMA is not modified.

How/why the TPM fails is important. If the TPM fails because of an
intermittent problem, then your solution of denying read/execute could
work, but what would happen if it was persistent? Would you be able to
quiesce the system?

As there is no way of differentiating a persistent from intermittent
failure, both need to be addressed in the same manor. For persistent
TPM failure, we can not access the TPM to modify the PCR. So what
options do we have left? My suggestion, though not optimal, prevents
the IMA PCR from being quoted.


Hi Mimi

the solution you are proposing is reasonable as the default
behaviour, because not all IMA users need the high confidence
in the measurements, as ensured by denying the execution of
system calls.

However, during the IMA initialization the TPM is tested
by issuing a PCR read (the test procedure may be extended
to better detect existing errors in advance). So, this means
that a TPM failure when the system is already powered on is
very unlikely and may cause serious issues as it could happen
if other devices are involved.

For this reason, also my extension seems helpful especially
in the situations where all events need to be measured properly.
In this case, IMA users are aware that a TPM failure could hang
their systems, because they need to manually insert the required
kernel parameter.

As you said a TPM failure is very unlikely, what type of attack are you
trying to defend against, that could possibly warrant causing the system
to hang?


I don't know if this can really happen, but an attacker may issue
a lot of commands to the TPM, so that the timeout limit is reached
when IMA is trying to extend the PCR.

Roberto Sassu

Processing lots of commands isn't an issue, as IMA takes the
ima_extend_list_mutex to synchronize adding the measurement to the
measurement list and extending the PCR. The TPM device driver takes the
tpm_mutex, in tpm_transmit(), before transmitting the command.


I mean issuing a lot of TPM commands, so that the TPM is unable
to process the IMA request.


So the issue remains whether an individual PCR extend can timeout/fail.
As you previously said, this is highly unlikely.


I think the question is whether or not an attacker can cause the
TPM to reach the timeout limit. If this is feasible and it cannot
be clearly detected by inspecting the measurements list, denying
the system call for which the measurement cannot be taken may be a solution.

Roberto Sassu


Mimi


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