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

[Mimi Zohar]

On Thu, 2011-11-17 at 11:57 +0100, Roberto Sassu wrote:
> On 11/16/2011 07:52 PM, Rajiv Andrade wrote:
> >
> > Thanks, Rajiv Andrade Security Development IBM Linux Technology Center
> >
> > On 16-11-2011 12:37, Roberto Sassu wrote:
> >> On 11/16/2011 02:38 PM, Mimi Zohar wrote:
> >>> On Wed, 2011-11-16 at 11:10 +0100, Roberto Sassu wrote:
> >>>> The ima_add_digest_entry() function has been split in order to avoid
> >>>> adding an entry in the measurements list for which the PCR extend
> >>>> operation subsequently fails. Required memory is allocated earlier
> >>>> in the
> >>>> new function ima_prepare_template_entry() and the template entry is
> >>>> added
> >>>> after ima_pcr_extend().
> >>>>
> >>>> Signed-off-by: Roberto Sassu<roberto.sassu@xxxxxxxxx>
> >>>
> >>
> >> Hi Mimi
> >>
> >> i don't know if this condition can happen, but suppose that
> >> for whatever reason the PCR extend fails. In this case, since
> >> the PCR is not extended, the measurements list can be modified,
> >> by removing the non-measured entry, without this fact being
> >> detected by the verifier. So, probably we can avoid to display
> >> the entry.
> >>
> >>
> > Hi Roberto,
> >
> > IMA's trustworthiness is built on the assumption that the TPM underneath
> > can
> > be trusted. If that can't be, the eventlog alone doesn't provide us any
> > security.
> > It's the TPM device driver's job though to workaround any HW bug so that
> > in the
> > end all its stakeholders have their commands processed successfully, as
> > we've
> > pursued in some changes here:
> >
> 
> Hi Rajiv
> 
> thanks for your comments.
> 
> I absolutely agree that we have to trust the TPM for the correct
> execution of IMA.
> 
> I think the principle that has been used to build IMA (according
> to the TGC specifications) is that we can trust the eventlog
> as long as the measurement infrastructure is reliable or it is
> possible to detect a threat from previous measurements.
> 
> For this reason, a system call is never executed before the
> inode measurement is inserted in the eventlog and the PCR
> is extended. Since these operations must be considered as
> atomic, their execution is protected by a mutex, that is
> released only after all tasks have been performed. This
> ensures that we begin with a measured kernel and we can
> reliably measure all further interactions. This also explains,
> in my view, why delaying the PCR extend operation may lead
> to security risks.
> 
> About my patch, i did not move out the protected region any
> of the above described operations. Instead, i'm preventing
> measurements for which the PCR extend failed to be added to
> the measurements list, because in any case it is impossible
> for a verifier to detect their removal from the list.
> 
> As i mentioned in the previous mail, one solution to overcome
> this issue is to deny, on the platform running IMA, the execution
> of those system calls for which the measurement process ended
> with an error.
> 
> Regards
> 
> Roberto Sassu

True, if the TPM failed to extend the PCR, a malicious user would be
able to remove the measurement from the measurement list without it
being detected.  However, according to the TPM specs, the PCR extend
operation is always suppose to succeed, even in the case when the TPM is
not enabled.

More importantly we need to be able to detect when the PCR has not been
extended appropriately in order to address it.  Otherwise we're just
covering it up.

thanks,

Mimi

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