Re: Fixing CVE-2017-15361

[Peter Huewe]

Am 25. Oktober 2017 20:53:49 MESZ schrieb Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>:
>On Wed, Oct 25, 2017 at 07:17:17AM -0700, Matthew Garrett wrote:
>> On Wed, Oct 25, 2017 at 6:44 AM, Jarkko Sakkinen
>> <jarkko.sakkinen@xxxxxxxxxxxxxxx> wrote:
>> > I'm implementing a fix for CVE-2017-15361 that simply blacklists
>> > vulnerable FW versions. I think this is the only responsible action
>from
>> > my side that I can do.
>> 
>> I'm not sure this is ideal - do Infineon have any Linux tooling for
>> performing firmware updates, and if so will that continue working if
>> the device is blacklisted? It's also a poor user experience to have
>> systems using TPM-backed disk encryption keys suddenly rendered
>> unbootable, and making it as easy as possible for people to do an
>> upgrade and then re-seal secrets with new keys feels like the correct
>> approach.
>
>I talked today with Alexander Steffen in the KS unconference and we
>concluded that this would be a terrible idea.
>
>Alexander stated the following things about FW updates (Alexander,
>please correct me if I state something incorrectly or if you have
>something to add):
>
>* FW update can be constructed either in a way that the keys in the
>  NVRAM are not cleared or in a way that they are cleared.
>* FW update cannot be directly applied to the TPM but must come as
>  part of the firmware update from the vendor.
>
>I proposed the following as an alternative:
>
>* Print a message to the klog (which log level would be appropriate?).
Info?
Maybe warn, definitely not err

>* Possibly sleep for few seconds. Is this a good idea?
Helps how?

>
>While writing this email yet another alternative popped into my mind:
>what if we allow only in-kernel use but disallow the use of /dev/tpm0?
>You could still use trusted keys.
>
No, same terrible idea since you block the upgrade path.
Upgrade tools work from userspace via the kernel driver. 
So /dev/tpm0 is necessary.

>Here are all the ideas that I have and I am open for better
>alternatives.
>
>/Jarkko

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