On Thu, 1 Aug 2019 at 13:30, Janne Karhunen [off-list ref] wrote:

On Thu, Aug 1, 2019 at 10:40 AM Sumit Garg [off-list ref] wrote:

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I chose the userspace plugin due to this, you can use userspace aids
to provide any type of service. Use the crypto library you desire to
do the magic you want.

Here TEE isn't similar to a user-space crypto library. In our case TEE
is based on ARM TrustZone which only allows TEE communications to be
initiated from privileged mode. So why would you like to route
communications via user-mode (which is less secure) when we have
standardised TEE interface available in kernel?

The physical access guards for reading/writing the involved critical
memory are identical as far as I know? Layered security is generally a
good thing, and the userspace pass actually adds a layer, so not sure
which is really safer?

AFAIK, layered security is better in case we move from lower privilege
level to higher privilege level rather than in reverse order.

-Sumit

In my case the rerouting was to done generalize it. Any type of trust
source, anywhere.

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Isn't actual purpose to have trusted keys is to protect user-space
from access to kernel keys in plain format? Doesn't user mode helper
defeat that purpose in one way or another?

Not really. CPU is in the user mode while running the code, but the
code or the secure keydata being is not available to the 'normal'
userspace. It's like microkernel service/driver this way. The usermode
driver is part of the kernel image and it runs on top of a invisible
rootfs.

Can you elaborate here with an example regarding how this user-mode
helper will securely communicate with a hardware based trust source
with other user-space processes denied access to that trust source?

The other user mode processes will never see the device node to open.
There is none in existence for them; it only exists in the ramfs based
root for the user mode helper.


--
Janne

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