Re: [PATCH] tpm: vtpm_proxy: Introduce flag to prevent sysfs entries

[Jason Gunthorpe]

On Fri, Jun 24, 2016 at 02:43:00PM -0400, Stefan Berger wrote:
> On 06/24/2016 01:48 PM, Jason Gunthorpe wrote:
> >On Fri, Jun 24, 2016 at 10:36:55AM -0400, Stefan Berger wrote:
> >>Introduce TPM_VTPM_PROXY_NO_SYSFS flag that prevents a vtpm_proxy driver
> >>instance from having the typical sysfs entries that shows the state of the
> >>TPM. The flag is to be set in the ioctl creating the vtpm_proxy device
> >>pair and maps on a new chip flags TPM_CHIP_FLAG_NO_SYSFS.
> >No other subsystem does something so goofy, this really needs to be
> >part of namespace support for TPM.
> 
> And I am not sure how to go about this. TPM2 by the way doesn't have such
> entries, so it's much better from that perspective.

Presumably this will become obvious when you figure out how to make
namespaces work for IMA.. tpm #0 will have to change, and that will
have to impact the sysfs layout.

> >Why can't you just make the sysfs files unreadable in user space?
> 
> There are actually ways to go about this. Likely bind-mounting over
> /sys/device/virtual/tpm would be one solution to hide all virtual TPM
> device. Another is applying an AppArmor policy to the container denying
> access to tpm directories or entries. SELinux would not be so easy.

Seems reasonable

> The flag in this patch seemed like a 'cheap' way to eliminate that
> problem as well.

Generally hacks like this are discouraged in the kernel.

> >If a container can make them readable again can't it also just create
> >the chardev node?
> 
> What do you mean by making them readable again? The chardev node can be
> created inside the container with the major and minor numbers as it was
> created with the ioctl.

Well, root in the container can undo all sorts of things. It can
unbind mount the sysfs, it could change file permissions and it could
manually create the char dev nodes. AFAIK the only way to protect
against root in the container is to have full kernel namespace support
for the device..

Jason