Re: [PATHC v2 0/9] ima: carry the measurement list across kexec

Hello Eric,

Am Dienstag, 20 September 2016, 11:07:29 schrieb Eric W. Biederman:

Thiago Jung Bauermann [off-list ref] writes:

quoted

Am Samstag, 17 September 2016, 00:17:37 schrieb Eric W. Biederman:

quoted

Thiago Jung Bauermann [off-list ref] writes:

Is this what you had in mind?

Sort of.

I was just thinking that instead of having the boot path verify your ima
list matches what is in the tpm and halting the boot there, we could
test that on reboot.  Which would give a clean failure without the nasty
poking into a prepared image.  The downside is that we have already run
the shutdown scripts so it wouldn't be much cleaner, than triggering a
machine reboot from elsewhere.

But I don't think we should spend too much time on that.  It was a
passing thought.  We should focus on getting a non-poked ima buffer
cleanly into kexec and we can worry about the rest later.

I was thinking of this as something orthogonal to the ima buffer feature.
But you're right, it's better not to discuss this now. I'll post a separate 
patch for this later.

quoted

quoted

So from 10,000 feet I think that is correct.

I am not quite certain why a new mechanism is being invented.  We have
other information that is already passed (much of it architecture
specific) like the flattened device tree.  If you remove the need to
update the information can you just append this information to the
flattened device tree without a new special mechanism to pass the data?

I am just reluctant to invent a new mechanism when there is an existing
mechanism that looks like it should work without problems.

Michael Ellerman suggested putting the buffer contents inside the device
tree itself, but the s390 people are also planning to implement this
feature. That architecture doesn't use device trees, so a solution that
depends on DTs won't help them.

With this mechanism each architecture will still need its own way of
communicating to the next kernel where the buffer is, but I think it's
easier to pass a base address and length than to pass a whole buffer.

A base address and length pair is fine.  There are several other pieces
of data that we pass that way.

quoted

I suppose we could piggyback the ima measurements buffer at the end of
one of the other segments such as the kernel or, in the case of
powerpc, the dtb but it looks hackish to me. I think it's cleaner to
put it in its own segment.

The boot protocol unfortunately is different on different architectures,
and for each one we will have to implement and document the change.
Because when you get into boot protocol issues you can't assume the
kernel you are booting is the same version as the kernel that is booting
it.

Where I run into a problem is you added a semi-generic concept a
hand-over buffer.  Not a ima data buffer but a hand-over buffer.

The data falling in it's own dedicated area of memory and being added
with kexec_add_buffer is completely fine.  I can see a dedicated pointer
in struct kimage if necessary.

A semi-generic concept called a hand-over buffer seems to be a
construction of infrustructure for no actual reason that will just
result in confusion.  There are lots of things that are handed over, the
flattend device tree, ramdisks, bootparams on x86, etc, etc.  ima is not
special in this execpt for being perhaps the first addition that we are
going to want the option of including on most architectures.

Ok, I understand. I decided to implement a generic concept because I thought 
that proposing a feature that is more useful than what I need it for would  
increase its chance of being accepted. It's interesting to see that it had 
the opposite effect.

I reworked and simplified the code and folded the hand-over buffer patches 
into Mimi's patch series to carry the measurement list across kexec. The 
kexec buffer code is in the following patches now:

[PATCH v5 01/10] powerpc: ima: Get the kexec buffer passed by the previous 
kernel
[PATCH v5 05/10] powerpc: ima: Send the kexec buffer to the next kernel

Each patch has a changelog listing what I changed to make it specific to 
IMA.

-- 
[]'s
Thiago Jung Bauermann
IBM Linux Technology Center