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

Thiago Jung Bauermann [off-list ref] writes:

Hello Eric,

Am Freitag, 16 September 2016, 14:47:13 schrieb Eric W. Biederman:

quoted

Mimi Zohar [off-list ref] writes:

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Hi Andrew,

On Wed, 2016-08-31 at 18:38 -0400, Mimi Zohar wrote:

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On Wed, 2016-08-31 at 13:50 -0700, Andrew Morton wrote:

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On Tue, 30 Aug 2016 18:40:02 -0400 Mimi Zohar 

[off-list ref] wrote:

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The TPM PCRs are only reset on a hard reboot.  In order to validate
a
TPM's quote after a soft reboot (eg. kexec -e), the IMA measurement
list
of the running kernel must be saved and then restored on the
subsequent
boot, possibly of a different architecture.

The existing securityfs binary_runtime_measurements file
conveniently
provides a serialized format of the IMA measurement list. This
patch
set serializes the measurement list in this format and restores it.

Up to now, the binary_runtime_measurements was defined as
architecture
native format.  The assumption being that userspace could and would
handle any architecture conversions.  With the ability of carrying
the
measurement list across kexec, possibly from one architecture to a
different one, the per boot architecture information is lost and
with it
the ability of recalculating the template digest hash.  To resolve
this
problem, without breaking the existing ABI, this patch set
introduces
the boot command line option "ima_canonical_fmt", which is
arbitrarily
defined as little endian.

The need for this boot command line option will be limited to the
existing version 1 format of the binary_runtime_measurements.
Subsequent formats will be defined as canonical format (eg. TPM 2.0
support for larger digests).

This patch set pre-req's Thiago Bauermann's "kexec_file: Add buffer
hand-over for the next kernel" patch set.

These patches can also be found in the next-kexec-restore branch
of:
git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity
.git

I'll merge these into -mm to get some linux-next exposure.  I don't
know what your upstream merge plans will be?

Sounds good.  I'm hoping to get some review/comments on this patch set
as well.  At the moment, I'm chasing down a kernel test robot report
from this afternoon.

My concern about changing the canonical format as originally defined in
patch 9/9 from big endian to little endian never materialized.  Andreas
Steffan, the patch author, is happy either way.

We proposed two methods of addressing Eric Biederman's concerns of not
including the IMA measurement list segment in the kexec hash as
described in  https://lkml.org/lkml/2016/9/9/355.

- defer calculating and verifying the serialized IMA measurement list
buffer hash to IMA
- calculate the kexec hash on load, verify it on the kexec execute,
before re-calculating and updating it.

I need to ask: How this is anticipated to interact with kexec on panic?
Because honestly I can't see this ever working in that case.  The
assumption is that the original kernel has gone crazy.  So from a
practical standpoint any trusted path should have been invalided.

We are not interested in carrying the measurement list in the case of kexec 
on panic. I see that the code is adding a hand-over buffer to the crash 
image as well, but that is a bug.

The fix is to do nothing in ima_add_kexec_buffer if image->type != 
KEXEC_TYPE_DEFAULT.
 

quoted

This entire idea of updating the kexec image makes me extremely
extremely nervious.  It feels like sticking a screw driver through the
spokes of your bicicle tires while ridding down the road.

I can see tracking to see if the list has changed at some
point and causing a reboot(LINUX_REBOOT_CMD_KEXEC) to fail.

Yes, that is an interesting feature that I can add using the checksum-
verifying part of my code. I can submit a patch for that if there's 
interest, adding a reboot notifier that verifies the checksum and causes a 
regular reboot instead of a kexec reboot if the checksum fails.

I was thinking an early failure instead of getting all of the way down
into a kernel an discovering the tpm/ima subsystem would not
initialized.  But where that falls in the reboot pathway I don't expect
there is much value in it.

quoted

At least the common bootloader cases that I know of using kexec are very
minimal distributions that live in a ramdisk and as such it should be
very straight forward to measure what is needed at or before
sys_kexec_load.  But that was completely dismissed as unrealistic so I
don't have a clue what actual problem you are trying to solve.

We are interested in solving the problem in a general way because it will be 
useful to us in the future for the case of an arbitrary number of kexecs 
(and thus not only a bootloader but also multiple full-blown distros may be 
involved in the chain).

But you are right that for the use case for which we currently need this 
feature it's feasible to measure everything upfront. We can cross the other 
bridge when we get there.

Then let's start there.  Passing the measurment list is something that
should not be controversial.

quoted

If there is anyway we can start small and not with this big scary
infrastructure change I would very much prefer it.

Sounds good. If we pre-measure everything then the following patches from my 
buffer hand-over series are enough:

[PATCH v5 2/5] kexec_file: Add buffer hand-over support for the next kernel
[PATCH v5 3/5] powerpc: kexec_file: Add buffer hand-over support for the 
next kernel

Would you consider including those two?

And like I mentioned in the cover letter, patch 1/5 is an interesting 
improvement that is worth considering.

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.

Eric

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

Thiago Jung Bauermann [off-list ref] writes:

quoted

Hello Eric,

Am Freitag, 16 September 2016, 14:47:13 schrieb Eric W. Biederman:

quoted

I can see tracking to see if the list has changed at some
point and causing a reboot(LINUX_REBOOT_CMD_KEXEC) to fail.

Yes, that is an interesting feature that I can add using the checksum-
verifying part of my code. I can submit a patch for that if there's
interest, adding a reboot notifier that verifies the checksum and causes
a regular reboot instead of a kexec reboot if the checksum fails.

I was thinking an early failure instead of getting all of the way down
into a kernel an discovering the tpm/ima subsystem would not
initialized.  But where that falls in the reboot pathway I don't expect
there is much value in it.

I'm not sure I understand. What I described doesn't involve the tpm or ima. 
I'm suggesting that if I take the parts of patch 4/5 in the kexec hand-over 
buffer series that verify the image checksum, I can submit a separate patch 
that checks the integrity of the kexec image early in kernel_kexec() and 
reverts to a regular reboot if the check fails. This would be orthogonal to 
ima carrying its measurement list across kexec.

I think there is value in that, because if the kexec image is corrupted the 
machine will just get stuck in the purgatory and (unless it's a platform 
where the purgatory can print to the console) without even an error message 
explaining what is going on. Whereas if we notice the corruption before 
jumping into the purgatory we can switch to a regular reboot and the machine 
will boot successfully.

To have an early failure, when would the checksum verification be done?
What I can think of is to have kexec_file_load accept a new flag 
KEXEC_FILE_VERIFY_IMAGE, which userspace could use to request an integrity 
check when it's about to start the reboot procedure. Then it can decide to 
either reload the kernel or use a regular reboot if the image is corrupted.

Is this what you had in mind?

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At least the common bootloader cases that I know of using kexec are
very
minimal distributions that live in a ramdisk and as such it should be
very straight forward to measure what is needed at or before
sys_kexec_load.  But that was completely dismissed as unrealistic so I
don't have a clue what actual problem you are trying to solve.

We are interested in solving the problem in a general way because it
will be useful to us in the future for the case of an arbitrary number
of kexecs (and thus not only a bootloader but also multiple full-blown
distros may be involved in the chain).

But you are right that for the use case for which we currently need this
feature it's feasible to measure everything upfront. We can cross the
other bridge when we get there.

Then let's start there.  Passing the measurment list is something that
should not be controversial.

Great!

quoted

quoted

If there is anyway we can start small and not with this big scary
infrastructure change I would very much prefer it.

Sounds good. If we pre-measure everything then the following patches
from my buffer hand-over series are enough:

[PATCH v5 2/5] kexec_file: Add buffer hand-over support for the next
kernel [PATCH v5 3/5] powerpc: kexec_file: Add buffer hand-over support
for the next kernel

Would you consider including those two?

And like I mentioned in the cover letter, patch 1/5 is an interesting
improvement that is worth considering.

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.

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.

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