Mimi Zohar [off-list ref] writes:
Hi Andrew,
On Wed, 2016-08-31 at 18:38 -0400, Mimi Zohar wrote:
quoted
On Wed, 2016-08-31 at 13:50 -0700, Andrew Morton wrote:
quoted
On Tue, 30 Aug 2016 18:40:02 -0400 Mimi Zohar [off-list ref] wrote:
quoted
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.
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.
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.
If there is anyway we can start small and not with this big scary
infrastructure change I would very much prefer it.
Eric
Hello Eric,
Am Freitag, 16 September 2016, 14:47:13 schrieb Eric W. Biederman:
Mimi Zohar [off-list ref] writes:
quoted
Hi Andrew,
On Wed, 2016-08-31 at 18:38 -0400, Mimi Zohar wrote:
quoted
On Wed, 2016-08-31 at 13:50 -0700, Andrew Morton wrote:
quoted
On Tue, 30 Aug 2016 18:40:02 -0400 Mimi Zohar
[off-list ref] wrote:
quoted
quoted
quoted
quoted
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.
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.
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.
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.
--
[]'s
Thiago Jung Bauermann
IBM Linux Technology Center