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OpenBSD Kernel Relinking Issue

CVE Category Price Severity
CVE-2015-2924 CWE-426 $10,000 High
Author Risk Exploitation Type Date
Maksymilian Arciemowicz High Local 2023-06-19
CVSS
CVSS:4.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS vector description

Our sensors found this exploit at: https://cxsecurity.com/ascii/WLB-2023060046

Below is a copy:

OpenBSD Kernel Relinking Issue
The automatic and mandatory-by-default reordering of OpenBSD kernels
is NOT transactional and as a result, a local unpatched exploit exists
which allows tampering or replacement of the kernel. Arbitrary build
artifacts are cyclically relinked with no data integrity or provenance
being maintained or verified for the objects being consumed with
respect to the running kernel before and during the execution of the
mandatory kernel_reorder process in the supplied /etc/rc and
/usr/libexec scripts. The reordering occurs at the end of installation
process and also automatically every reboot cycle thereafter unless
manually bypassed by a knowledgable party.

The kernel_reorder routine verifies a SHA256 signature for the linked
kernel from last boot but does not verify the integrity or provenance
of any objects kept in the kernel "link kit" installed in
/usr/share/relink, so arbitrary objects can be injected and
automatically relinked at the next startup. I have verified that it is
indeed the case that both valid kernels with a different uname and
kernels which cause data destruction due to over-tuning of a subset of
the components which were compiled manually and copied into
/usr/share/relink and crash the system after being booted once
relinked but which do not match the build of the running kernel at the
time they were copied into /usr/share/relink as working
proof-of-concept exploits.

Install media are also open to tampering and exploitation as signed
checksum data are not carried with the install sets inside the
installation image and an improperly-encapsulated poorly-documented
tarball of unverifiable (in the sense of SLSA) kernel objects is
embedded in the base distribution and then relinked with a new random
ordering of the objects cyclically between boot cycles.

Sites with a strong security posture are advised that this is a
critical vulnerability and likely deliberate back door into the
system. Additionally, OpenBSD leaks the state of the pseudorandom
number generator to predictable locations on disk and in system memory
at a fixed point during every start up and shutdown procedure. The
lack of build process hardening has been on-going for over three
years. Theo de Raadt is disinterested in improving or reviewing the
design or providing any further clarification, as he has stated on the
mailing list when shortfalls in the relinking process were reported
over the past ~3 years. I hope that this can come to the attention of
a third-party technical expert with standing in the computer security
industry.

Workaround:

As the link kit is embedded in the base distribution and automatically
relinked without an option to disable it in the provided installation
script it requires manual removal at present.

Cf.

https://marc.info/?l=openbsd-bugs&m=159074964523007&w=2 (noted lack of
idempotency)
https://marc.info/?l=openbsd-bugs&m=168688579123005&w=2 (noted lack of
integrity or provenance verification and the consumption of invalid
objects)

https://slsa.dev/spec/v1.0/levels#build-l2-hosted-build-platform:

"Track/Level Requirements                 Focus
 Build L3       Hardened build platform      Tampering during the build"

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