---===[ Qubes Security Bulletin 067 ]===---

                             2021-03-19


                     Multiple RPM vulnerabilities


User action required
=====================

Users must install the following specific packages in order to address
the issues discussed in this bulletin:

  For Qubes 4.0:
  - rpm 4.14.2.1 (plus rebuilt packages to link with the new rpm)
  - qubes-core-dom0-linux 4.0.29
  - qubes-mgmt-salt-dom0-update 4.0.10

  For Qubes 4.1:
  - qubes-core-dom0-linux 4.1.10
  - qubes-mgmt-salt-dom0-update 4.1.6

The packages are to be installed in dom0 via the Qubes Update tool [4]
or via the qubes-dom0-update command as follows:

  For updates from the stable repository (not immediately available):
  $ sudo qubes-dom0-update

  For updates from the security-testing repository:
  $ sudo qubes-dom0-update --enablerepo=qubes-dom0-security-testing

After installing the updates in dom0, it is necessary to install updates
in Fedora-based TemplateVMs and StandaloneVMs. This can be
done via the Qubes Update tool [4] or using qubesctl (salt) as follows:

  $ sudo qubesctl --skip-dom0 --templates --standalones state.sls update.qubes-vm

These packages will migrate from the security-testing repository to the
current (stable) repository over the next two weeks after being tested
by the community.


Summary
========

Demi M. Obenour has discovered several issues in the RPM package
manager:

- CVE-2021-20271[1] RPM: Signature checks bypass via corrupted RPM
  package
- CVE-2021-3421[2] RPM: unsigned signature header leads to string
  injection into an RPM database
- CVE-2021-20266[3] RPM: missing length checks in hdrblobInit()

These issues allow an attacker who controls packages the user downloads
to inject malicious content that, under some conditions, may not be
detected by signature verification. Specifically, they allow the
attacker to modify parts of the package header that are not protected by
the signature and that are later integrated into the RPM database. This
allows for corrupting the RPM database and preventing further updates of
select packages.  In the case of Fedora TemplateVMs, this also allows
for arbitrary code execution.

The CVE-2021-20271 exploit takes advantage of multiple headers in the
RPM package format. In a proper RPM package, the signature is placed in
a separate header (called the "signature header") and, if present, is
verified by librpm when loading the file (according to the requested
verification level). An RPM package also contains a "main header" that
includes all the other package metadata. The main header is protected by
a signature in the signature header. The payload is protected either by
a signature in the signature header or by a SHA-256 hash located in the
main header. The ability to distinguish between these two headers is
available to librpm internals but not to external librpm users.

A malformed package may contain a signature in the main header instead
of the signature header. Librpm will reject such a package only if a
strict signature check was requested. Otherwise, it will treat the
package as unsigned. DNF, on the other hand, has no way to check whether
the signature was in the correct header.  It will load the package and,
seeing a signature, will assume that it was verified by librpm. This
allows for bypassing package signature verification.

The other bugs (CVE-2021-20266, CVE-2021-3421) concern incorrect parsing
of the signature header (which, while it contains the signature, is
itself unsigned). These bugs lead either to crashing or to corrupting
the RPM database (if such a malformed package is installed).

While Fedora will release its own patches in due course, we apply a
mitigation that prevents the privilege escalation aspect of these
issues. We configure RPM to always verify package signatures, even if a
higher level package manager (like DNF) does not explicitly request it.
This way, bypassing the signature check in DNF is not enough to
compromise an entire TemplateVM. Note that this change also prevents the
installation of unsigned RPM packages, even when explicitly requested.
See the "Side effects" section below.

For the dom0 aspect of these issues in Qubes 4.0, we update RPM to a
version that is not vulnerable. We have decided to update to the next
major version of RPM (from 4.13 to 4.14). This is because, besides the
security fix itself (which could be backported), version 4.14 has
significantly improved integrity verification. In 4.14, the macro
_pkgverify_level can be used to require that all packages be signed by a
trusted key.  It defaults to "digest", meaning that only checksums must
be present, but we have set it to "all", requiring that all packages be
signed.  Additionally, the checks performed before importing a package
have been significantly enhanced, which substantially reduces the attack
surface prior to integrity verification.

In the near future, we will also deploy an extra tool to perform
preliminary validation of all RPM packages in dom0 before handing them
over to RPM.


Impact
=======

The impact differs between Fedora templates and dom0:

1. For Fedora templates, an attacker who controls packages that the user
   downloads can completely bypass package signature verification and
   fully compromise Fedora templates.

2. For dom0, an attacker who controls packages that the user downloads
   can corrupt the RPM database and (almost silently) prevent further
   updates of select packages.

The attack requires control over the contents of downloaded packages.
This requirement differs slightly between Fedora templates and dom0:

1. For Fedora templates, the attacker would either have to
   a. compromise the Fedora infrastructure that is serving updates or
   b. perform a man-in-the-middle attack on the HTTPS connection used to
      download the repository metadata (which contains package hashes).

2. For dom0, the attacker would either have to attack the user's
   repository access (as in the Fedora template case) or compromise the
   UpdateVM (sys-firewall in the default configuration).


Side effects
=============

The mitigation forces signature verification in RPM regardless of other
options. This means that RPM will refuse to install packages that are
unsigned (or signed with an untrusted signature), even when explicitly
requested to do so. This breaks use cases such as installing
locally-built packages and installing manually-downloaded packages the
integrity of which was verified separately (which is often the case for
closed-source software).

In such cases, neither `dnf install /path/to/the/package.rpm` nor `rpm
-i /path/to/the/package.rpm` will work any longer. Instead, to install a
package without a trusted signature (that has been verified by other
means), use the following command:

    rpm --define '_pkgverify_level digest' -i /path/to/the/package.rpm

If the package has any dependencies, the above command will list them,
and they will have to be installed with `dnf` manually.


Credits
========

These issues were discovered and reported by Demi M. Obenour.


References
===========

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1934125
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1927747
[3] https://bugzilla.redhat.com/show_bug.cgi?id=1927741
[4] https://www.qubes-os.org/doc/updating-qubes-os/

--
The Qubes Security Team
https://www.qubes-os.org/security/