F37 Change: Signed RPM Contents (System-Wide Change proposal)

Friday, 1 April 2022

https://fedoraproject.org/wiki/Changes/Signed_RPM_Contents

== Summary ==
We want to add signatures to individual files that are part of shipped RPMs.

These signatures will use the Linux IMA (Integrity Measurement
Architecture) scheme, which means they can be used to enforce runtime
policies to ensure execution of only trusted files.

== Owner ==
* Name: [[User:Pbrobinson| Peter Robinson]]
* Email: pbrobinson(a)gmail.com
* Name: [[User:Puiterwijk| Patrick Uiterwijk]]
* Email: puiterwijk(a)redhat.com

== Detailed Description ==

During signing builds, the files in it will be signed with IMA signatures.
These signatures will be made with a key that's kept by the Fedora
Infrastructure team, and installed on the sign vaults.

These signature can then be used with the Linux Integrity Measurement
Architecture (IMA) kernel subsystem to verify files on execution based
on a policy.

The IMA subsystem is described on
[https://sourceforge.net/p/linux-ima/wiki/Home/ the project page].

IMA allows users to extend the trust of their system to the OS and
processes. It allows the users, if they so wish, to set polices to
ensure their machine and their resources are used the way they
intended it to be not about restricting the use for the average Fedora
user.

Like all security pieces IMA doesn't solve the whole security problem
with a single option, it's not intended to, but that does not mean it
doesn't provide additional value and tools for Fedora users to protect
their systems. To quote a section of [https://lwn.net/Articles/753276/
an LWN article]:

<pre>
The goals of the integrity subsystem are to detect files that have
been altered, either accidentally or maliciously, by comparing a
measurement of the current file contents with that of a stored "good"
value; it can then enforce various file integrity policies (e.g. no
access or no execution). IMA is three separate pieces: measurement,
which calculates a hash of file contents; appraisal, which verifies
file signatures made using the measured hashes; and audit, which
records hashes and other information in the audit logs. There is also
the extended verification module (EVM), which targets the measurement
and protection of the file metadata, though that is not included in
what she would be presenting.

It is important to note that IMA does not protect against attacks on
objects in memory, it can only be used to thwart attacks that change
files.
</pre>

The intention here is not to ship a default policies for users but
rather have sample policies that users can modify and use themselves.
The Fedora IoT Edition intends to have sample policies and
documentation for a number of IoT and Edge use cases.

This means that they can configure a policy based on which the kernel
will determine whether to verify (or measure) files before opening
them.
You could for example make a policy that appraises (verifies) all
files that are executed by root: `appraise uid=1000
appraise_type=imasig`.

Note explicitly that we do not intend to install a default policy as
part of this change, and users will need to deploy their own policy
before anything is measured or appraised.
This means that after this is done, users will have the option to
enable a policy and have that be enforced, but there will be nothing
automatic.
We will, however, document various example policies people can adapt
to their needs.

By default, the signatures will not be deployed to the file system.
That will only be done once rpm-plugin-ima is installed.
After that, RPM will put the signatures on the "security.ima" extended
attribute on the files.

== Feedback ==

=== RPM Size ===
One of the main concerns that have been voiced is the RPM size, both
on disk (mirrors) and on an installed system.
For this comparison, I have cached all the RPMs installed in a Fedora
Rawhide 20210118.n.1 default Server install (server netinstall disk,
and then no changes to the group selection).
After creating two copies of that data, one with just resigned (to
exclude rpm size difference resulting from different key lengths), and
one resigned with IMA file signatures inserted, I then installed two
blank VMs by using the standard virt-manager settings, changing only
the name and the system type to "EFI".

This is using a prime256v1 file signing key. This is the same key
format supported by the Fedora signing system.

==== Binary RPMs on disk ====
Resigned: 462524  (452M)

Resigned+IMA: 467812  (457M)

This comes down to a 1.1% increase on size of the binary RPMs.

==== Installed size ====
On installation of two different VMs, one with the resigned RPMs, and
one with the resigned+ima RPMs, the `/usr` directory size does not
change at all (both are exactly 1417064 bytes).

The size of the rpmdb increases from 22952 to 28416 bytes, a 20% increase.
This is on an install size of 1.7GB in total, so this 5MB increase is
a 0.3% size increase on the final installed system.

Note that both of those VMs did not have rpm-plugins-ima installed,
which means the file signatures are not put in place.

When I install the rpm-plugin-ima, and run "dnf reinstall *", the
`/usr` directory increases by 0.002% to 1417104.

==== RPM header size bug ====

There was a bug in rpm from when file signatures were moved over from
the header into the sigheader. Details
[https://pagure.io/fesco/issue/2547#comment-711253 here]. This bug is
fixed in Fedora and RHEL-8.

=== Tie-in to Secure Boot ===

While using the IMA subsystem in combination with Secure Boot enables
users to extend trust on the system out into the binaries executed,
there is no requirement at all to use secure boot.
This means that if you have Secure Boot enabled, that enables you to
extend the trust, but if you don't want to use secure boot, that has
no impact on the IMA subsystem.

=== Why in the RPM header, instead of a side-package like -debuginfo? ===

These signatures would need to get deployed as a filesystem extended
attribute in order to be used by the IMA subsystem.
While it is possible to generate the signatures as separate files in a
side package, that means that RPM would then need to add code to
install extended attributes based on contents of another RPM package.
This would mean that the contents of an e.g. `-file-signature`
subpackage won't be deployed to the file systems as normal files, but
instead as xattr's on existing files.

That, together with the minimal size increases of the actual RPMs,
made us decide to go with the signature header approach and not
implement the side-package method.

=== Comparison with MAC's ===

The IMA subsystem is orthogonal to the Mandatory Access Control
systems like selinux, though it is integrated with them.

Where with selinux you can limit who can read from or write to to
which file, IMA will allow you to set policies that enforce file
contents to be as expected (signature validated) before they can be
read/executed.

The integration is in the form that you can write a policy line that
matches on certain selinux contexts, like for example `dont_appraise
obj_type=var_log_t`, to ensure that files of `var_log_t` do not need
to be verified, and `appraise obj_type=shell_exec_t` to ensure that
any shell that is executed (e.g. bash) is signed with a trusted key.

=== Why not ... ===

==== fsverity ====

As one of the fsverity maintainers has said on
[https://lwn.net/Articles/842326/|LWN]: IMA and FS-Verity are not
mutually exclusive, but at this moment IMA has a possibility to
enforce a system-wide policy instead of needing the userspace binaries
to check the signature status themselves.

There's currently competing proposals for upstream kernel for
functionality like integration with IMA
[https://www.spinics.net/lists/linux-integrity/msg21056.html this one]
and [https://www.spinics.net/lists/linux-integrity/msg20947.html this
one]. The proposal also requires
[https://www.spinics.net/lists/linux-integrity/msg20713.html Adding
PGP support to the kernel] which isn't upstream and may take some
time.

The [https://fedoraproject.org/wiki/Changes/FsVerityRPM FsVerity]
feature also needs enhancements across Fedora including changes to
koji, additions to rpm and well as filesystems that support it. It's
also got limited filesystem support, for example it's not supported on
XFS which is the default on the Server Edition.

==== DIGLIM ====

The Digest Lists Integrity Module (DIGLIM) isn't actually independent
of this feature, it still requires digests of the files such as the
ones generated here in this proposal, it just consumes them in a
different method than IMA on the end system. Rather than embedding
them in xattr on the filesystem as IMA does the kernel loads them into
memory.

Ultimately DIGLIM is complementary and has the ability to build on top
of this functionality once this actually makes it upstream.

==== rpm -V ====

rpm -V will tell you whether a file matches the digest that's in the
RPM Database, but that is useful if you think a file might have been
accidentally changed. If an attacker has the opportunity to change a
file on the file system, it is very likely they could also update the
RPM binary (or the rpmdb) to make it not report the change.

IMA signatures will be able to identify the file to have been changed,
and importantly also enable a policy which enforces the signatures,
which means that if a change was made to a file that is matched by a
policy, the kernel would actively refuse loading it, instead of
depending on the administrator to check its validity with a known-good
rpm database and binary.

== Benefit to Fedora ==

Having all files signed with a verifiable key means that system owners
can use the kernel Integrity and Measurement Architecture (IMA) to
enforce only verified files can be executed, or define other policies.

Having all files signed with Fedora keys would enable integration with
for example [https://keylime.dev/ Keylime], which is a CNCF project
that implements remote system attestation, based on which a system may
or may not get access to secrets and other consequences.

This feature is wanted by the IoT Edition, for enabling both
attestation and local policy verification.
Many IoT users have expressed interest in this functionality and are
already working to build on it.

IMA doesn't solve all security issues but it provides an extra useful
tool to those that wish provide higher levels of security on their
devices. From other proposals such as the fsverity and DGLIM ones
there is clearly a desire to be able to provide this style of extra
security to Fedora users.

An advantage that this proposal has, that the others do not currently
provide, is that all the functionality is in place in koji, the
signing infrastructure, the various packages and kernel now and
doesn't depend on out of tree patches to get into various upstream
projects or work on Fedora infrastructure to be scheduled.

== Scope ==
* Proposal owners:
The proposal owners will generate the keys in Infrastructure and get
them deployed to the sign vaults, and enable the configuration options
for this in robosignatory.
Support for file signatures is already in the deployed versions of
sigul and robosignatory.

* Other developers:
Nothing needed from other developers

* Release engineering: [https://pagure.io/releng/issue/10731 #10731]
A mass rebuild would be nice (as it ensures all packages are signed),
but is not required to implement the change itself.

* Policies and guidelines:
No impact

* Trademark approval:
No impact

* Alignment with Objectives:
This aligns with the Internet of Things objective.

== Upgrade/compatibility impact ==
For standard Fedora users there will be a very tiny increase in rpmdb
size. If an advanced user was already signing their own files (for the
Fedora shipped RPMs) for IMA functionality, they will just overwrite
the existing signature.

== How To Test ==
You can verify that a signature has been put in place by looking at
the extended attribute by running: `getfattr -d -m security.ima
/usr/bin/bash` (change `/usr/bin/bash` with the file to check).

The signatures can be tested “in vitro” by running `evmctl ima_verify
--key publiccert.der -v myfile.txt`.
This should result in the system reporting “<filename>: verification is OK”.
The full system could be tested by enrolling the Fedora IMA key to the
kernel `_ima` keyring, and adding a policy that verifies (some) files
to be verified against the key.  (instructions to follow).

== User Experience ==
If the user deploys an IMA policy to verify all or some files, they
should be able to trust the signatures made by the Fedora build
system.

== Dependencies ==
No external package dependencies.

== Contingency Plan ==

* Contingency mechanism: None. The development work is already
complete and all the infrastructure functionality is upstream.
* Contingency deadline: The signing should be turned on before a mass
rebuild, and is ready to go if the change is approved.
* Blocks release? No
* Blocks product? N/A

== Documentation ==
We intend to write documentation on how to use the IMA subsystem for
docs.fedoraproject.org, but that is orthogonal to this feature itself.
We expect to provide example policies users can use to base their policy on.

== Release Notes ==

-- 
Ben Cotton
He / Him / His
Fedora Program Manager
Red Hat
TZ=America/Indiana/Indianapolis

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