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Update docs/ima.md

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Co-authored-by: Jonathan Lebon <jonathan@jlebon.com>
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Colin Walters 2022-04-21 16:19:14 -04:00
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@ -25,14 +25,17 @@ files would (depending on policy) not be readable or executable.
## IMA signatures and OSTree checksum ## IMA signatures and OSTree checksum
Mechanically, IMA signatures appear as a `security.ima` extended attribute Mechanically, IMA signatures appear as a `security.ima` extended attribute
on the file. This is a signed digest of just the file content. on the file. This is a signed digest of just the file content (and not
any metadata)
OSTree has first-class support for extended attributes; OSTree's checksums in contrast include not just the file content, but also
they are included in the object digest along with other key file attributes metadata such as uid, gid and mode and extended attributes;
such as uid, gid and mode.
Together, this means that adding an IMA signature to a file in the OSTree Together, this means that adding an IMA signature to a file in the OSTree
model appears as a new object (with a new digest). model appears as a new object (with a new digest). A nice property is that
this enables the transactional addition (or removal) of IMA signatures.
However, adding IMA signatures to files that were previously unsigned
also today duplicates disk space.
## Signing ## Signing
@ -51,7 +54,7 @@ page; we will not replicate it here.
- An OSTree repository (could be any mode; `archive` or e.g. `bare-user`) - An OSTree repository (could be any mode; `archive` or e.g. `bare-user`)
- A ref or commit digest (e.g. `exampleos/x86_64/stable`) - A ref or commit digest (e.g. `exampleos/x86_64/stable`)
- A digest algorthim (usually `sha256`, but you may use e.g. `sha512` as well) - A digest algorithm (usually `sha256`, but you may use e.g. `sha512` as well)
- An RSA private key - An RSA private key
You can then add IMA signatures to all regular files in the commit: You can then add IMA signatures to all regular files in the commit:
@ -79,8 +82,9 @@ The EVM subsystem builds on IMA, and adds another signature which
covers most file data, such as the uid, gid and mode and selected covers most file data, such as the uid, gid and mode and selected
security-relevant extended attributes. security-relevant extended attributes.
If you've been following along, note this is very, very close to what ostree This is quite close to the ostree native checksum - the ordering
checksums as well! of the fields is different so the checksums are physically different, but
logically they are very close.
However, the focus of the EVM design seems to mostly However, the focus of the EVM design seems to mostly
be on machine-specific signatures with keys stored in a TPM. be on machine-specific signatures with keys stored in a TPM.
@ -106,6 +110,5 @@ signing commits with these "portable" EVM signatures in addition to IMA.
- https://fedoraproject.org/wiki/Changes/Signed_RPM_Contents - https://fedoraproject.org/wiki/Changes/Signed_RPM_Contents
- https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/managing_monitoring_and_updating_the_kernel/enhancing-security-with-the-kernel-integrity-subsystem_managing-monitoring-and-updating-the-kernel - https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/managing_monitoring_and_updating_the_kernel/enhancing-security-with-the-kernel-integrity-subsystem_managing-monitoring-and-updating-the-kernel
<!-- SPDX-License-Identifier: (CC-BY-SA-3.0 OR GFDL-1.3-or-later) -->
###### Licensing for this document:
`SPDX-License-Identifier: (CC-BY-SA-3.0 OR GFDL-1.3-or-later)`