improve secure boot article after review #3

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fmaury merged 2 commits from secure_boot_2 into master 2022-12-16 16:09:06 +00:00
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@ -30,12 +30,13 @@ software.
## Secure Boot, the technology
Secure Boot[^spec] is an attempt at ensuring that only authorized software is run on a
machine. It does so by bootstrapping the security of the system at boot time,
giving way to other technologies to keep the system secure, later on. Secure
boot works by authorizing only select executables to be run. Authorized
executables are signed using public cryptography, and the keys used to verify
those signatures are stored securely in UEFI "databases".
Secure Boot[^spec] helps running only authorized software on a machine. It
does so by bootstrapping the security of the system at boot time, by verifying
signatures on various software components, before giving way to other
technologies to keep the system secure, later on. Secure boot works by
authorizing only select executables to be run. Authorized executables are
signed using public cryptography, and the keys used to verify those signatures
are stored securely in UEFI "databases".
[^spec]: [UEFI Specifications](https://uefi.org/specs/access)
@ -48,17 +49,19 @@ the case of GNU/Systemd/Linux, the bootloader runs the Linux kernel, which
shuts down all UEFI Boot Services, before dropping its privileges and then
doing "Linux stuff" (like starting the userland part of the operating system).
This privilege drop is very important because this is what ensures that no code
past that point is able to tamper with the UEFI environment, including UEFI
variables, which contains sensitive data.
past that point is able to tamper with the UEFI sensitive information,
including UEFI variables, which contains sensitive data.
Secure Boot aims at securing "everything" that is executed prior to that
privilege drop. Once the privileges are drop, it is up to the operating system
to ensure that only authorized software is run. Most Linux distros do not even
try to do it, although there are notable exceptions (Chrome OS, Android, just
to name a few). If we run one of the distros that do not leverage technologies
privilege drop. Once the privileges are drop, Secure Boot is done and it is up
to the operating system to extend that integrity/authenticity protection and
ensure that only authorized software is run. Most Linux distros do not even try
to do it, although there are notable exceptions (Chrome OS, Android, just to
name a few). If we run one of the distros that do not leverage technologies
such as dm-verity, fs-verity (with signatures) or Linux IMA (Integrity
Management Architecture), then Secure Boot is strictly insufficient to protect
user data integrity or even system integrity in general.
Management Architecture), then Secure Boot protected the boot integrity for
basically nothing, because the security chain is broken by the operating
system, and user data is at risk from possibly any tainted userland executable.
Nevertheless, ANSSI, the French Infosec Agency recommends in its GNU/Linux
security guide[^linuxguide] to enable Secure Boot (R3) for all systems
@ -88,11 +91,12 @@ attempts to prove that it is not only useless but incoherent and misleading.
Secure boot relies on a set of public keys to verify authorized software
authenticity. By default, most vendors ship Microsoft public keys. These keys
sign all Microsoft Windows version, of course, but to avoid a monopoly, other
executables were signed. The list is rather short because with each signature
and authorized software, the attack surface grows and the probability of a
vulnerability raises. Several were already found in the recent past (e.g.
CVE-2020-10713[^CVE-2020-10713], CVE-2022-34301[^CVE-2022-34301],
CVE-2022-34302[^CVE-2022-34302], CVE-2022-34303[^CVE-2022-34303].
executables were signed. The list is ought to be short (and unfortunately, it
is not) because with each signature and authorized software, the attack surface
grows and the probability of a vulnerability raises. Several were already found
in the recent past (e.g. CVE-2020-10713[^CVE-2020-10713],
CVE-2022-34301[^CVE-2022-34301], CVE-2022-34302[^CVE-2022-34302],
CVE-2022-34303[^CVE-2022-34303].
[^CVE-2020-10713]: [CVE-2020-10713](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-10713)
@ -110,15 +114,16 @@ Intel[^intelUpgrade] or Lenovo[^lenovoUpgrade].
[^lenovoUpgrade]: [Lenovo flash BIOS with UEFI tool](https://support.lenovo.com/us/en/solutions/ht118103-flash-bios-with-uefi-tool-ideacentre-stick-300)
Grub and the kernels are not directly authorized by Microsoft, as it would
Grub and the kernels are not directly authorized by Microsoft, as it would
require for each and every single version to be signed individually by
Microsoft. Instead, a binary called Shim[^shim] was developed. This rather
small, auditable, innocuous-looking program is signed by Microsoft. Its role is
basically that of a trojan horse. Indeed, its only purpose is to
cryptographically verify the authenticity of any executable. However, this
time, the list of public keys used to verify these executables is not directly
under the control of Microsoft. These public keys are either built in Shim
itself or stored in a EFI variable serving as a database.
basically that of a trojan horse (or a security pivot, depending on the way you
look at it). Indeed, its only purpose is to cryptographically verify the
authenticity of any executable. However, this time, the list of public keys
used to verify these executables is not directly under the control of
Microsoft. These public keys are either built in Shim itself or stored in a EFI
variable serving as a database.
[^shim]: [shim source code](https://github.com/rhboot/shim)