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, the technology
Secure Boot[^spec] is an attempt at ensuring that only authorized software is run on a Secure Boot[^spec] helps running only authorized software on a machine. It
machine. It does so by bootstrapping the security of the system at boot time, does so by bootstrapping the security of the system at boot time, by verifying
giving way to other technologies to keep the system secure, later on. Secure signatures on various software components, before giving way to other
boot works by authorizing only select executables to be run. Authorized technologies to keep the system secure, later on. Secure boot works by
executables are signed using public cryptography, and the keys used to verify authorizing only select executables to be run. Authorized executables are
those signatures are stored securely in UEFI "databases". 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) [^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 shuts down all UEFI Boot Services, before dropping its privileges and then
doing "Linux stuff" (like starting the userland part of the operating system). 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 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 past that point is able to tamper with the UEFI sensitive information,
variables, which contains sensitive data. including UEFI variables, which contains sensitive data.
Secure Boot aims at securing "everything" that is executed prior to that 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 privilege drop. Once the privileges are drop, Secure Boot is done and it is up
to ensure that only authorized software is run. Most Linux distros do not even to the operating system to extend that integrity/authenticity protection and
try to do it, although there are notable exceptions (Chrome OS, Android, just ensure that only authorized software is run. Most Linux distros do not even try
to name a few). If we run one of the distros that do not leverage technologies 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 such as dm-verity, fs-verity (with signatures) or Linux IMA (Integrity
Management Architecture), then Secure Boot is strictly insufficient to protect Management Architecture), then Secure Boot protected the boot integrity for
user data integrity or even system integrity in general. 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 Nevertheless, ANSSI, the French Infosec Agency recommends in its GNU/Linux
security guide[^linuxguide] to enable Secure Boot (R3) for all systems 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 Secure boot relies on a set of public keys to verify authorized software
authenticity. By default, most vendors ship Microsoft public keys. These keys authenticity. By default, most vendors ship Microsoft public keys. These keys
sign all Microsoft Windows version, of course, but to avoid a monopoly, other 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 executables were signed. The list is ought to be short (and unfortunately, it
and authorized software, the attack surface grows and the probability of a is not) because with each signature and authorized software, the attack surface
vulnerability raises. Several were already found in the recent past (e.g. grows and the probability of a vulnerability raises. Several were already found
CVE-2020-10713[^CVE-2020-10713], CVE-2022-34301[^CVE-2022-34301], in the recent past (e.g. CVE-2020-10713[^CVE-2020-10713],
CVE-2022-34302[^CVE-2022-34302], CVE-2022-34303[^CVE-2022-34303]. 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) [^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) [^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 require for each and every single version to be signed individually by
Microsoft. Instead, a binary called Shim[^shim] was developed. This rather Microsoft. Instead, a binary called Shim[^shim] was developed. This rather
small, auditable, innocuous-looking program is signed by Microsoft. Its role is small, auditable, innocuous-looking program is signed by Microsoft. Its role is
basically that of a trojan horse. Indeed, its only purpose is to basically that of a trojan horse (or a security pivot, depending on the way you
cryptographically verify the authenticity of any executable. However, this look at it). Indeed, its only purpose is to cryptographically verify the
time, the list of public keys used to verify these executables is not directly authenticity of any executable. However, this time, the list of public keys
under the control of Microsoft. These public keys are either built in Shim used to verify these executables is not directly under the control of
itself or stored in a EFI variable serving as a database. 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) [^shim]: [shim source code](https://github.com/rhboot/shim)