path: root/lectures/01-memory-management-and-booting.org

#+title: Memory Management and Booting
#+date: 2026-03-02 Mon
#+author: W. Kosior
#+email: wkosior@agh.edu.pl

* A Sample Program
#+begin_src C
  #include <stdio.h>

  int main(int argc, char * * argv) {
    unsigned char * memory = (char *) 0;

    for (int i = 0; i < 100; i++) {
      for (int j = 0; j < 20; j++) {
        printf("%hhx ", memory[20 * i + j]);
      }
      putchar('\n');
    }

    return 0;
  }
#+end_src

* Virtual Memory
| address process uses | real address accessed |
|----------------------+-----------------------|
| 76bd91485000         | 1030ba                |
| 76bd91486000         | 1030b9                |
| 76bd91487000         | 10347b                |
| 76bd91488000         | 1030b8                |
| 76bd91489000         | 44e2c1                |
| 76bd9148a000         | 40e5ff                |
| 76bd9148b000         | 430272                |
| 76bd9148c000         | 42848a                |

* Mapping of Physical Memory Pages
- hardware-backed
- paging set up by kernel
- *per-process* address space
- used by all modern non-embedded OSes
- not the only address translation in a typical computer system

* MMU
- required* by Linux
- present in x86, ARM, MIPS, PowerPC, etc. processors meant "for
    application-level computing"
- no MMU in, e.g., ARM Cortex-M
- *MPU* instead of MMU in ARM Cortex-R

* Other Restrictrions to Non-Kernel Code
- privileged CPU instructions, IO
- rings 0-3 in x86
- returning control to kernel (sw interrupts)

* Privilege Levels in x86 and ARM
|           | x86    | ARM 7 | ARM 8 |
|-----------+--------+-------+-------|
| OS kernel | ring 0 | PL1   | EL1   |
|           | ring 1 |       |       |
|           | ring 2 |       |       |
| userspace | ring 3 | PL0   | EL0   |

* Software in a Computer
- code typically on hard drive
  - bootloader
  - OS kernel & userspace apps
- code typically on flash memory chip
  - firmware (BIOS)
    - services provided to other code

* Privilege Levels in x86 and ARM (corrected)
|           | x86    | ARM 7 | ARM 8 |
|-----------+--------+-------+-------|
| firmware  | SMM    |       | EL3   |
| OS kernel | ring 0 | PL1   | EL1   |
|           | ring 1 |       |       |
|           | ring 2 |       |       |
| userspace | ring 3 | PL0   | EL0   |

* Software in a Computer, cont.
- code typically on hard drive
  - hypervisor
- code typically on flash memory chip
  - EC ← not very interesting in our case
  - microcode
  - Intel ME
  - AMD PSP
  - Google Titan M2

* Privilege Levels in x86 and ARM (corrected)
|                      | x86               | ARM 7 | ARM 8  |
|----------------------+-------------------+-------+--------|
| "security" subsystem |                varies              |
| firmware             | SMM               |       | EL3    |
| hypervisor           | Intel VT-x, AMD-V | PL2   | EL2    |
| OS kernel            | ring 0            | PL1   | EL1    |
|                      | ring 1            |       |        |
|                      | ring 2            |       |        |
| userspace            | ring 3            | PL0   | EL0    |

* About "Subsystems" and "Coprocessors"
- some execeute on separate SoCs
  - often with a different instruction set
- some do have full control over the system
- some do not

* System Startup in x86 (Legacy)
- BIOS initializes RAM
- BIOS initializes other peripherals
- boot sequence
  - initial 512 bytes of drive's data loaded
  - boot signature presence check
  - initial MBR bytes executed as code
    - 16-bit real mode
  - bootstrap code loads the bootloader
  - bootloader loads the OS kernel
    - use of ramdisks
    - paging gets set up by the kernel

* UEFI
- "Unified Extensible Firmware Interface"
- replacement for the "Legacy BIOS"
- developed from Intel's EFI
- open standard
- freely-licensed and proprietary implementations
  - TianoCore

* System Startup in x86 (UEFI)
- BIOS initializes RAM
- BIOS initializes other peripherals
- boot sequence
  - EFI System Partition
    - FAT 12/16/32
  - .efi executables
    - PE format :(
    - bootloader → .efi
    - OS kernel → .efi
- ties to GPT partitioning

* UEFI BIOS Services
- EFI variables
  - name+GUID+value
  - typically in NVRAM
  - configuration (e.g., boot order)
- network stack
- more

* EFI Variables
- types
  - bootservice access
  - runtime access
  - "secure"
  - volatile / nonvolatile
- =/sys/firmware/efi/efivars=

* Signing
- X.509
  - standard for certs
  - TLS
  - certificates, signatures, key exchange
- PKCS #7
  - standard and format for certs, keys, etc.
  - can be used as a wrapper for X.509
- DER, PEM
- PE binaries
- PGP
  - GPG — a PGP implementation
  - private and public keys

* Secure Boot
- assumption: all ring 0 code signed
  - stop ring 0 rootkits
  - securing network booting
  - an attacker with hard drive access
  - what about, e.g., malicious non-kernel code?
- verification: UEFI BIOS → bootloader → kernel → modules
- root public key held by BIOS

* Keys, Certs and Hashes
- EFI variables
  - "secure" — not normally writable by an OS
  - can be made writable in some scenarios
- PK → KEK (1 or more cert) → db & dbx
- db: certs *&* hashes

* EFI Verification
- PE binary verified with cert from db: OK
- PE binary with hash in db: OK
- PE binary with hash in dbx: not OK
- on mainstream systems:
  - motherboard vendor's cert in PK
  - MS cert in KEK
  - MS cert in db
  - Windows bootloader signed by known key

* We Want Other OSes!
Go into BIOS settings, enroll your own PK, KEK and db.

- *usually* doable
- we'll go through that as well
- not needed for popular distributions

* shim
- an EFI application
- can load other EFIs
  - verification
  - GRUB, etc.
- gives machine owner control
  - Machine Owner Key
- binaries in many GNU+Linux distros
  - signed by Microsoft
    - *most* hardware respects it
  - often built reproducibly

* MokList EFI Variable
- managed and used by shim
  - made inaccessible when OS is booted
  - contents exposed in MokListRT
- holds certs/hashes added be hardware *owner*
  - owner-created certs
  - OS distribution certs
- modification → physical access needed

* Secure Boot with Debian, Ubuntu, Fedora, etc.
- shim → EFI application → bootloader → OS kernel
  - GRUB *still* the most popular
- when software stack follows the Secure Boot concepts
  - bootloader verifies kernel
  - kernel verifies its modules
  - SB → Linux kernel lockdown

* Post-shim Verification
- GRUB
  - typically signed by distro
  - can call back to shim to verify kernel using MokList
  - more on GRUB later ;)
- Linux kernel
  - uses (signed) modules
  - in-tree modules
    - the "official" ones
    - public key in the kernel itself
  - out-of-tree modules
    - proprietary NVidia, etc.

* Secure Boot and Out-of-Tree Modules
- public key cannot be stored on-disk
- public key → PE file → verified using MokList
  - patch rejected by Linus Torvalds…
  - …yet applied by some distros

* Lockdown Mode
- Secure Boot → automatically enabled
- kernel mandates module verification
- =/dev/mem=
- hibernation
- more…
- SysRq+x

* Other Systems / Configurations
- Das U-Boot
  - many ARM boards
  - has "Verified Boot"
  - UEFI & Secure Boot also available
- Coreboot
  - includes a payload in firmware
  - GRUB as payload
    - verification with PGP (public key in GRUB image)
  - Depthcharge as payload (Chromebooks)
    - verification present
  - TianoCore as payload
    - UEFI & Secure Boot as described

* Dangers
- does hw ship with MS cert for shim?
  - if not, does hw allow replacing the entire key chain?
    - "Restricted Boots™® It's for your safety!"
- are cert dates checked?
  - key expiration