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iBoot

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iBoot
Developer(s)Apple Inc.
Operating systemDarwin, macOS,[1] iPadOS and iOS[2]
Platformx86, ARM
TypeBoot loader
LicenseProprietary software

iBoot is the stage 2 bootloader for all Apple products.[3] It replaces the older EFI-based bootloader on Intel-based Macs. Compared with its predecessor, iBoot improves authentication performed in the boot chain.[2]

For x86-based Macs, the boot process starts by running code stored in secured UEFI boot ROM (stage 1). The boot ROM has two primary responsibilities: to initialize system hardware and to select an operating system to run (the POST and UEFI component). For ARM-based Macs, the boot ROM does not include UEFI.[4]

For iPhones, iPads and ARM-based Macs, the boot process starts by running the device's boot ROM. The boot ROM loads the Low-Level Bootloader (LLB), which is the stage 1 bootloader and loads iBoot. If all goes well, iBoot will then proceed to load the iOS, iPadOS or macOS kernel as well as the rest of the operating system.[5][6] If the iBoot fails to load or fails to verify iOS, iPadOS or macOS, the bootloader jumps to DFU (Device Firmware Update)[7] mode; otherwise it loads the remaining kernel modules.[2] Since Apple A7 and Apple M1, the LLB is stored on NAND flash of iPhone or iPad, or SSD of Apple Silicon Mac.

On x86 Macs, iBoot is located in /System/Library/CoreServices/boot.efi.[8] Once the kernel and all drivers necessary for booting are loaded, the boot loader starts the kernel’s initialization procedure. At this point, enough drivers are loaded for the kernel to find the root device.[9]

Features

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For iBoot, it features a command prompt when in recovery, DFU, or restore mode (it's also in "DEBUG" builds of iBoot, but was never seen in future builds). Command availability depends on the type of iBoot being used, especially the build style.

When using iBoot's command prompt, you use the included commands to manage the behaviour, such as its boot arguments (internally called the "boot-args" in the NVRAM), or if the startup command (fsboot) should be used when iBoot is automatically loaded (known as auto-boot).[10][11]

Memory safety

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Apple has modified the C compiler toolchain that is used to build iBoot in order to advance memory safety since iOS 14. This advancement is designed to mitigate entire classes of common memory corruption vulnerabilities such as buffer overflows, heap exploitations, type confusion vulnerabilities, and use-after-free attacks. These modifications can potentially prevent attackers from successfully escalating their privileges to run malicious code, such as an attack involving arbitrary code execution.[12]

Source code leak incident

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In 2018, a portion of iBoot source code for iOS 9 was leaked on GitHub,[13] Apple then issued a copyright takedown request (DMCA) to GitHub to remove the repository. It was believed an Apple employee was responsible for the leak. However, this was not confirmed by Apple.

iBoot-87.1, the earliest known version of iBoot running on production hardware over serial. Screenshot by mcg29 on X.

History

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The earliest known version of iBoot was iBoot-87.1, seen on very early prototypes during the iPhone's production in 2006-2007. It had the same features as the first known version of iBoot (iBoot-99), except it not having features before the final release. This version of iBoot could be considered the "first beta" of iBoot. [14]

References

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  1. ^ "Darwin 9.2 Source Code". Apple Inc. Archived from the original on September 21, 2020. Retrieved January 19, 2020.
  2. ^ a b c Ryan, Peter Y. A.; Naccache, David; Quisquater, Jean-Jacques (2016-03-17). The New Codebreakers: Essays Dedicated to David Kahn on the Occasion of His 85th Birthday. Springer. ISBN 9783662493014.
  3. ^ Hayes, Darren R. (2014-12-17). A Practical Guide to Computer Forensics Investigations. Pearson IT Certification. ISBN 9780132756150.
  4. ^ "boot process for T2, M1, and iOS devices".
  5. ^ Apple Inc. (May 2016). "iOS Security Guide" (PDF). apple.com. Archived (PDF) from the original on February 27, 2016.
  6. ^ "Boot process for a Mac with Apple silicon - Apple Support". Jan 2024.
  7. ^ "iFixit Support: DFU Restore". iFixit. Retrieved 2019-09-29.
  8. ^ "rEFIt - The Intel Mac boot process". refit.sourceforge.net. Retrieved 2017-08-26.
  9. ^ "The Early Boot Process". developer.apple.com. Retrieved 2017-08-26.
  10. ^ "iRecovery on GitHub".
  11. ^ "iBoot information from the Apple Wiki".
  12. ^ "Memory safe iBoot implementation". Apple Platform Security. Apple. Retrieved 25 January 2023.
  13. ^ "Apple confirms iPhone source code leak". BBC News. 9 February 2018.
  14. ^ "iBoot-87.1 on the iPhone 2G by mcg29 on Twitter". 6 March 2024.
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