9/25/2023 0 Comments Systemrescuecd uefi create![]() ![]() Super UEFIinSecureBoot Disk is a bootable image with GRUB2 bootloader designed to be used as a base for recovery USB flash drives. ![]() This is how Super UEFIinSecureBoot Disk has been made. Let's modify PreLoader by removing all unnecessary features and patch verification code to allow everything.ĭisk architecture is as follows: _ _ _ The first method does not provide this, allowing only GRUB to execute arbitrary files. The second method is preferable as executed software can load and start another software, for example, UEFI shell can execute any program. Use custom pre-loader (the second one) which hook UEFI file vertification functions (EFI_SECURITY_ARCH_PROTOCOL.FileAuthenticationState, EFI_SECURITY2_ARCH_PROTOCOL.FileAuthentication).Use modded GRUB with internal EFI loader, without digital signature vertification or module restrictions.There are two ways to create a universal bootable flash drive that would not require adding the keys of each executable file to the trusted files: It's not exactly clear why this method is preferable-UEFI allows one to redefine (hook) UEFI verification functions, this is how PreLoader works, and indeed the very shim feature is present but disabled by default.Īnyway, using the signed GRUB from some Linux distribution does not suit our needs. efi (PE) loader without using the UEFI LoadImage/StartImage functions, as well as the validation code of the loaded files via shim, in order to preserve the ability to load files trusted by shim but not trusted in terms of UEFI. efi-files, introduced its own custom internal. ![]() ![]() The chainloader module, which loads arbitrary. GRUB2To prevent signed bootloader abuse with malicious intentions, Red Hat created patches for GRUB2 that block «dangerous» functions when Secure Boot is enabled: insmod/rmmod, appleloader, linux (replaced by linuxefi), multiboot, xnu, memrw, iorw. In general, shim is used to run GRUB2 - the most popular bootloader in Linux. Untrusted software first boot with shim.Īll modern popular Linux distributions use shim due to certificate support, which makes it easy to provide updates for the main bootloader without the need for user interaction. Files become trusted only for these pre-loaders, not for Secure Boot in general, and still couldn't be loaded without PreLoader or shim. When executed for the first time, you need to select a certificate to be added or the file to be hashed in the graphical interface, after which the data is added into a special NVRAM variable on the motherboard which is not accessible from the loaded operating system. PreLoader and shim do not use UEFI db certificate store, but contain a database of allowed hashes (PreLoader) or certificates (shim) inside the executable file.īoth programs, in addition to automatically executing trusted files, allow you to run any previously untrusted programs in Secure Boot mode, but require the physical presence of the user. To address this issue, Linux Foundation released PreLoader and Matthew Garrett made shim-small bootloaders that verify the signature or hash of a single file and execute it. Microsoft forbid to sign software licensed under GPLv3 because of tivoization restriction license rule, therefore GRUB cannot be signed. Signed bootloaders of bootloadersSo, to boot Linux with Secure Boot enabled, you need a signed bootloader. I wanted to make a bootable USB flash drive with various computer recovery software that would boot without disabling Secure Boot. Linux distributions, hypervisors, antivirus boot disks, computer recovery software authors all have to sign their bootloaders in Microsoft. This process include code audit procedure and justification for the need to sign their file with globally trusted key if they want the disk or USB flash to work in Secure Boot mode without adding their key on each computer manually. Most motherboards include only Microsoft keys as trusted, which forces bootable software vendors to ask Microsoft to sign their bootloaders. It is necessary to enter UEFI settings when the computer boots, and only then it's possible to change Secure Boot settings. Secure Boot can be disabled on any retail motherboard, but a mandatory requirement for changing its state is physical presence of the user at the computer. Secure Boot prevents the execution of unsigned or untrusted program code (.efi programs and operating system boot loaders, additional hardware firmware like video card and network adapter OPROMs). In 2013, a new technology called Secure Boot appeared, intended to prevent bootkits from being installed and run. Modern PC motherboards' firmware follow UEFI specification since 2010. ![]()
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