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https://git.proxmox.com/git/efi-boot-shim
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It works like this: during startup of shim, we hook into the system's ExitBootServices() and StartImage(). If the system's StartImage() is called, we automatically unhook, because we're chainloading to something the system can verify. When shim's verify is called, we record what kind of certificate the image was verified against. If the call /succeeds/, we remove our hooks. If ExitBootServices() is called, we check how the bootloader verified whatever it is loading. If it was verified by its hash, we unhook everything and call the system's EBS(). If it was verified by certificate, we check if it has called shim_verify(). If it has, we unhook everything and call the system's EBS() If the bootloader has not verified anything, and is itself verified by a certificate, we display a security violation warning and halt the machine. |
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|---|---|---|
| Cryptlib | ||
| include | ||
| lib | ||
| .gitignore | ||
| cert.S | ||
| console_control.h | ||
| COPYRIGHT | ||
| crypt_blowfish.c | ||
| crypt_blowfish.h | ||
| elf_ia32_efi.lds | ||
| elf_ia64_efi.lds | ||
| elf_x86_64_efi.lds | ||
| fallback.c | ||
| make-certs | ||
| Makefile | ||
| MokManager.c | ||
| MokVars.txt | ||
| netboot.c | ||
| netboot.h | ||
| PasswordCrypt.c | ||
| PasswordCrypt.h | ||
| README | ||
| replacements.c | ||
| replacements.h | ||
| shim.c | ||
| shim.h | ||
| TODO | ||
| ucs2.h | ||
shim is a trivial EFI application that, when run, attempts to open and execute another application. It will initially attempt to do this via the standard EFI LoadImage() and StartImage() calls. If these fail (because secure boot is enabled and the binary is not signed with an appropriate key, for instance) it will then validate the binary against a built-in certificate. If this succeeds and if the binary or signing key are not blacklisted then shim will relocate and execute the binary. shim will also install a protocol which permits the second-stage bootloader to perform similar binary validation. This protocol has a GUID as described in the shim.h header file and provides a single entry point. On 64-bit systems this entry point expects to be called with SysV ABI rather than MSABI, and so calls to it should not be wrapped. To use shim, simply place a hex dump of the public certificate in cert.h and build it with make.