TPM, UEFI,
Trusted Boot, Secure Boot
How Does a PC Boot?
• First: History
– http://en.wikipedia.org/wiki/Booting
– Code entered on Hand Switches
– Non-Volatile Memory; infrequent cold boot
– Development of ROM-based IPL
– PC developed multi-stage boot process
Power Up Sequence
• POST – Power On Self Test
• http://en.wikipedia.org/wiki/Power-on_self-test
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First code that runs
Part of the BIOS
Checks hardware to prevent boot failure
Runs BIOS/POST code in other devices
BIOS must know how to operate devices
Each BIOS crafted for each motherboard
Initial Program Load - IPL
• Original PC booted each device in fixed order
– Floppy, CD, HDD, Net
• Newer BIOS now can change boot order
• Loads boot code from device
– Secondary boot loader
– Stored in particular track of FDD/HDD
– Independent of software architecture
Boot Loader
• Knows about device architecture
• HDD – Partition tables, block size,
– File Structure: FAT16, FAT32, NTFS etc
• Finds the OS boot program
– MS Boot loader
• From a particular named file
– Linux loader
OS Loader
• What you see when the Operating System
Starts
• Launches and configures the OS for the
hardware
• Installs relevant device drivers
• Checks license and file system
• Gets network operating etc
Security
• BIOS is in Flash – can be changed
– User code can write to flash memory
• Attacker can control machine from 1st power
• HDD Boot sector can be changed
– (boot sector virus)
• OS Loader can be changed
TPM and BIOS
• BIOS is in control from POST
• BIOS asks TPM for verification
• BIOS can still be attacked
– OS/CPU must protect BIOS
• BIOS chip could still be changed
• BIOS asks TPM to verify Boot Loader
• BIOS could still boot something if verify failed
– Hardware still usable if BIOS permits
Secure Boot
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BIOS will only boot authenticated boot loader
Boot loader only boots authentic OS
BitLocker must be enabled to secure disk
BIOS cannot then override TPM signing
BIOS could still boot another device unless
locked
• BIOS could be replaced to use another disc
• Original disc still cryptographically secured
TPM Weakness
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Should be tamperproof
Used in Games Console
Gamers motivated to break-in
Hardware attack details publish
Split chip open and collect key from Bus
Use brute force decryption for game keys
Manufacture problem not design problem?
UEFI
• Duplication between BIOS and OS
– Both need device drivers
• BIOS – TPM secure boot is optional
• UEFI addresses this
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UEFI is part of BIOS
Performs Boot Loader function
Checks OS signature without TPM
Will refuse to boot
Knows file structure
Drivers come from OS disk
Features
• Larger disks / Newer Hardware
• Eliminates boot virus attacks
• Secures OS
– Gives manufactures' control of hardware use
• Can still work with BitLocker/TPM
• Still vulnerable
– Contains buffer overflow error (Samsung)
– BIOS reflashable/rechipable
Unified Extensible Firmware Interface
Fully Trustworthy?
• Anti-Evil Maid method
• Store signatures of BIOS etc in TPM
• User checks TPM signature using:
– Password
– Physical device (USB stick with crypto key)
• All signatures checked
– Detects any non-authentic code
– BitLocker not needed
– Detects compromise on any component