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Prashant Pol
Boot veri
f
ication in Android
To ensure integrity of os and partitions
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Introduction
Android Engineer at LINE
Multi device experience team
@prashantspol
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Agenda
- Boot veri
f
ication signi
f
icance
- Concepts
- Veri
f
ication sequence
- Veri
f
ication in A/B device updates
- Rollback protection
- Veri
f
ied boot 2.0
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Why boot veri
f
ication
signi
f
icant
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Boot Android OS
Anti-virus Anti-malware
Google Play Protect
periodic scans
Phone Security
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Boot Veri
f
ication signi
f
icance
- Security
- Unveri
f
ied
f
irmware update
- Data breach
- Lack of trust
Boot Veri
fi
cation is important
What if boot veri
f
ication was not implemented
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What is Boot?
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Boot process overview
Some of piece of
software
Android
launcher UI
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boot ROM bootloader linux kernel
init process
zygote
process
system
server
android
launcher UI
Boot process overview
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In simple terms,
Ensures that all code executed during boot up was from a trusted source
and nothing was compromised.
Android BootVeri
f
ication
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History
4.4
7.0
8.0
Boot Veri
f
ication was introduced
Warn user
Prevent boot
Standardise boot veri
f
ication
Android OS
11
almost a decade ago
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Root of trust
Bootloader
boot partition
other partitions
BootVeri
f
ication
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Root of trust
Android perspective,
- A cryptographic key used to sign hashes
- Private key / Public key
- Same key is used for future version releases
- Foundation for secure operations
- Designed in a way that it must be inherently trusted.
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Custom root of trust:
- Tamper evident storage
- Boot warning
- User’s physical con
f
irmation required
Hardware backed root of trust is considered to be most secure.
- in processor chip / system on chip (SoC)
Root of trust (cont…)
Why ??
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- Approved by manufacturer
- Signed with manufacturer’s root of trust
- Boot veri
f
ication applied
Original OS
What if want to install custom os ?
- Custom OS => Custom Root of Trust
- Relax boot veri
f
ication
- How boot loader know about it ?
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Device State
LOCKED
UNLOCKED
- Prevents flashing custom os version
- Checks and warns during boot
- Can flash custom os version
- Boot checks are relaxed
Device state
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- Physical interaction required
- User’s data partition is wiped out
- RAM will be reset
- User can flash custom os image
- Settings > System > Developer Options > OEM unlocking (ON)
- fastboot flashing unlock
LOCKED UNLOCKED
State changes
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- fastboot flashing lock
-Settings > System > Developer Options > OEM unlocking (OFF)
- Cannot flash any custom os image
- Device reset
- Reselling
LOCKED
UNLOCKED
State changes (cont…)
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Root of trust
Bootloader
Boot partition
Other Partitions
BootVeri
f
ication
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Partitions
boot
system
Kernel image
recovery
Android framework
Stores recovery image during OTA
userdata User installed apps and data
vendor Vendor speci
f
ic binary not distributable with AOSP
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Partition veri
f
ication
boot
Not only boot up code or kernel. But need to verify partitions as well.
dtbo
system
vendor
• Small partitions
• Can be loaded into RAM to calculate hash
• Big partitions
• Can not be loaded into RAM entirely to calculate hash
Each partition’s hash is calculated and saved at the start or end of the partition.
)PXUPIBTI#*(QBSUJUJPOT
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Big partitions are read from disk using tree hashing.
Tree hashing
partition data in bytes
Random
salt
Partition
Hash
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Tree hashing (cont…)
partition data in bytes
4K 4K 4K 4K 4K 4K 4K 4K 4K 4K 4K 4K
appended hashes
4K 4K 4K 4K 4K 4K 4K 4K 4K
Layer 1
Layer 2
Layer 3 appended hashes
4K 4K 4K 4K 4K 4K
Layer 4 4K
Root Hash
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Agenda
- Boot veri
- Concepts
- Veri
f
ication sequence
- Veri
- Rollback Protection
- Veri
f
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Veri
f
ication sequence
Root of trust
Start booting
Calculate hash of next code to execute
Compare with pre-calculated hash
Is hash comparison success Veri
f
ication Error
Fail
Success
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Handling Veri
f
ication Error
Boot
Start Run time
Set error
f
lag
Restart
User handling required
Success
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Device State
LOCKED ?
Valid OS ? Valid OS ?
Cannot
Boot
Warn user eio mode?
Show eio
screen
BOOT OS
N Y
N
Y
Boot
f
low (simpli
f
ied)
N
Y
Y
N
Y
N
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Agenda
- Boot veri
- Concepts
- Veri
f
- Veri
f
ication in A/B device updates
- Rollback Protection
- Veri
f
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Slot A Slot B
A/B device updates
Over-the-air (OTA) update.
- update_engine daemon
- Streamed installation
Slot A Slot B
In Use
Idle
New version update
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SLOT A SLOT B
OTA update
Active
isBootable = true
Boot from slot A
Boot from
slot B
isActive = true
Passive
Active isSuccessful = true
Success
Success
Failure
Failure
Passive
A/B device update (cont…)
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Agenda
- Boot veri
- Concepts
- Veri
- Veri
f
- Rollback Protection
- Veri
f
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Rollback Protection
Android Phone
OS Other OS version
?
v10 v8
Installed version (rollback index) is important.
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>
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Agenda
- Boot veri
- Concepts
- Veri
- Veri
f
- Veri
f
ied boot 2.0
- Rollback Protection
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vbmeta
hash of
boot
Hash tree metadata
of system
Hash tree metadata
of vendor
(Signed by key)
Veri
f
ied boot 2.0
- Android veri
f
ied boot
- Android 8.0
- Standardised partition footer
vbmeta struct:
- Central data structure
- Rollback indexes
- Chained partition support 34
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VBMeta digest
Digest = hash over all vbmeta structs (including chained partitions).
Digest is used to verify authenticity
Digest & keys from hardware backed attestation used to cross verify.
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Provides boot state with enum Veri
f
iedBootState.
HW Attestation
- Veri
f
ied
- SelfSigned
- Unveri
f
ied
- Failed
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Conclusion
• Boot veri
f
ication is important to trust device.
• If boot was compromised then any additional security application or any piece of
software cannot be trusted.
• Cryptographic hashing & signing is used to ensure authenticity of executed code.
• After boot up, hardware backed key attestation data could be used in user space to
decide authenticity of boot up.
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References
-https://source.android.com/docs/security/features/veri
f
iedboot
-https://source.android.com/docs/security/features/veri
f
iedboot/device-state
-https://source.android.com/docs/security/features/veri
f
iedboot/veri
f
ied-boot
-https://source.android.com/docs/security/features/veri
f
iedboot/boot-flow
-https://android.googlesource.com/platform/external/avb/+/master/README.md
-https://source.android.com/docs/security/features/veri
f
iedboot/dm-verity
-https://android.googlesource.com/platform/hardware/interfaces/+/master/boot/1.0/IBootControl.hal
-https://developer.android.com/training/articles/security-key-attestation
-https://source.android.com/docs/core/architecture/partitions
-https://source.android.com/docs/core/ota/ab
-https://android.googlesource.com/platform/external/avb/+/master/README.md#the-vbmeta-digest
and many more…
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Thank you
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