Secure Monitor
Secure Monitor Calls
The secure monitor provides two top level handlers of which each provides a range of sub handlers.
Secure Monitor Calls follow the ARM SMC calling convention up to a small change:
| Bit number | Bit mask | Description |
|---|---|---|
| 31 | 0x80000000 | Set to 0 means Yielding Call; Set to 1 means Fast Call. |
| 30 | 0x40000000 | Set to 0 means SMC32 convention; Set to 1 means SMC64. |
| 29-24 | 0x3F000000 | Service Call ranges. |
| 23-16 | 0x00FF0000 | Must be zero. |
| 15-8 | 0x0000FF00 | Argument type. This is different from the ARM SMC calling convention. |
| 7-0 | 0x000000FF | Function number within the range call type. |
If bit n is set in the argument type then parameter Xn is treated as a pointer and the kernel will setup address translation for it in svcCallSecureMonitor.
Id 0
Functions exposed to user-mode processes using svcCallSecureMonitor.
| Sub-Id | Name | In | Out |
|---|---|---|---|
| 0xC3000401 | SetConfig | ||
| 0xC3000002 | GetConfig (Same as Id 1 Sub-Id 4.) | ||
| 0xC3000003 | CheckStatus | ||
| 0xC3000404 | GetResult | ||
| 0xC3000E05 | ExpMod | ||
| 0xC3000006 | GetRandomBytes (Same as Id 1 Sub-Id 5.) | ||
| 0xC3000007 | #GenerateAesKek | ||
| 0xC3000008 | #LoadAesKey | ||
| 0xC3000009 | #CryptAes | ||
| 0xC300000A | #GenerateSpecificAesKey | ||
| 0xC300040B | #ComputeCmac | ||
| 0xC300100C | #LoadRsaPrivateKey | ||
| 0xC300100D | #PrivateRsa | ||
| 0xC300100E | #LoadRsaPublicKey | ||
| 0xC300060F | #PublicRsa | ||
| 0xC3000610 | #UnwrapRsaEncryptedAesKey | ||
| 0xC3000011 | #LoadRsaWrappedAesKey | ||
| 0xC3000012 | [2.0.0+] GenerateRsaWrappedAesKek |
The overall concept here is the following:
- All key material (AES and RSA) is stored in userspace, but it's encrypted with random AES kek's ("key encryption keys").
- Each kek is generated as a function of an access key (picked at random).
- The kek is generated differently depending on the #CryptoUsecase the key is used for.
- This means: Each key is "locked" to the #CryptoUsecase it was designated for.
- You can use a key for a different usecase, but you will only get garbage output.
- After the kek has been generated, it is wrapped with a session-specific key and given back to userspace.
- This means: Plaintext kek keys never leave TrustZone.
- Further, this means: Actual AES/RSA keys never leave TrustZone.
GenerateAesKek
Takes an "access key" as input, an #CryptoUsecase.
Returns a session-unique kek for said usecase.
LoadAesKey
Takes a session kek created with #GenerateAesKek, and a wrapped AES key.
The session kek must have been created with CryptoUsecase_Aes.
CryptAes
Encrypts/decrypts using Aes (CTR and CBC).
Key must be set prior using one of the #LoadAesKey, #GenerateSpecificAesKey or #LoadRsaWrappedAesKey commands.
GenerateSpecificAesKey
Todo: This one seems unrelated to #CryptoUsecase.
LoadRsaPrivateKey
Takes a session kek created with #GenerateAesKek, and a wrapped RSA private key.
The session kek must have been created with CryptoUsecase_PrivateRsa.
PrivateRsa
Encrypts using Rsa private key.
Key must be set prior using the #LoadRsaPrivateKey command.
LoadRsaPublicKey
Takes a session kek created with #GenerateAesKek, and a wrapped RSA public key.
The session kek must have been created with CryptoUsecase_PublicRsa.
PublicRsa
Encrypts using Rsa public key.
Key must be set prior using the #LoadRsaPublicKey command.
UnwrapRsaEncryptedAesKey
Takes a session kek created with #GenerateAesKek, and a wrapped RSA public key.
Returns a session-unique AES key especially for use in #LoadRsaWrappedAesKey.
The session kek must have been created with CryptoUsecase_RsaWrappedAesKey.
LoadRsaWrappedAesKey
Takes a session-unique AES key from #UnwrapRsaEncryptedAesKey.
enum CryptoUsecase
| Value | Name |
|---|---|
| 0 | CryptoUsecase_Aes |
| 1 | CryptoUsecase_PrivateRsa |
| 2 | CryptoUsecase_PublicRsa |
| 3 | CryptoUsecase_RsaWrappedAesKey |
Id 1
Functions exposed to the kernel internally.
| Sub-Id | Name | In | Out |
|---|---|---|---|
| 0xC4000001 | CpuSuspend | X1=power_state, X2=entrypoint_addr, X3=context_addr | None |
| 0x84000002 | CpuOff | None | None |
| 0xC4000003 | CpuOn | ||
| 0xC3000004 | GetConfig (Same as Id 0 Sub-Id 2.) | W1=config_item, X2,X3,X4,X5,X6,X7=0 | X0=result, X1,X2,X3,X4=config_val |
| 0xC3000005 | GetRandomBytes (Same as Id 0 Sub-Id 6.) | X1=dst_addr, X2,X3,X4,X5,X6,X7=0 | |
| 0xC3000006 | Panic | W1=unk, X2,X3,X4,X5,X6,X7=0 | X0=result |
| 0xC3000007 | [2.0.0+] ProtectKernelRegion | ||
| 0xC3000008 | [2.0.0+] ReadWriteRegister |
Errors
2: Invalid input 3: Busy