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.

SMC arguments are passed using registers X0-X7 with X0 always sending the call sub-id and returning the result of the call.

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	#DecryptRsaPrivateKey
0xC300100E	#LoadRsaPublicKey
0xC300060F	#PublicRsa
0xC3000610	#UnwrapPreparedAesKey
0xC3000011	#LoadPreparedAesKey
0xC3000012	[2.0.0+] GeneratePreparedAesKek

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.

Note: The CryptoUsecase_PreparedAesKey represents a RSA wrapped AES key.

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.

DecryptRsaPrivateKey

Takes a session kek created with #GenerateAesKek, a wrapped AES key, and a wrapped RSA private key.

The session kek must have been created with CryptoUsecase_PrivateRsa.

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.

UnwrapPreparedAesKey

Takes a session kek created with #GenerateAesKek, and a wrapped RSA public key.

Returns a session-unique AES key especially for use in #LoadPreparedAesKey.

The session kek must have been created with CryptoUsecase_PreparedAesKey.

LoadPreparedAesKey

Takes a session-unique AES key from #UnwrapPreparedAesKey.

enum CryptoUsecase

Value	Name
0	CryptoUsecase_Aes
1	CryptoUsecase_PrivateRsa
2	CryptoUsecase_PublicRsa
3	CryptoUsecase_PreparedAesKey

ID 1

Functions exposed to the kernel internally.

Sub-ID	Name	In	Out
0xC4000001	#CpuSuspend	X1=power_state, X2=entrypoint_addr, X3=context_id	None
0x84000002	#CpuOff	None	None
0xC4000003	#CpuOn	X1=target_cpu, X2=entrypoint_addr, X3=context_id, X4,X5,X6,X7=0	X0=result
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=size, X2,X3,X4,X5,X6,X7=0	X0=result, X1,X2,X3,X4,X5,X6,X7=rand_bytes
0xC3000006	#Panic	W1=unk, X2,X3,X4,X5,X6,X7=0	X0=result
0xC3000007	[2.0.0+] ProtectKernelRegion
0xC3000008	[2.0.0+] ReadWriteRegister

CpuSuspend

Standard ARM PCSI SMC. Suspends the CPU (CPU0).

The kernel calls this SMC on shutdown with power_state set to 0x0201001B (power level: 0x02==system; power type: 0x01==powerdown; ID: 0x1B).

CpuOff

Standard ARM PCSI SMC. Turns off the CPU (CPU1, CPU2 or CPU3).

CpuOn

Standard ARM PCSI SMC. Turns on the CPU (CPU1, CPU2 or CPU3).

GetConfig

Takes a config_item and returns an associated config_val.

GetRandomBytes

Takes a size and returns rand_bytes.

The kernel limits size to 0x38 (for fitting in return registers).

Panic

Issues a system panic.

The kernel always calls this with unk set to 0xF00.

Errors

Value	Description
2	Invalid input
3	Busy