EMODT — Change the Type of an EPC Page

Opcode/Instruction	Op/En	64/32 bit Mode Support	CPUID Feature Flag	Description
EAX = 0FH ENCLS[EMODT]	IR	V/V	SGX2	This leaf function changes the type of an existing EPC page.

Instruction Operand Encoding ¶

Op/En	EAX		RBX	RCX
IR	EMODT (In)	Return Error Code (Out)	Address of a SECINFO (In)	Address of the destination EPC page (In)

Description ¶

This leaf function modifies the type of an EPC page. The security attributes are configured to prevent access to the EPC page at its new type until a corresponding invocation of the EACCEPT leaf confirms the modification. This instruction can only be executed when current privilege level is 0.

RBX contains the effective address of a SECINFO structure while RCX contains the effective address of an EPC page. The table below provides additional information on the memory parameter of the EMODT leaf function.

EMODT Memory Parameter Semantics ¶

SECINFO	EPCPAGE
Read access permitted by Non Enclave	Read/Write access permitted by Enclave

The instruction faults if any of the following:

EMODT Faulting Conditions ¶

The operands are not properly aligned.	If unsupported security attributes are set.
The Enclave is not initialized.	SECS is locked by another thread.
The EPC page is locked by another thread.	RCX does not contain an effective address of an EPC page in the running enclave.
The EPC page is not valid.

The error codes are:

Error Code (see Table 40-4)	Description
No Error	EMODT successful.
SGX_PAGE_NOT_MODIFIABLE	The EPC page cannot be modified because it is in the PENDING or MODIFIED state.
SGX_EPC_PAGE_CONFLICT	Page is being written by EADD, EAUG, ECREATE, ELDU/B, EMODPR, or EWB.

Table 40-34. EMODT Return Value in RAX

Concurrency Restrictions ¶

Leaf	Parameter	Base Concurrency Restrictions
Leaf	Parameter	Access	On Conflict	SGX_CONFLICT VM Exit Qualification
EMODT	Target [DS:RCX]	Exclusive	SGX_EPC_PAGE_ CONFLICT	EPC_PAGE_CONFLICT_ERROR

Table 40-35. Base Concurrency Restrictions of EMODT

Leaf	Parameter	Additional Concurrency Restrictions
		vs. EACCEPT, EACCEPTCOPY, EMODPE, EMODPR, EMODT		vs. EADD, EEXTEND, EINIT		vs. ETRACK, ETRACKC
		Access	On Conflict	Access	On Conflict	Access	On Conflict
EMODT	Target [DS:RCX]	Exclusive	SGX_EPC_PAGE _CONFLICT	Concurrent		Concurrent

Table 40-36. Additional Concurrency Restrictions of EMODT

Operation ¶

Temp Variables in EMODT Operational Flow ¶

Name	Type	Size (bits)	Description
TMP_SECS	Effective Address	32/64	Physical address of SECS to which EPC operand belongs.
SCRATCH_SECINFO	SECINFO	512	Scratch storage for holding the contents of DS:RBX.

IF (DS:RBX is not 64Byte Aligned)

THEN #GP(0); FI;

IF (DS:RCX is not 4KByte Aligned)

THEN #GP(0); FI;

IF (DS:RCX does not resolve within an EPC)

THEN #PF(DS:RCX); FI;

SCRATCH_SECINFO ← DS:RBX;

(* Check for misconfigured SECINFO flags*)

IF ( (SCRATCH_SECINFO reserved fields are not zero ) or

!(SCRATCH_SECINFO.FLAGS.PT is PT_TCS or SCRATCH_SECINFO.FLAGS.PT is PT_TRIM) )

THEN #GP(0); FI;

(* Check concurrency with SGX1 instructions on the EPC page *)

IF (other SGX1 instructions accessing EPC page)

THEN

RFLAGS.ZF ← 1;

RAX ← SGX_EPC_PAGE_CONFLICT;

GOTO DONE;

FI;

IF (EPCM(DS:RCX).VALID is 0)

THEN #PF(DS:RCX); FI;

(* Check the EPC page for concurrency *)

IF (EPC page in use by another SGX2 instruction)

THEN

RFLAGS.ZF ← 1;

RAX ← SGX_EPC_PAGE_CONFLICT;

GOTO DONE;

FI;

IF (!(EPCM(DS:RCX).PT is PT_REG or

(EPCM(DS:RCX).PT is PT_TCS and SCRATCH_SECINFO.FLAGS.PT is PT_TRIM)))

THEN #PF(DS:RCX); FI;

IF (EPCM(DS:RCX).PENDING is not 0 or (EPCM(DS:RCX).MODIFIED is not 0) )

THEN

RFLAGS.ZF ← 1;

RAX ← SGX_PAGE_NOT_MODIFIABLE;

GOTO DONE;

FI;

TMP_SECS ← GET_SECS_ADDRESS

IF (TMP_SECS.ATTRIBUTES.INIT = 0)

THEN #GP(0); FI;

(* Update EPCM fields *)

EPCM(DS:RCX).PR ← 0;

EPCM(DS:RCX).MODIFIED ← 1;

EPCM(DS:RCX).R ← 0;

EPCM(DS:RCX).W ← 0;

EPCM(DS:RCX).X ← 0;

EPCM(DS:RCX).PT ← SCRATCH_SECINFO.FLAGS.PT;

RFLAGS.ZF ← 0;

RAX←0;

DONE:

RFLAGS.CF,PF,AF,OF,SF ← 0;

Flags Affected ¶

Sets ZF if page is not modifiable or if other SGX2 instructions are executing concurrently, otherwise cleared. Clears CF, PF, AF, OF, SF.

Protected Mode Exceptions ¶

#GP(0)	If a memory operand effective address is outside the DS segment limit.
	If a memory operand is not properly aligned.
	If a memory operand is locked.
#PF(error	code) If a page fault occurs in accessing memory operands.
#PF(error	If a memory operand is not an EPC page.

64-Bit Mode Exceptions ¶

#GP(0)	If a memory operand is non-canonical form.
	If a memory operand is not properly aligned.
	If a memory operand is locked.
#PF(error	code) If a page fault occurs in accessing memory operands.
#PF(error	If a memory operand is not an EPC page.