EMODPR — Restrict the Permissions of an EPC Page

Opcode/Instruction	Op/En	64/32 bit Mode Support	CPUID Feature Flag	Description
EAX = 0EH ENCLS[EMODPR]	IR	V/V	SGX2	This leaf function restricts the access rights associated with a EPC page in an initialized enclave.

Instruction Operand Encoding ¶

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

Description ¶

This leaf function restricts the access rights associated with an EPC page in an initialized enclave. THE RWX bits of the SECINFO parameter are treated as a permissions mask; supplying a value that does not restrict the page permissions will have no effect. 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 EMODPR leaf function.

EMODPR 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:

EMODPR 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 38-4)	Description
No Error	EMODPR 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, EMODT, or EWB.

Table 38-31. EMODPR Return Value in RAX

Concurrency Restrictions ¶

Leaf	Parameter	Base Concurrency Restrictions
			On Conflict
EMODPR EMODPR Target [DS:RCX] Shared EMODPR Target [DS:RCX]	Target [DS:RCX]

Table 38-32. Base Concurrency Restrictions of EMODPR

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
EMODPR	Target [DS:RCX]	Exclusive	SGX_EPC_PAGE _CONFLICT	Concurrent		Concurrent

Table 38-33. Additional Concurrency Restrictions of EMODPR

Operation ¶

Temp Variables in EMODPR 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.R is 0 and SCRATCH_SECINFO.FLAGS.W is not 0) )

THEN #GP(0); FI;

(* Check concurrency with SGX1 or SGX2 instructions on the EPC page *)

IF (SGX1 or other SGX2 instructions accessing EPC page)

THEN #GP(0); 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).PENDING is not 0 or (EPCM(DS:RCX).MODIFIED is not 0) )

THEN

RFLAGS.ZF := 1;

RAX := SGX_PAGE_NOT_MODIFIABLE;

GOTO DONE;

FI;

IF (EPCM(DS:RCX).PT is not PT_REG)

THEN #PF(DS:RCX); FI;

TMP_SECS := GET_SECS_ADDRESS

IF (TMP_SECS.ATTRIBUTES.INIT = 0)

THEN #GP(0); FI;

(* Set the PR bit to indicate that permission restriction is in progress *)

EPCM(DS:RCX).PR := 1;

(* Update EPCM permissions *)

EPCM(DS:RCX).R := EPCM(DS:RCX).R & SCRATCH_SECINFO.FLAGS.R;

EPCM(DS:RCX).W := EPCM(DS:RCX).W & SCRATCH_SECINFO.FLAGS.W;

EPCM(DS:RCX).X := EPCM(DS:RCX).X & SCRATCH_SECINFO.FLAGS.X;

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.
	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.
	If a memory operand is not an EPC page.