GETSEC[EXITAC] — Exit Authenticated Code Execution Mode
| Opcode | Instruction | Description |
|---|---|---|
| NP 0F 37 (EAX=3) | GETSEC[EXITAC] | Exit authenticated code execution mode. RBX holds the Near Absolute Indirect jump target and EDX hold the exit parameter flags. |
Description ¶
The GETSEC[EXITAC] leaf function exits the ILP out of authenticated code execution mode established by GETSEC[ENTERACCS] or GETSEC[SENTER]. The EXITAC leaf of GETSEC is selected with EAX set to 3 at entry. EBX (or RBX, if in 64-bit mode) holds the near jump target offset for where the processor execution resumes upon exiting authenticated code execution mode. EDX contains additional parameter control information. Currently only an input value of 0 in EDX is supported. All other EDX settings are considered reserved and result in a general protection violation.
GETSEC[EXITAC] can only be executed if the processor is in protected mode with CPL = 0 and EFLAGS.VM = 0. The processor must also be in authenticated code execution mode. To avoid potential operability conflicts between modes, the processor is not allowed to execute this instruction if it is in SMM or in VMX operation. A violation of these conditions results in a general protection violation.
Upon completion of the GETSEC[EXITAC] operation, the processor unmasks responses to external event signals INIT#, NMI#, and SMI#. This unmasking is performed conditionally, based on whether the authenticated code execution mode was entered via execution of GETSEC[SENTER] or GETSEC[ENTERACCS]. If the processor is in authenticated code execution mode due to the execution of GETSEC[SENTER], then these external event signals will remain masked. In this case, A20M is kept disabled in the measured environment until the measured environment executes GETSEC[SEXIT]. INIT# is unconditionally unmasked by EXITAC. Note that any events that are pending, but have been blocked while in authenticated code execution mode, will be recognized at the completion of the GETSEC[EXITAC] instruction if the pin event is unmasked.
The intent of providing the ability to optionally leave the pin events SMI#, and NMI# masked is to support the completion of a measured environment bring-up that makes use of VMX. In this envisioned security usage scenario, these events will remain masked until an appropriate virtual machine has been established in order to field servicing of these events in a safer manner. Details on when and how events are masked and unmasked in VMX operation are described in Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3C. It should be cautioned that if no VMX environment is to be activated following GETSEC[EXITAC], that these events will remain masked until the measured environment is exited with GETSEC[SEXIT]. If this is not desired then the GETSEC function SMCTRL(0) can be used for unmasking SMI# in this context. NMI# can be correspondingly unmasked by execution of IRET.
A successful exit of the authenticated code execution mode requires the ILP to perform additional steps as outlined below:
- Invalidate the contents of the internal authenticated code execution area.
- Invalidate processor TLBs.
- Clear the internal processor AC Mode indicator flag.
- Re-lock the TPM locality 3 space.
- Unlock the Intel® TXT-capable chipset memory and I/O protections to allow memory and I/O activity by other processor agents.
- Perform a near absolute indirect jump to the designated instruction location.
The content of the authenticated code execution area is invalidated by hardware in order to protect it from further use or visibility. This internal processor storage area can no longer be used or relied upon after GETSEC[EXITAC]. Data structures need to be re-established outside of the authenticated code execution area if they are to be referenced after EXITAC. Since addressed memory content formerly mapped to the authenticated code execution area may no longer be coherent with external system memory after EXITAC, processor TLBs in support of linear to physical address translation are also invalidated.
Upon completion of GETSEC[EXITAC] a near absolute indirect transfer is performed with EIP loaded with the contents of EBX (based on the current operating mode size). In 64-bit mode, all 64 bits of RBX are loaded into RIP if REX.W precedes GETSEC[EXITAC]. Otherwise RBX is treated as 32 bits even while in 64-bit mode. Conventional CS limit checking is performed as part of this control transfer. Any exception conditions generated as part of this control transfer will be directed to the existing IDT; thus it is recommended that an IDTR should also be established prior to execution of the EXITAC function if there is a need for fault handling. In addition, any segmentation related (and paging) data structures to be used after EXITAC should be re-established or validated by the authenticated code prior to EXITAC.
In addition, any segmentation related (and paging) data structures to be used after EXITAC need to be re-established and mapped outside of the authenticated RAM designated area by the authenticated code prior to EXITAC. Any data structure held within the authenticated RAM allocated area will no longer be accessible after completion by EXITAC.
Operation ¶
(* The state of the internal flag ACMODEFLAG and SENTERFLAG persist across instruction boundary *)
IF (CR4.SMXE=0)
THEN #UD;
ELSIF ( in VMX non-root operation)
THEN VM Exit (reason=”GETSEC instruction”);
ELSIF (GETSEC leaf unsupported)
THEN #UD;
ELSIF ((in VMX operation) or ( (in 64-bit mode) and ( RBX is non-canonical) )
(CR0.PE=0) or (CPL>0) or (EFLAGS.VM=1) or
(ACMODEFLAG=0) or (IN_SMM=1)) or (EDX ≠ 0))
THEN #GP(0);
IF (OperandSize = 32)
THEN tempEIP := EBX;
ELSIF (OperandSize = 64)
THEN tempEIP := RBX;
ELSE
tempEIP := EBX AND 0000FFFFH;
IF (tempEIP > code segment limit)
THEN #GP(0);
Invalidate ACRAM contents;
Invalidate processor TLB(s);
Drain outgoing messages;
SignalTXTMsg(CloseLocality3);
SignalTXTMsg(LockSMRAM);
SignalTXTMsg(ProcessorRelease);
Unmask INIT;
IF (SENTERFLAG=0)
THEN Unmask SMI, INIT, NMI, and A20M pin event;
ELSEIF (IA32_SMM_MONITOR_CTL[0] = 0)
THEN Unmask SMI pin event;
ACMODEFLAG := 0;
IF IA32_EFER.LMA == 1
THEN CR3 := R8;
EIP := tempEIP;
END;
Flags Affected ¶
None.
Use of Prefixes ¶
LOCK Causes #UD.
REP* Cause #UD (includes REPNE/REPNZ and REP/REPE/REPZ).
Operand size Causes #UD.
NP 66/F2/F3 prefixes are not allowed.
Segmentoverrides Ignored.
Address size Ignored.
REX.W Sets 64-bit mode Operand size attribute.
Protected Mode Exceptions ¶
| #UD | If CR4.SMXE = 0. |
| If GETSEC[EXITAC] is not reported as supported by GETSEC[CAPABILITIES]. | |
| #GP(0) | If CR0.PE = 0 or CPL>0 or EFLAGS.VM =1. |
| If in VMX root operation. | |
| If the processor is not currently in authenticated code execution mode. | |
| If the processor is in SMM. | |
| If any reserved bit position is set in the EDX parameter register. |
Real-Address Mode Exceptions ¶
| #UD | If CR4.SMXE = 0. |
| If GETSEC[EXITAC] is not reported as supported by GETSEC[CAPABILITIES]. | |
| #GP(0) | GETSEC[EXITAC] is not recognized in real-address mode. |
Virtual-8086 Mode Exceptions ¶
| #UD | If CR4.SMXE = 0. |
| If GETSEC[EXITAC] is not reported as supported by GETSEC[CAPABILITIES]. | |
| #GP(0) | GETSEC[EXITAC] is not recognized in virtual-8086 mode. |
Compatibility Mode Exceptions ¶
All protected mode exceptions apply.
64-Bit Mode Exceptions ¶
All protected mode exceptions apply.
| #GP(0) | If the target address in RBX is not in a canonical form. |
VM-Exit Condition ¶
Reason (GETSEC) If in VMX non-root operation.