SAVEPREVSSP - Save Previous Shadow Stack Pointer

Opcode/ Instruction

Op/ En

64/32 bit Mode Support

CPUID Feature Flag

Description

F3 0F 01 EA (mod!=11, /5, RM=010) SAVEPREVSSP

ZO

V/V

CET_SS

Save a restore-shadow-stack token on previous shadow stack.

Instruction Operand Encoding

Op/En

Operand 1

Operand 2

Operand 3

Operand 4

ZO

N/A

N/A

N/A

N/A

Description

Push a restore-shadow-stack token on the previous shadow stack at the next 8 byte aligned boundary. The previous SSP is obtained from the previous-ssp token at the top of the current shadow stack.

Operation 

IF CPL = 3
   IF (CR4.CET & IA32_U_CET.SH_STK_EN) = 0
       THEN #UD; FI;
ELSE
   IF (CR4.CET & IA32_S_CET.SH_STK_EN) = 0
       THEN #UD; FI;
FI;
IF SSP not aligned to 8 bytes
   THEN #GP(0); FI;
(* Pop the "previous-ssp" token from current shadow stack *)
previous_ssp_token = ShadowStackPop8B(SSP)
(* If the CF flag indicates there was a alignment hole on current shadow stack then pop that alignment hole *)
(* Note that the alignment hole must be zero and can be present only when in legacy/compatibility mode *)
IF RFLAGS.CF == 1 AND (IA32_EFER.LMA AND CS.L)
   #GP(0)
FI;
IF RFLAGS.CF == 1
   must_be_zero = ShadowStackPop4B(SSP)
   IF must_be_zero != 0 THEN #GP(0)
FI;
(* Previous SSP token must have the bit 1 set *)
IF ((previous_ssp_token & 0x02) == 0)
   THEN #GP(0); (* bit 1 was 0 *)
IF ((IA32_EFER.LMA AND CS.L) = 0 AND previous_ssp_token [63:32] != 0)
THEN #GP(0); FI; (* If compatibility/legacy mode and SSP not in 4G *)
(* Save Prev SSP from previous_ssp_token to the old shadow stack at next 8 byte aligned address *)
old_SSP = previous_ssp_token & ~0x03
temp := (old_SSP | (IA32_EFER.LMA & CS.L));
Shadow_stack_store 4 bytes of 0 to (old_SSP - 4)
old_SSP := old_SSP & ~0x07;
Shadow_stack_store 8 bytes of temp to (old_SSP - 8)

Flags Affected

None.

C/C++ Compiler Intrinsic Equivalent 

SAVEPREVSSP void _saveprevssp(void);

Protected Mode Exceptions

#UD

If the LOCK prefix is used. If CR4.CET = 0. IF CPL = 3 and IA32_U_CET.SH_STK_EN = 0. IF CPL < 3 and IA32_S_CET.SH_STK_EN = 0.

#GP(0)

If SSP not 8 byte aligned. If alignment hole on shadow stack is not 0. If bit 1 of the previous-ssp token is not set to 1. If in 32-bit/compatibility mode and SSP recorded in previous-ssp token is beyond 4G.

#PF(fault-code)

If a page fault occurs.

Real-Address Mode Exceptions

#UD

The SAVEPREVSSP instruction is not recognized in real-address mode.

Virtual-8086 Mode Exceptions

#UD

The SAVEPREVSSP instruction is not recognized in virtual-8086 mode.

Compatibility Mode Exceptions

Same as protected mode exceptions.

64-Bit Mode Exceptions

#UD

If the LOCK prefix is used. If CR4.CET = 0. If CPL = 3 and IA32_U_CET.SH_STK_EN = 0. If CPL < 3 and IA32_S_CET.SH_STK_EN = 0.

#GP(0)

If SSP not 8 byte aligned. If carry flag is set. If bit 1 of the previous-ssp token is not set to 1.

#PF(fault-code)

If a page fault occurs.