SETSSBSY - Mark Shadow Stack Busy

Opcode/ Instruction

Op/ En

64/32 bit Mode Support

CPUID Feature Flag

Description

F3 0F 01 E8 SETSSBSY

ZO

V/V

CET_SS

Set busy flag in supervisor shadow stack token reference by IA32_PL0_SSP.

Instruction Operand Encoding

Op/En

Operand 1

Operand 2

Operand 3

Operand 4

ZO

N/A

N/A

N/A

N/A

Description

The SETSSBSY instruction verifies the presence of a non-busy supervisor shadow stack token at the address in the IA32_PL0_SSP MSR and marks it busy. Following successful execution of the instruction, the SSP is set to the value of the IA32_PL0_SSP MSR.

Operation 

IF (CR4.CET = 0)
   THEN #UD; FI;
IF (IA32_S_CET.SH_STK_EN = 0)
   THEN #UD; FI;
IF CPL > 0
   THEN GP(0); FI;
SSP_LA = IA32_PL0_SSP
If SSP_LA not aligned to 8 bytes
   THEN #GP(0); FI;
expected_token_value = SSP_LA                  (* busy bit must not be set *)
new_token_value          = SSP_LA | BUSY_BIT      (* set busy bit; bit position 0 *)
IF shadow_stack_lock_cmpxchg8B(SSP_LA, new_token_value, expected_token_value) != expected_token_value
   THEN #CP(SETSSBSY); FI;
SSP = SSP_LA

Flags Affected

None.

C/C++ Compiler Intrinsic Equivalent 

SETSSBSYvoid _setssbsy(void);

Protected Mode Exceptions

#UD

If the LOCK prefix is used. If CR4.CET = 0. IF IA32_S_CET.SH_STK_EN = 0.

#GP(0)

If IA32_PL0_SSP not aligned to 8 bytes. If CPL is not 0.

#CP(setssbsy)

If busy bit in token is set. If in 32-bit or compatibility mode, and the address in token is not below 4G.

#PF(fault-code)

If a page fault occurs.

Real-Address Mode Exceptions

#UD

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

Virtual-8086 Mode Exceptions

#UD

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

Compatibility Mode Exceptions

Same as protected mode exceptions.

64-Bit Mode Exceptions

Same as protected mode exceptions.