WRSSD/WRSSQ—Write to Shadow Stack

Opcode/Instruction	Op/En	64/32 bit Mode Support	CPUID Feature Flag	Description
0F 38 F6 !(11):rrr:bbb WRSSD m32, r32	MR	V/V	CET_SS	Write 4 bytes to shadow stack.
REX.W 0F 38 F6 !(11):rrr:bbb WRSSQ m64, r64	MR	V/N.E.	CET_SS	Write 8 bytes to shadow stack.

Instruction Operand Encoding

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
MR	ModRM:r/m (w)	ModRM:reg (r)	N/A	N/A

Description

Writes bytes in register source to the shadow stack.

Operation

IF CPL = 3
    IF (CR4.CET & IA32_U_CET.SH_STK_EN) = 0
         THEN #UD; FI;
    IF (IA32_U_CET.WR_SHSTK_EN) = 0
         THEN #UD; FI;
ELSE
    IF (CR4.CET & IA32_S_CET.SH_STK_EN) = 0
         THEN #UD; FI;
    IF (IA32_S_CET.WR_SHSTK_EN) = 0
         THEN #UD; FI;
FI;
DEST_LA = Linear_Address(mem operand)
IF (operand size is 64 bit)
    THEN
         (* Destination not 8B aligned *)
         IF DEST_LA[2:0]
              THEN GP(0); FI;
         Shadow_stack_store 8 bytes of SRC to DEST_LA;
    ELSE
         (* Destination not 4B aligned *)
         IF DEST_LA[1:0]
              THEN GP(0); FI;
         Shadow_stack_store 4 bytes of SRC[31:0] to DEST_LA;
FI;

Flags Affected

None.

C/C++ Compiler Intrinsic Equivalent

WRSSD void _wrssd(__int32, void *);

WRSSQ void _wrssq(__int64, 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. If CPL = 3 and IA32_U_CET.WR_SHSTK_EN = 0. If CPL < 3 and IA32_S_CET.WR_SHSTK_EN = 0.
#GP(0)	If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If destination is located in a non-writeable segment. If the DS, ES, FS, or GS register is used to access memory and it contains a NULL segment selector. If linear address of destination is not 4 byte aligned.
#SS(0)	If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)	If a page fault occurs if destination is not a user shadow stack when CPL3 and not a supervisor shadow stack when CPL < 3. Other terminal and non-terminal faults.

Real-Address Mode Exceptions

#UD	The WRSS instruction is not recognized in real-address mode.

Virtual-8086 Mode Exceptions

#UD	The WRSS instruction is not recognized in virtual-8086 mode.

Compatibility 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.

If CPL = 3 and IA32_U_CET.WR_SHSTK_EN = 0.

If CPL < 3 and IA32_S_CET.WR_SHSTK_EN = 0.

#PF(fault-code)

If a page fault occurs if destination is not a user shadow stack when CPL3 and not a supervisor shadow stack when CPL < 3.

Other terminal and non-terminal faults.

64-Bit Mode Exceptions

#UD