WRUSSD/WRUSSQ - Write to User Shadow Stack

Opcode/ Instruction

Op/ En

64/32 bit Mode Support

CPUID Feature Flag

Description

66 0F 38 F5 !(11):rrr:bbb WRUSSD m32, r32

MR

V/V

CET_SS

Write 4 bytes to shadow stack.

66 REX.W 0F 38 F5 !(11):rrr:bbb WRUSSQ 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 a user shadow stack page. The WRUSS instruction can be executed only if CPL = 0, however the processor treats its shadow-stack accesses as user accesses.

Operation 

IF CR4.CET = 0
   THEN #UD; FI;
IF CPL > 0
   THEN #GP(0); 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 as user-mode access;
   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 as user-mode access;
FI;

Flags Affected

None.

C/C++ Compiler Intrinsic Equivalent 

WRUSSD void _wrussd(__int32, void *);
WRUSSQ void _wrussq(__int64, void *);

Protected Mode Exceptions

#UD

If the LOCK prefix is used. If CR4.CET = 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. If CPL is not 0.

#SS(0)

If a memory operand effective address is outside the SS segment limit.

#PF(fault-code)

If destination is not a user shadow stack. Other terminal and non-terminal faults.

Real-Address Mode Exceptions

#UD

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

Virtual-8086 Mode Exceptions

#UD

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

Compatibility Mode Exceptions

#UD

If the LOCK prefix is used. If CR4.CET = 0.

#GP(0)

If a memory address is in a non-canonical form. If linear address of destination is not 4 byte aligned. If CPL is not 0.

#SS(0)

If a memory address referencing the SS segment is in a non-canonical form.

#PF(fault-code)

If destination is not a user shadow stack. Other terminal and non-terminal faults.

64-Bit Mode Exceptions

#UD

If the LOCK prefix is used. If CR4.CET = 0.

#GP(0)

If a memory address is in a non-canonical form. If linear address of destination is not 4 byte aligned. If CPL is not 0.

#PF(fault-code)

If destination is not a user shadow stack. Other terminal and non-terminal faults.