WRUSSD/WRUSSQ—Write to User Shadow Stack

Opcode/

Op/

64/32

CPUID

Description

Instruction En bit Mode Support Feature Flag
66 0F 38 F5 WRUSSD r/m32, r32 MR V/V CET_SS Write 4 bytes to shadow stack.
66 REX.W 0F 38 F5 WRUSSQ r/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) NA NA

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.