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.