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.