IA32SYSENTERESP

IA32SYSENTERESP is a Model-Specific Register (MSR) in the IA-32 architecture used by the SYSENTER/SYSEXIT fast system call mechanism. It stores the address of the kernel-mode stack pointer (the top of the kernel stack) that the processor loads into the ESP register when a user-space thread executes SYSENTER to enter the kernel.

During a SYSENTER, the processor switches from user mode to ring 0, loading the kernel code segment from IA32_SYSENTER_CS, the kernel entry point address from IA32_SYSENTER_EIP, and the kernel stack pointer from IA32_SYSENTER_ESP. The operating system must initialize and update IA32SYSENTERESP to point to a valid kernel stack for the current CPU or task, ensuring proper isolation between processes.

Access to this MSR is privileged; only ring-0 software can read or write it via RDMSR or WRMSR. User-space code cannot modify it directly, which helps enforce the security boundary between user and kernel space.

Relation and context: IA32_SYSENTER_ESP is one of a trio of MSRs (IA32_SYSENTER_CS, IA32_SYSENTER_ESP, IA32_SYSENTER_EIP) that configure the SYSENTER entry path. In 64-bit operating systems, SYSENTER remains available for 32-bit code, but long-mode syscalls generally use the SYSCALL/SYSRET mechanism instead.

Security and correctness considerations: The kernel must ensure IA32SYSENTERESP points to a valid, properly aligned kernel stack with appropriate guards to prevent stack corruption or leakage during transitions. Misconfiguration can lead to crashes or privilege escalation.