[PATCH] Fix sysenter disabling in vm86 mode

From: Brian Gerst <bgerst@didntduck.org>

The current code disables sysenter when first entering vm86 mode, but does
not disable it again when coming back to a vm86 task after a task switch.

arch/i386/kernel/cpu/common.c

@@ -510,7 +510,7 @@ void __init cpu_init (void)
 		BUG();
 	enter_lazy_tlb(&init_mm, current);
 
-	load_esp0(t, thread->esp0);
+	load_esp0(t, thread);
 	set_tss_desc(cpu,t);
 	cpu_gdt_table[cpu][GDT_ENTRY_TSS].b &= 0xfffffdff;
 	load_TR_desc();

arch/i386/kernel/process.c

@@ -507,7 +507,7 @@ struct task_struct * __switch_to(struct task_struct *prev_p, struct task_struct
 	/*
 	 * Reload esp0, LDT and the page table pointer:
 	 */
-	load_esp0(tss, next->esp0);
+	load_esp0(tss, next);
 
 	/*
 	 * Load the per-thread Thread-Local Storage descriptor.

arch/i386/kernel/vm86.c

@@ -124,7 +124,8 @@ struct pt_regs * save_v86_state(struct kernel_vm86_regs * regs)
 
 	tss = init_tss + get_cpu();
 	current->thread.esp0 = current->thread.saved_esp0;
-	load_esp0(tss, current->thread.esp0);
+	current->thread.sysenter_cs = __KERNEL_CS;
+	load_esp0(tss, &current->thread);
 	current->thread.saved_esp0 = 0;
 	put_cpu();
 
@@ -301,8 +302,10 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	asm volatile("movl %%gs,%0":"=m" (tsk->thread.saved_gs));
 
 	tss = init_tss + get_cpu();
-	tss->esp0 = tsk->thread.esp0 = (unsigned long) &info->VM86_TSS_ESP0;
-	disable_sysenter(tss);
+	tsk->thread.esp0 = (unsigned long) &info->VM86_TSS_ESP0;
+	if (cpu_has_sep)
+		tsk->thread.sysenter_cs = 0;
+	load_esp0(tss, &tsk->thread);
 	put_cpu();
 
 	tsk->thread.screen_bitmap = info->screen_bitmap;

include/asm-i386/processor.h

@@ -407,6 +407,7 @@ struct thread_struct {
 /* cached TLS descriptors. */
 	struct desc_struct tls_array[GDT_ENTRY_TLS_ENTRIES];
 	unsigned long	esp0;
+	unsigned long	sysenter_cs;
 	unsigned long	eip;
 	unsigned long	esp;
 	unsigned long	fs;
@@ -428,6 +429,7 @@ struct thread_struct {
 
 #define INIT_THREAD  {							\
 	.vm86_info = NULL,						\
+	.sysenter_cs = __KERNEL_CS,					\
 	.io_bitmap_ptr = NULL,						\
 }
 
@@ -447,21 +449,13 @@ struct thread_struct {
 	.io_bitmap	= { [ 0 ... IO_BITMAP_LONGS] = ~0 },		\
 }
 
-static inline void load_esp0(struct tss_struct *tss, unsigned long esp0)
+static inline void load_esp0(struct tss_struct *tss, struct thread_struct *thread)
 {
-	tss->esp0 = esp0;
+	tss->esp0 = thread->esp0;
 	/* This can only happen when SEP is enabled, no need to test "SEP"arately */
-	if ((unlikely(tss->ss1 != __KERNEL_CS))) {
-		tss->ss1 = __KERNEL_CS;
-		wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
-	}
-}
-
-static inline void disable_sysenter(struct tss_struct *tss)
-{
-	if (cpu_has_sep)  {
-		tss->ss1 = 0;
-		wrmsr(MSR_IA32_SYSENTER_CS, 0, 0);
+	if (unlikely(tss->ss1 != thread->sysenter_cs)) {
+		tss->ss1 = thread->sysenter_cs;
+		wrmsr(MSR_IA32_SYSENTER_CS, thread->sysenter_cs, 0);
 	}
 }