x86: Fix non-lazy GS handling in sys_vm86()

This fixes a stack corruption panic or null dereference oops
due to a bad GS in resume_userspace() when returning from
sys_vm86() and calling lockdep_sys_exit().

Only a problem when CONFIG_LOCKDEP and CONFIG_CC_STACKPROTECTOR
enabled.
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <1244384628.2323.4.camel@bimbo>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/vm86_32.c

@@ -287,10 +287,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	info->regs.pt.ds = 0;
 	info->regs.pt.es = 0;
 	info->regs.pt.fs = 0;
-
-/* we are clearing gs later just before "jmp resume_userspace",
- * because it is not saved/restored.
- */
+#ifndef CONFIG_X86_32_LAZY_GS
+	info->regs.pt.gs = 0;
+#endif
 
 /*
  * The flags register is also special: we cannot trust that the user
@@ -343,7 +342,9 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 	__asm__ __volatile__(
 		"movl %0,%%esp\n\t"
 		"movl %1,%%ebp\n\t"
+#ifdef CONFIG_X86_32_LAZY_GS
 		"mov  %2, %%gs\n\t"
+#endif
 		"jmp resume_userspace"
 		: /* no outputs */
 		:"r" (&info->regs), "r" (task_thread_info(tsk)), "r" (0));