1 files changed, 1 insertions, 0 deletions
diff --git a/vm.c b/vm.c
index 16fb03b..d4982ee 100644
--- a/vm.c
+++ b/vm.c
@@ -168,6 +168,7 @@ switchuvm(struct proc *p)
   cpu->gdt[SEG_TSS].s = 0;
   cpu->ts.ss0 = SEG_KDATA << 3;
   cpu->ts.esp0 = (uint)proc->kstack + KSTACKSIZE;
+  cpu->ts.iomb = (ushort) 0xFFFF;  // forbid I/O instructions from user space
   ltr(SEG_TSS << 3);
   if(p->pgdir == 0)
     panic("switchuvm: no pgdir");