diff --git a/exec.c b/exec.c
index 8dbbdb6..e5d1816 100644
--- a/exec.c
+++ b/exec.c
@@ -46,6 +46,8 @@ exec(char *path, char **argv)
       goto bad;
     if((sz = allocuvm(pgdir, sz, ph.vaddr + ph.memsz)) == 0)
       goto bad;
+    if(ph.vaddr % PGSIZE != 0)
+      goto bad;
     if(loaduvm(pgdir, (char*)ph.vaddr, ip, ph.off, ph.filesz) < 0)
       goto bad;
   }