add bounds check to prevent memory corruption and integer overflow

This change adds a bounds check to make sure userland can't overflow a kernel stack buffer. In addition the change also prevents an integer overflow from occurring.

minix/kernel/system/do_safecopy.c

@@ -412,6 +412,9 @@ int do_vsafecopy(struct proc * caller, message * m_ptr)
 
 	/* No. of vector elements. */
 	els = m_ptr->m_lsys_kern_vsafecopy.vec_size;
+	if (els > SCPVEC_NR) {
+		return EINVAL;
+	}
 	bytes = els * sizeof(struct vscp_vec);
 
 	/* Obtain vector of copies. */