clear passwords with explicit_bzero

Make sure to explicitly clear memory that is used for password input. memset
is often optimized out by the compiler.

Brought to attention by the OpenBSD community, see:
https://marc.info/?t=146989502600003&r=1&w=2
Thread subject: x11/slock: clear passwords with explicit_bzero

Changes:

- explicit_bzero.c import from libressl-portable.
- Makefile: add COMPATSRC for compatibility src.
- config.mk: add separate *BSD section in config.mk to simply uncomment it on
  these platforms.

Makefile

@@ -3,7 +3,7 @@
 
 include config.mk
 
-SRC = slock.c
+SRC = slock.c ${COMPATSRC}
 OBJ = ${SRC:.c=.o}
 
 all: options slock
@@ -35,8 +35,8 @@ clean:
 dist: clean
 	@echo creating dist tarball
 	@mkdir -p slock-${VERSION}
-	@cp -R LICENSE Makefile README config.def.h config.mk ${SRC} slock.1 \
-		slock-${VERSION}
+	@cp -R LICENSE Makefile README config.def.h config.mk ${SRC} \
+		explicit_bzero.c slock.1 slock-${VERSION}
 	@tar -cf slock-${VERSION}.tar slock-${VERSION}
 	@gzip slock-${VERSION}.tar
 	@rm -rf slock-${VERSION}

config.mk

@@ -18,9 +18,13 @@ LIBS = -L/usr/lib -lc -lcrypt -L${X11LIB} -lX11 -lXext -lXrandr
 CPPFLAGS = -DVERSION=\"${VERSION}\" -DHAVE_SHADOW_H
 CFLAGS = -std=c99 -pedantic -Wall -Os ${INCS} ${CPPFLAGS}
 LDFLAGS = -s ${LIBS}
+COMPATSRC = explicit_bzero.c
 
 # On *BSD remove -DHAVE_SHADOW_H from CPPFLAGS and add -DHAVE_BSD_AUTH
 # On OpenBSD and Darwin remove -lcrypt from LIBS
+#LIBS = -L/usr/lib -lc -L${X11LIB} -lX11 -lXext -lXrandr
+#CPPFLAGS = -DVERSION=\"${VERSION}\" -DHAVE_BSD_AUTH -D_BSD_SOURCE
+#COMPATSRC =
 
 # compiler and linker
 CC = cc

explicit_bzero.c

@@ -0,0 +1,19 @@
+/*	$OpenBSD: explicit_bzero.c,v 1.3 2014/06/21 02:34:26 matthew Exp $ */
+/*
+ * Public domain.
+ * Written by Matthew Dempsky.
+ */
+
+#include <string.h>
+
+__attribute__((weak)) void
+__explicit_bzero_hook(void *buf, size_t len)
+{
+}
+
+void
+explicit_bzero(void *buf, size_t len)
+{
+	memset(buf, 0, len);
+	__explicit_bzero_hook(buf, len);
+}

slock.c

@@ -23,6 +23,8 @@
 #include <bsd_auth.h>
 #endif
 
+#include "util.h"
+
 enum {
 	INIT,
 	INPUT,
@@ -135,7 +137,7 @@ readpw(Display *dpy, const char *pws)
 	 * timeout. */
 	while (running && !XNextEvent(dpy, &ev)) {
 		if (ev.type == KeyPress) {
-			buf[0] = 0;
+			explicit_bzero(&buf, sizeof(buf));
 			num = XLookupString(&ev.xkey, buf, sizeof(buf), &ksym, 0);
 			if (IsKeypadKey(ksym)) {
 				if (ksym == XK_KP_Enter)
@@ -161,14 +163,16 @@ readpw(Display *dpy, const char *pws)
 					XBell(dpy, 100);
 					failure = True;
 				}
+				explicit_bzero(&passwd, sizeof(passwd));
 				len = 0;
 				break;
 			case XK_Escape:
+				explicit_bzero(&passwd, sizeof(passwd));
 				len = 0;
 				break;
 			case XK_BackSpace:
 				if (len)
-					--len;
+					passwd[len--] = 0;
 				break;
 			default:
 				if (num && !iscntrl((int)buf[0]) && (len + num < sizeof(passwd))) {

util.h

@@ -0,0 +1,2 @@
+#undef explicit_bzero
+void explicit_bzero(void *, size_t);