fixed a potential buffer overflow bug on the stack (thanks to Ghassan Misherg)

[slock.git] / slock.c

diff --git a/slock.c b/slock.c
index 0f4c3cc0bc7ea0c0bc2dbf8bb84327c93d46eb7b..94cb26ba61c3fa76a6ab448c99ea097cd87bcdaa 100644 (file)
--- a/slock.c
+++ b/slock.c
@@ -1,15 +1,13 @@
-/* (C)opyright MMIV-MMV Anselm R. Garbe <garbeam at gmail dot com>
+/* (C)opyright MMVI-MMVII Anselm R. Garbe <garbeam at gmail dot com>
  * See LICENSE file for license details.
  */
 #define _XOPEN_SOURCE 500
-
 #if HAVE_SHADOW_H
 #include <shadow.h>
-#else
-#include <pwd.h>
 #endif
 
 #include <ctype.h>
+#include <pwd.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
@@ -19,16 +17,41 @@
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>
 
+const char *
+get_password() { /* only run as root */
+       const char *rval;
+       struct passwd *pw;
+
+       if(geteuid() != 0) {
+               fputs("slock: cannot retrieve password entry (make sure to suid slock)\n", stderr);
+               exit(EXIT_FAILURE);
+       }
+       pw = getpwuid(getuid());
+       endpwent();
+       rval =  pw->pw_passwd;
+
+#if HAVE_SHADOW_H
+       {
+               struct spwd *sp;
+               sp = getspnam(getenv("USER"));
+               endspent();
+               rval = sp->sp_pwdp;
+       }
+#endif
+       /* drop privileges */
+       if(setgid(pw->pw_gid) < 0 || setuid(pw->pw_uid) < 0) {
+               fputs("slock: cannot drop privileges\n",stdout);
+               exit(EXIT_FAILURE);
+       }
+       return rval;
+}
+
 int
 main(int argc, char **argv) {
        char curs[] = {0, 0, 0, 0, 0, 0, 0, 0};
        char buf[32], passwd[256];
        int num, screen;
-#if HAVE_SHADOW_H
-       struct spwd *sp;
-#else
-       struct passwd *pw;
-#endif
+       const char *pws;
        unsigned int len;
        Bool running = True;
        Cursor invisible;
@@ -44,17 +67,7 @@ main(int argc, char **argv) {
                fputs("slock-"VERSION", (C)opyright MMVI Anselm R. Garbe\n", stdout);
                exit(EXIT_SUCCESS);
        }
-       if(geteuid() != 0) {
-               fputs("slock: cannot retrieve password entry (make sure to suid slock)\n", stderr);
-               exit(EXIT_FAILURE);
-       }
-#if HAVE_SHADOW_H
-       sp = getspnam(getenv("USER"));
-       endspent();
-#else
-       pw = getpwuid(getuid());
-       endpwent();
-#endif
+       pws = get_password();
        if(!(dpy = XOpenDisplay(0))) {
                fputs("slock: cannot open display\n", stderr);
                exit(EXIT_FAILURE);
@@ -62,11 +75,6 @@ main(int argc, char **argv) {
        screen = DefaultScreen(dpy);
 
        /* init */
-       passwd[0] = 0;
-       while(XGrabKeyboard(dpy, RootWindow(dpy, screen), True, GrabModeAsync,
-                        GrabModeAsync, CurrentTime) != GrabSuccess)
-               usleep(1000);
-
        wa.override_redirect = 1;
        wa.background_pixel = BlackPixel(dpy, screen);
        w = XCreateWindow(dpy, RootWindow(dpy, screen), 0, 0,
@@ -78,47 +86,50 @@ main(int argc, char **argv) {
        pmap = XCreateBitmapFromData(dpy, w, curs, 8, 8);
        invisible = XCreatePixmapCursor(dpy, pmap, pmap, &black, &black, 0, 0);
        XDefineCursor(dpy, w, invisible);
+       running = XGrabPointer(dpy, RootWindow(dpy, screen), False,
+                       ButtonPressMask | ButtonReleaseMask | PointerMotionMask,
+                       GrabModeAsync, GrabModeSync, None, invisible, CurrentTime) == GrabSuccess;
+       len = 10;
+       for(len = 10; len && (XGrabKeyboard(dpy, RootWindow(dpy, screen), True, GrabModeAsync,
+               GrabModeAsync, CurrentTime) != GrabSuccess); len--)
+               usleep(1000);
+       running = running && (len > 0);
+       len = 0;
        XMapRaised(dpy, w);
        XSync(dpy, False);
 
        /* main event loop */
        while(running && !XNextEvent(dpy, &ev))
                if(ev.type == KeyPress) {
-                       len = strlen(passwd);
                        buf[0] = 0;
-                       num = XLookupString(&ev.xkey, buf, sizeof(buf), &ksym, 0);
+                       num = XLookupString(&ev.xkey, buf, sizeof buf, &ksym, 0);
                        if(IsFunctionKey(ksym) || IsKeypadKey(ksym)
                                        || IsMiscFunctionKey(ksym) || IsPFKey(ksym)
                                        || IsPrivateKeypadKey(ksym))
                                continue;
                        switch(ksym) {
                        case XK_Return:
-#if HAVE_SHADOW_H
-                               if((running = strncmp(crypt(passwd, sp->sp_pwdp), sp->sp_pwdp, sizeof(passwd))))
-#else
-                               if((running = strncmp(crypt(passwd, pw->pw_passwd), pw->pw_passwd, sizeof(passwd))))
-#endif
+                               passwd[len] = 0;
+                               if((running = strcmp(crypt(passwd, pws), pws)) != 0)
                                        XBell(dpy, 100);
-                               passwd[0] = 0;
+                               len = 0;
                                break;
                        case XK_Escape:
-                               passwd[0] = 0;
+                               len = 0;
                                break;
                        case XK_BackSpace:
                                if(len)
-                                       passwd[--len] = 0;
+                                       --len;
                                break;
                        default:
-                               if(num && !iscntrl((int) buf[0])) {
-                                       buf[num] = 0;
-                                       if(len)
-                                               strncat(passwd, buf, sizeof(passwd));
-                                       else
-                                               strncpy(passwd, buf, sizeof(passwd));
+                               if(num && !iscntrl((int) buf[0]) && (len + num < sizeof passwd)) { 
+                                       memcpy(passwd + len, buf, num);
+                                       len += num;
                                }
                                break;
                        }
                }
+       XUngrabPointer(dpy, CurrentTime);
        XFreePixmap(dpy, pmap);
        XDestroyWindow(dpy, w);
        XCloseDisplay(dpy);