3 files changed, 30 insertions, 6 deletions

diff --git a/config.def.h b/config.def.h
index eae2d9a..6fba2b6 100644
--- a/config.def.h
+++ b/config.def.h
@@ -1,3 +1,7 @@
+/* user and group to drop privileges to */
+static const char *user  = "nobody";
+static const char *group = "nogroup";
+
 static const char *colorname[NUMCOLS] = {
         "black",     /* after initialization */
         "#005577",   /* during input */
diff --git a/config.mk b/config.mk
index 049305c..11357a7 100644
--- a/config.mk
+++ b/config.mk
@@ -15,7 +15,7 @@ INCS = -I. -I/usr/include -I${X11INC}
 LIBS = -L/usr/lib -lc -lcrypt -L${X11LIB} -lX11 -lXext -lXrandr
 
 # flags
-CPPFLAGS = -DVERSION=\"${VERSION}\" -DHAVE_SHADOW_H
+CPPFLAGS = -DVERSION=\"${VERSION}\" -D_DEFAULT_SOURCE -DHAVE_SHADOW_H
 CFLAGS = -std=c99 -pedantic -Wall -Os ${INCS} ${CPPFLAGS}
 LDFLAGS = -s ${LIBS}
 COMPATSRC = explicit_bzero.c
diff --git a/slock.c b/slock.c
index da4b099..7127ebe 100644
--- a/slock.c
+++ b/slock.c
@@ -6,6 +6,7 @@
 
 #include <ctype.h>
 #include <errno.h>
+#include <grp.h>
 #include <pwd.h>
 #include <stdarg.h>
 #include <stdlib.h>
@@ -83,7 +84,6 @@ dontkillme(void)
 }
 #endif
 
-/* only run as root */
 static const char *
 getpw(void)
 {
@@ -119,10 +119,6 @@ getpw(void)
         }
 #endif /* HAVE_SHADOW_H */
 
-        /* drop privileges */
-        if (geteuid() == 0 &&
-            ((getegid() != pw->pw_gid && setgid(pw->pw_gid) < 0) || setuid(pw->pw_uid) < 0))
-                die("slock: cannot drop privileges\n");
         return rval;
 }
 
@@ -316,6 +312,10 @@ usage(void)
 
 int
 main(int argc, char **argv) {
+        struct passwd *pwd;
+        struct group *grp;
+        uid_t duid;
+        gid_t dgid;
         const char *pws;
         Display *dpy;
         int s, nlocks;
@@ -328,6 +328,18 @@ main(int argc, char **argv) {
                 usage();
         } ARGEND
 
+        /* validate drop-user and -group */
+        errno = 0;
+        if (!(pwd = getpwnam(user)))
+                die("slock: getpwnam %s: %s\n", user, errno ?
+                    strerror(errno) : "user entry not found");
+        duid = pwd->pw_uid;
+        errno = 0;
+        if (!(grp = getgrnam(group)))
+                die("slock: getgrnam %s: %s\n", group, errno ?
+                    strerror(errno) : "group entry not found");
+        dgid = grp->gr_gid;
+
 #ifdef __linux__
         dontkillme();
 #endif
@@ -339,6 +351,14 @@ main(int argc, char **argv) {
         if (!(dpy = XOpenDisplay(NULL)))
                 die("slock: cannot open display\n");
 
+        /* drop privileges */
+        if (setgroups(0, NULL) < 0)
+                die("slock: setgroups: %s\n", strerror(errno));
+        if (setgid(dgid) < 0)
+                die("slock: setgid: %s\n", strerror(errno));
+        if (setuid(duid) < 0)
+                die("slock: setuid: %s\n", strerror(errno));
+
         /* check for Xrandr support */
         rr = XRRQueryExtension(dpy, &rrevbase, &rrerrbase);