xserver: check for elevated privileges not uid=0

This allows us to run the server as a normal user whilst still
being able to use the -modulepath, -logfile and -config switches
We define a xf86PrivsElevated which will do the checks and cache
the result in case it is called more than once.
Also renamed the paths #defines to match their new meaning.
Original discussion which led to this patch can be found here:
http://lists.freedesktop.org/archives/xorg-devel/2011-September/025853.html

Signed-off-by: Antoine Martin <antoine@nagafix.co.uk>
Tested-by: Michal Suchanek <hramrach at centrum.cz>
Reviewed-by: Jamey Sharp <jamey at minilop.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>

3 files changed, 84 insertions, 23 deletions

diff --git a/hw/xfree86/common/xf86Config.c b/hw/xfree86/common/xf86Config.c
index 569695c8a..f51be7e9c 100644
--- a/hw/xfree86/common/xf86Config.c
+++ b/hw/xfree86/common/xf86Config.c
@@ -72,8 +72,8 @@
  * These paths define the way the config file search is done.  The escape
  * sequences are documented in parser/scan.c.
  */
-#ifndef ROOT_CONFIGPATH
-#define ROOT_CONFIGPATH	"%A," "%R," \
+#ifndef ALL_CONFIGPATH
+#define ALL_CONFIGPATH	"%A," "%R," \
 			"/etc/X11/%R," "%P/etc/X11/%R," \
 			"%E," "%F," \
 			"/etc/X11/%F," "%P/etc/X11/%F," \
@@ -83,8 +83,8 @@
 			"%P/lib/X11/%X.%H," \
 			"%P/lib/X11/%X"
 #endif
-#ifndef USER_CONFIGPATH
-#define USER_CONFIGPATH	"/etc/X11/%S," "%P/etc/X11/%S," \
+#ifndef RESTRICTED_CONFIGPATH
+#define RESTRICTED_CONFIGPATH	"/etc/X11/%S," "%P/etc/X11/%S," \
 			"/etc/X11/%G," "%P/etc/X11/%G," \
 			"/etc/X11/%X," "/etc/%X," \
 			"%P/etc/X11/%X.%H," \
@@ -92,14 +92,14 @@
 			"%P/lib/X11/%X.%H," \
 			"%P/lib/X11/%X"
 #endif
-#ifndef ROOT_CONFIGDIRPATH
-#define ROOT_CONFIGDIRPATH	"%A," "%R," \
+#ifndef ALL_CONFIGDIRPATH
+#define ALL_CONFIGDIRPATH	"%A," "%R," \
 				"/etc/X11/%R," "%C/X11/%R," \
 				"/etc/X11/%X," "%C/X11/%X"
 #endif
-#ifndef USER_CONFIGDIRPATH
-#define USER_CONFIGDIRPATH	"/etc/X11/%R," "%C/X11/%R," \
-				"/etc/X11/%X," "%C/X11/%X"
+#ifndef RESTRICTED_CONFIGDIRPATH
+#define RESTRICTED_CONFIGDIRPATH	"/etc/X11/%R," "%C/X11/%R," \
+					"/etc/X11/%X," "%C/X11/%X"
 #endif
 #ifndef SYS_CONFIGDIRPATH
 #define SYS_CONFIGDIRPATH	"/usr/share/X11/%X," "%D/X11/%X"
@@ -2310,12 +2310,12 @@ xf86HandleConfigFile(Bool autoconfig)
 	MessageType filefrom = X_DEFAULT;
 	MessageType dirfrom = X_DEFAULT;
 
-	if (getuid() == 0) {
-	    filesearch = ROOT_CONFIGPATH;
-	    dirsearch = ROOT_CONFIGDIRPATH;
+	if (!xf86PrivsElevated()) {
+	    filesearch = ALL_CONFIGPATH;
+	    dirsearch = ALL_CONFIGDIRPATH;
 	} else {
-	    filesearch = USER_CONFIGPATH;
-	    dirsearch = USER_CONFIGDIRPATH;
+	    filesearch = RESTRICTED_CONFIGPATH;
+	    dirsearch = RESTRICTED_CONFIGDIRPATH;
 	}
 
 	if (xf86ConfigFile)
diff --git a/hw/xfree86/common/xf86Init.c b/hw/xfree86/common/xf86Init.c
index c1e48eed2..5263b5f73 100644
--- a/hw/xfree86/common/xf86Init.c
+++ b/hw/xfree86/common/xf86Init.c
@@ -238,6 +238,65 @@ xf86PrintMarkers(void)
   LogPrintMarkers();
 }
 
+Bool xf86PrivsElevated(void)
+{
+  static Bool privsTested = FALSE;
+  static Bool privsElevated = TRUE;
+
+  if (!privsTested) {
+#if defined(WIN32)
+    privsElevated = FALSE;
+#else
+    if ((getuid() != geteuid()) || (getgid() != getegid())) {
+      privsElevated = TRUE;
+    } else {
+#if defined(HAVE_ISSETUGID)
+      privsElevated = issetugid();
+#elif defined(HAVE_GETRESUID)
+      uid_t ruid, euid, suid;
+      gid_t rgid, egid, sgid;
+
+      if ((getresuid(&ruid, &euid, &suid) == 0) &&
+          (getresgid(&rgid, &egid, &sgid) == 0)) {
+        privsElevated = (euid != suid) || (egid != sgid);
+      }
+      else {
+        printf("Failed getresuid or getresgid");
+        /* Something went wrong, make defensive assumption */
+        privsElevated = TRUE;
+      }
+#else
+      if (getuid()==0) {
+        /* running as root: uid==euid==0 */
+        privsElevated = FALSE;
+      }
+      else {
+        /*
+         * If there are saved ID's the process might still be privileged
+         * even though the above test succeeded. If issetugid() and
+         * getresgid() aren't available, test this by trying to set
+         * euid to 0.
+         */
+        unsigned int oldeuid;
+        oldeuid = geteuid();
+
+        if (seteuid(0) != 0) {
+          privsElevated = FALSE;
+        } else {
+          if (seteuid(oldeuid) != 0) {
+            FatalError("Failed to drop privileges.  Exiting\n");
+          }
+          privsElevated = TRUE;
+        }
+      }
+#endif
+    }
+#endif
+    privsTested = TRUE;
+  }
+  return privsElevated;
+}
+
 static Bool
 xf86CreateRootWindow(WindowPtr pWin)
 {
@@ -872,7 +931,7 @@ OsVendorInit(void)
 
 #ifdef O_NONBLOCK
   if (!beenHere) {
-    if (geteuid() == 0 && getuid() != geteuid())
+    if (xf86PrivsElevated())
     {
       int status;
 
@@ -1043,10 +1102,11 @@ ddxProcessArgument(int argc, char **argv, int i)
       FatalError("Required argument to %s not specified\n", argv[i]);	\
     }
 
-  /* First the options that are only allowed for root */
+  /* First the options that are not allowed with elevated privileges */
   if (!strcmp(argv[i], "-modulepath") || !strcmp(argv[i], "-logfile")) {
-    if ( (geteuid() == 0) && (getuid() != 0) ) {
-      FatalError("The '%s' option can only be used by root.\n", argv[i]);
+    if (xf86PrivsElevated()) {
+      FatalError("The '%s' option cannot be used with "
+                 "elevated privileges.\n", argv[i]);
     }
     else if (!strcmp(argv[i], "-modulepath"))
     {
@@ -1074,9 +1134,9 @@ ddxProcessArgument(int argc, char **argv, int i)
   if (!strcmp(argv[i], "-config") || !strcmp(argv[i], "-xf86config"))
   {
     CHECK_FOR_REQUIRED_ARGUMENT();
-    if (getuid() != 0 && !xf86PathIsSafe(argv[i + 1])) {
+    if (xf86PrivsElevated() && !xf86PathIsSafe(argv[i + 1])) {
       FatalError("\nInvalid argument for %s\n"
-	  "\tFor non-root users, the file specified with %s must be\n"
+	  "\tWith elevated privileges, the file specified with %s must be\n"
 	  "\ta relative path and must not contain any \"..\" elements.\n"
 	  "\tUsing default "__XCONFIGFILE__" search path.\n\n",
 	  argv[i], argv[i]);
@@ -1087,9 +1147,9 @@ ddxProcessArgument(int argc, char **argv, int i)
   if (!strcmp(argv[i], "-configdir"))
   {
     CHECK_FOR_REQUIRED_ARGUMENT();
-    if (getuid() != 0 && !xf86PathIsSafe(argv[i + 1])) {
+    if (xf86PrivsElevated() && !xf86PathIsSafe(argv[i + 1])) {
       FatalError("\nInvalid argument for %s\n"
-	  "\tFor non-root users, the file specified with %s must be\n"
+	  "\tWith elevated privileges, the file specified with %s must be\n"
 	  "\ta relative path and must not contain any \"..\" elements.\n"
 	  "\tUsing default "__XCONFIGDIR__" search path.\n\n",
 	  argv[i], argv[i]);
@@ -1375,7 +1435,7 @@ ddxUseMsg(void)
   ErrorF("\n");
   ErrorF("\n");
   ErrorF("Device Dependent Usage\n");
-  if (getuid() == 0 || geteuid() != 0)
+  if (!xf86PrivsElevated())
   {
     ErrorF("-modulepath paths      specify the module search path\n");
     ErrorF("-logfile file          specify a log file name\n");
diff --git a/hw/xfree86/common/xf86Priv.h b/hw/xfree86/common/xf86Priv.h
index e0b18098e..255407b48 100644
--- a/hw/xfree86/common/xf86Priv.h
+++ b/hw/xfree86/common/xf86Priv.h
@@ -145,6 +145,7 @@ extern _X_EXPORT Bool xf86LoadModules(char **list, pointer *optlist);
 extern _X_EXPORT int xf86SetVerbosity(int verb);
 extern _X_EXPORT int xf86SetLogVerbosity(int verb);
 extern _X_EXPORT Bool xf86CallDriverProbe( struct _DriverRec * drv, Bool detect_only );
+extern _X_EXPORT Bool xf86PrivsElevated(void);
 
 #endif /* _NO_XF86_PROTOTYPES */