os-posix: set groups properly for -runas

Andrew Griffiths reports that -runas does not set supplementary group
IDs.  This means that gid 0 (root) is not dropped when switching to an
unprivileged user.

Add an initgroups(3) call to use the -runas user's /etc/groups
membership to update the supplementary group IDs.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

os-posix.c

@@ -31,6 +31,7 @@
 /*needed for MAP_POPULATE before including qemu-options.h */
 #include <sys/mman.h>
 #include <pwd.h>
+#include <grp.h>
 #include <libgen.h>
 
 /* Needed early for CONFIG_BSD etc. */
@@ -199,6 +200,11 @@ static void change_process_uid(void)
             fprintf(stderr, "Failed to setgid(%d)\n", user_pwd->pw_gid);
             exit(1);
         }
+        if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
+            fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
+                    user_pwd->pw_name, user_pwd->pw_gid);
+            exit(1);
+        }
         if (setuid(user_pwd->pw_uid) < 0) {
             fprintf(stderr, "Failed to setuid(%d)\n", user_pwd->pw_uid);
             exit(1);