1 files changed, 81 insertions, 4 deletions

diff --git a/apt-pkg/contrib/fileutl.cc b/apt-pkg/contrib/fileutl.cc
index 6b8f04dea..e81f32a52 100644
--- a/apt-pkg/contrib/fileutl.cc
+++ b/apt-pkg/contrib/fileutl.cc
@@ -48,6 +48,7 @@
 #include <errno.h>
 #include <glob.h>
 #include <pwd.h>
+#include <grp.h>
 
 #include <set>
 #include <algorithm>
@@ -64,6 +65,10 @@
 #include <endian.h>
 #include <stdint.h>
 
+#if __gnu_linux__
+#include <sys/prctl.h>
+#endif
+
 #include <apti18n.h>
 									/*}}}*/
 
@@ -2170,17 +2175,89 @@ bool Popen(const char* Args[], FileFd &Fd, pid_t &Child, FileFd::OpenMode Mode)
 
 bool DropPrivs()
 {
-   if (getuid() != 0)
+   // uid will be 0 in the end, but gid might be different anyway
+   uid_t old_uid = getuid();
+   gid_t old_gid = getgid();
+
+   if (old_uid != 0)
+      return true;
+   if(_config->FindB("Debug::NoDropPrivs", false) == true)
       return true;
 
-   const std::string nobody = _config->Find("APT::User::Nobody", "nobody");
-   struct passwd *pw = getpwnam(nobody.c_str());
+   const std::string toUser = _config->Find("APT::Sandbox::User", "_apt");
+   struct passwd *pw = getpwnam(toUser.c_str());
    if (pw == NULL)
-      return _error->Warning("No user %s, can not drop rights", nobody.c_str());
+      return _error->Error("No user %s, can not drop rights", toUser.c_str());
+
+#if __gnu_linux__
+   // see prctl(2), needs linux3.5
+   int ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0,0, 0);
+   if(ret < 0)
+      _error->Warning("PR_SET_NO_NEW_PRIVS failed with %i", ret);
+#endif
+   // Do not change the order here, it might break things
+   if (setgroups(1, &pw->pw_gid))
+      return _error->Errno("setgroups", "Failed to setgroups");
+
+   if (setegid(pw->pw_gid) != 0)
+      return _error->Errno("setegid", "Failed to setegid");
+
    if (setgid(pw->pw_gid) != 0)
       return _error->Errno("setgid", "Failed to setgid");
+
    if (setuid(pw->pw_uid) != 0)
       return _error->Errno("setuid", "Failed to setuid");
 
+   // the seteuid() is probably uneeded (at least thats what the linux
+   // man-page says about setuid(2)) but we cargo culted it anyway
+   if (seteuid(pw->pw_uid) != 0)
+      return _error->Errno("seteuid", "Failed to seteuid");
+
+   // Verify that the user has only a single group, and the correct one
+   gid_t groups[1];
+   if (getgroups(1, groups) != 1)
+      return _error->Errno("getgroups", "Could not get new groups");
+   if (groups[0] != pw->pw_gid)
+      return _error->Error("Could not switch group");
+
+   // Verify that gid, egid, uid, and euid changed
+   if (getgid() != pw->pw_gid)
+      return _error->Error("Could not switch group");
+   if (getegid() != pw->pw_gid)
+      return _error->Error("Could not switch effective group");
+   if (getuid() != pw->pw_uid)
+      return _error->Error("Could not switch user");
+   if (geteuid() != pw->pw_uid)
+      return _error->Error("Could not switch effective user");
+
+#ifdef HAVE_GETRESUID
+   // verify that the saved set-user-id was changed as well
+   uid_t ruid = 0;
+   uid_t euid = 0;
+   uid_t suid = 0;
+   if (getresuid(&ruid, &euid, &suid))
+      return _error->Errno("getresuid", "Could not get saved set-user-ID");
+   if (suid != pw->pw_uid)
+      return _error->Error("Could not switch saved set-user-ID");
+#endif
+
+#ifdef HAVE_GETRESGID
+   // verify that the saved set-group-id was changed as well
+   gid_t rgid = 0;
+   gid_t egid = 0;
+   gid_t sgid = 0;
+   if (getresgid(&rgid, &egid, &sgid))
+      return _error->Errno("getresuid", "Could not get saved set-group-ID");
+   if (sgid != pw->pw_gid)
+      return _error->Error("Could not switch saved set-group-ID");
+#endif
+
+   // Check that uid and gid changes do not work anymore
+   if (pw->pw_gid != old_gid && (setgid(old_gid) != -1 || setegid(old_gid) != -1))
+      return _error->Error("Could restore a gid to root, privilege dropping did not work");
+
+   if (pw->pw_uid != old_uid && (setuid(old_uid) != -1 || seteuid(old_uid) != -1))
+      return _error->Error("Could restore a uid to root, privilege dropping did not work");
+
    return true;
 }