1 files changed, 131 insertions, 0 deletions
diff --git a/gl/lib/idpriv-drop.c b/gl/lib/idpriv-drop.c
new file mode 100644
index 0000000..b059944
--- /dev/null
+++ b/gl/lib/idpriv-drop.c
@@ -0,0 +1,131 @@
+/* Dropping uid/gid privileges of the current process permanently.
+   Copyright (C) 2009-2023 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include "idpriv.h"
+
+#include <stdlib.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+int
+idpriv_drop (void)
+{
+#if HAVE_GETUID
+  int uid = getuid ();
+#endif
+#if HAVE_GETGID
+  int gid = getgid ();
+#endif
+
+  /* Drop the gid privilege first, because in some cases the gid privilege
+     cannot be dropped after the uid privilege has been dropped.  */
+
+  /* This is for executables that have the setgid bit set.  */
+#if HAVE_SETRESGID /* glibc, FreeBSD, OpenBSD, HP-UX */
+  /* This code is needed: In particular, on HP-UX 11.11, setregid (gid, gid)
+     may leave the saved gid as 0.  See also the comment below regarding
+     setresuid.  */
+  if (setresgid (gid, gid, gid) < 0)
+    return -1;
+#elif HAVE_SETREGID /* Mac OS X, NetBSD, AIX, IRIX, Solaris, OSF/1, Cygwin */
+  if (setregid (gid, gid) < 0)
+    return -1;
+#elif HAVE_SETEGID /* Solaris 2.4 */
+  if (setegid (gid) < 0)
+    return -1;
+#endif
+
+  /* This is for executables that have the setuid bit set.  */
+#if HAVE_SETRESUID /* glibc, FreeBSD, OpenBSD, HP-UX */
+  /* On systems which have setresuid(), we use it instead of setreuid(),
+     because
+       Hao Chen, David Wagner, Drew Dean: Setuid Demystified
+       <https://www.usenix.org/legacy/publications/library/proceedings/sec02/full_papers/chen/chen.pdf>
+     says about setreuid(): "The rule by which the saved uid id is modified
+     is complicated." Similarly, <https://unixpapa.com/incnote/setuid.html>
+     says about setreuid(): "What exactly happens to the saved UID when this
+     is used seems to vary a lot."  */
+  if (setresuid (uid, uid, uid) < 0)
+    return -1;
+#elif HAVE_SETREUID /* Mac OS X, NetBSD, AIX, IRIX, Solaris, OSF/1, Cygwin */
+  if (setreuid (uid, uid) < 0)
+    return -1;
+#elif HAVE_SETEUID /* Solaris 2.4 */
+  if (seteuid (uid) < 0)
+    return -1;
+#endif
+
+  /* Verify that the privileges have really been dropped.
+     This verification is here for security reasons.  Doesn't matter if it
+     takes a couple of system calls.
+     On Solaris (which has saved uids and gids but no getresuid, getresgid
+     functions), we could read /proc/<pid>/cred and verify the saved uid and
+     gid found there. But it's not clear to me when to interpret the file as a
+     'prcred_t' and when as a 'prcred32_t'.
+     Hao Chen, David Wagner, Drew Dean: Setuid Demystified
+     <https://www.usenix.org/legacy/publications/library/proceedings/sec02/full_papers/chen/chen.pdf>
+     section 8.1.3 also recommends to use a setreuid call as a probe, but
+     this call would unexpectedly succeed (and the verification thus fail)
+     on Linux if the process has the CAP_SETUID capability.
+     When the verification fails, it indicates that we need to use different
+     API in the code above. Therefore 'abort ()', not 'return -1'.  */
+#if HAVE_GETRESUID /* glibc, FreeBSD, OpenBSD, HP-UX */
+  {
+    uid_t real;
+    uid_t effective;
+    uid_t saved;
+    if (getresuid (&real, &effective, &saved) < 0
+        || real != uid
+        || effective != uid
+        || saved != uid)
+      abort ();
+  }
+#else
+# if HAVE_GETEUID
+  if (geteuid () != uid)
+    abort ();
+# endif
+# if HAVE_GETUID
+  if (getuid () != uid)
+    abort ();
+# endif
+#endif
+#if HAVE_GETRESGID /* glibc, FreeBSD, OpenBSD, HP-UX */
+  {
+    gid_t real;
+    gid_t effective;
+    gid_t saved;
+    if (getresgid (&real, &effective, &saved) < 0
+        || real != gid
+        || effective != gid
+        || saved != gid)
+      abort ();
+  }
+#else
+# if HAVE_GETEGID
+  if (getegid () != gid)
+    abort ();
+# endif
+# if HAVE_GETGID
+  if (getgid () != gid)
+    abort ();
+# endif
+#endif
+
+  return 0;
+}