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/* Dropping uid/gid privileges of the current process permanently.
Copyright (C) 2009-2024 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;
}
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