suid_exec.c revision da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968
/***********************************************************************
* *
* This software is part of the ast package *
* Copyright (c) 1982-2007 AT&T Knowledge Ventures *
* and is licensed under the *
* Common Public License, Version 1.0 *
* by AT&T Knowledge Ventures *
* *
* A copy of the License is available at *
* http://www.opensource.org/licenses/cpl1.0.txt *
* (with md5 checksum 059e8cd6165cb4c31e351f2b69388fd9) *
* *
* Information and Software Systems Research *
* AT&T Research *
* Florham Park NJ *
* *
* David Korn <dgk@research.att.com> *
* *
***********************************************************************/
#pragma prototyped
/*
* This is a program to execute 'execute only' and suid/sgid shell scripts.
* This program must be owned by root and must have the set uid bit set.
* It must not have the set group id bit set. This program must be installed
* where the define parameter THISPROG indicates to work correctly on system V
*
* Written by David Korn
* AT&T Labs
* Enhanced by Rob Stampfli
*/
/* The file name of the script to execute is argv[0]
* Argv[1] is the program name
* The basic idea is to open the script as standard input, set the effective
* user and group id correctly, and then exec the shell.
* The complicated part is getting the effective uid of the caller and
* setting the effective uid/gid. The program which execs this program
* may pass file descriptor FDIN as an open file with mode SPECIAL if
* the effective user id is not the real user id. The effective
* user id for authentication purposes will be the owner of this
* open file. On systems without the setreuid() call, e[ug]id is set
* by copying this program to a /tmp/file, making it a suid and/or sgid
* program, and then execing this program.
* A forked version of this program waits until it can unlink the /tmp
* file and then exits. Actually, we fork() twice so the parent can
* wait for the child to complete. A pipe is used to guarantee that we
* do not remove the /tmp file too soon.
*/
#include <ast.h>
#include "FEATURE/externs"
#include <ls.h>
#include <sig.h>
#include <error.h>
#include <sys/wait.h>
#include "version.h"
#define SPECIAL 04100 /* setuid execute only by owner */
#define FDIN 10 /* must be same as /dev/fd below */
#undef FDSYNC
#define FDSYNC 11 /* used on sys5 to synchronize cleanup */
#define FDVERIFY 12 /* used to validate /tmp process */
#undef BLKSIZE
#define BLKSIZE sizeof(char*)*1024
#define THISPROG "/etc/suid_exec"
#define DEFSHELL "/bin/sh"
static void error_exit(const char*);
static int in_dir(const char*, const char*);
static int endsh(const char*);
#ifndef _lib_setregid
# undef _lib_setreuid
#endif
#ifndef _lib_setreuid
static void setids(int,uid_t,gid_t);
static int mycopy(int, int);
static void maketemp(char*);
#else
static void setids(int,int,int);
#endif /* _lib_setreuid */
static const char version[] = "\n@(#)$Id: suid_exec "SH_RELEASE" $\n";
static const char badopen[] = "cannot open";
static const char badexec[] = "cannot exec";
static const char devfd[] = "/dev/fd/10"; /* must match FDIN above */
static char tmpname[] = "/tmp/SUIDXXXXXX";
static char **arglist;
static char *shell;
static char *command;
static uid_t ruserid;
static uid_t euserid;
static gid_t rgroupid;
static gid_t egroupid;
static struct stat statb;
int main(int argc,char *argv[])
{
register int m,n;
register char *p;
struct stat statx;
int mode;
uid_t effuid;
gid_t effgid;
NOT_USED(argc);
arglist = argv;
if((command = argv[1]) == 0)
error_exit(badexec);
ruserid = getuid();
euserid = geteuid();
rgroupid = getgid();
egroupid = getegid();
p = argv[0];
#ifndef _lib_setreuid
maketemp(tmpname);
if(strcmp(p,tmpname)==0)
{
/* At this point, the presumption is that we are the
* version of THISPROG copied into /tmp, with the owner,
* group, and setuid/gid bits correctly set. This copy of
* the program is executable by anyone, so we must be careful
* not to allow just any invocation of it to succeed, since
* it is setuid/gid. Validate the proper execution by
* examining the FDVERIFY file descriptor -- if it is owned
* by root and is mode SPECIAL, then this is proof that it was
* passed by a program with superuser privileges -- hence we
* can presume legitimacy. Otherwise, bail out, as we suspect
* an impostor.
*/
if(fstat(FDVERIFY,&statb) < 0 || statb.st_uid != 0 ||
(statb.st_mode & ~S_IFMT) != SPECIAL || close(FDVERIFY)<0)
error_exit(badexec);
/* This enables the grandchild to clean up /tmp file */
close(FDSYNC);
/* Make sure that this is a valid invocation of the clone.
* Perhaps unnecessary, given FDVERIFY, but what the heck...
*/
if(stat(tmpname,&statb) < 0 || statb.st_nlink != 1 ||
!S_ISREG(statb.st_mode))
error_exit(badexec);
if(ruserid != euserid &&
((statb.st_mode & S_ISUID) == 0 || statb.st_uid != euserid))
error_exit(badexec);
goto exec;
}
/* Make sure that this is the real setuid program, not the clone.
* It is possible by clever hacking to get past this point in the
* clone, but it doesn't do the hacker any good that I can see.
*/
if(euserid)
error_exit(badexec);
#endif /* _lib_setreuid */
/* Open the script for reading first and then validate it. This
* prevents someone from pulling a switcheroo while we are validating.
*/
n = open(p,0);
if(n == FDIN)
{
n = dup(n);
close(FDIN);
}
if(n < 0)
error_exit(badopen);
/* validate execution rights to this script */
if(fstat(FDIN,&statb) < 0 || (statb.st_mode & ~S_IFMT) != SPECIAL)
euserid = ruserid;
else
euserid = statb.st_uid;
/* do it the easy way if you can */
if(euserid == ruserid && egroupid == rgroupid)
{
if(access(p,X_OK) < 0)
error_exit(badexec);
}
else
{
/* have to check access on each component */
while(*p++)
{
if(*p == '/' || *p == 0)
{
m = *p;
*p = 0;
if(eaccess(argv[0],X_OK) < 0)
error_exit(badexec);
*p = m;
}
}
p = argv[0];
}
if(fstat(n, &statb) < 0 || !S_ISREG(statb.st_mode))
error_exit(badopen);
if(stat(p, &statx) < 0 ||
statb.st_ino != statx.st_ino || statb.st_dev != statx.st_dev)
error_exit(badexec);
if(stat(THISPROG, &statx) < 0 ||
(statb.st_ino == statx.st_ino && statb.st_dev == statx.st_dev))
error_exit(badexec);
close(FDIN);
if(fcntl(n,F_DUPFD,FDIN) != FDIN)
error_exit(badexec);
close(n);
/* compute the desired new effective user and group id */
effuid = euserid;
effgid = egroupid;
mode = 0;
if(statb.st_mode & S_ISUID)
effuid = statb.st_uid;
if(statb.st_mode & S_ISGID)
effgid = statb.st_gid;
/* see if group needs setting */
if(effgid != egroupid)
if(effgid != rgroupid || setgid(rgroupid) < 0)
mode = S_ISGID;
/* now see if the uid needs setting */
if(mode)
{
if(effuid != ruserid)
mode |= S_ISUID;
}
else if(effuid)
{
if(effuid != ruserid || setuid(ruserid) < 0)
mode = S_ISUID;
}
if(mode)
setids(mode, effuid, effgid);
#ifndef _lib_setreuid
exec:
#endif /* _lib_setreuid */
/* only use SHELL if file is in trusted directory and ends in sh */
shell = getenv("SHELL");
if(shell == 0 || !endsh(shell) || (
!in_dir("/bin",shell) &&
!in_dir("/usr/bin",shell) &&
!in_dir("/usr/lbin",shell) &&
!in_dir("/usr/local/bin",shell)))
shell = DEFSHELL;
argv[0] = command;
argv[1] = (char*)devfd;
execv(shell,argv);
error_exit(badexec);
}
/*
* return true of shell ends in sh of ksh
*/
static int endsh(register const char *shell)
{
while(*shell)
shell++;
if(*--shell != 'h' || *--shell != 's')
return(0);
if(*--shell=='/')
return(1);
if(*shell=='k' && *--shell=='/')
return(1);
return(0);
}
/*
* return true of shell is in <dir> directory
*/
static int in_dir(register const char *dir,register const char *shell)
{
while(*dir)
{
if(*dir++ != *shell++)
return(0);
}
/* return true if next character is a '/' */
return(*shell=='/');
}
static void error_exit(const char *message)
{
sfprintf(sfstdout,"%s: %s\n",command,message);
exit(126);
}
/*
* This version of access checks against effective uid and effective gid
*/
int eaccess(register const char *name, register int mode)
{
struct stat statb;
if (stat(name, &statb) == 0)
{
if(euserid == 0)
{
if(!S_ISREG(statb.st_mode) || mode != 1)
return(0);
/* root needs execute permission for someone */
mode = (S_IXUSR|S_IXGRP|S_IXOTH);
}
else if(euserid == statb.st_uid)
mode <<= 6;
else if(egroupid == statb.st_gid)
mode <<= 3;
#ifdef _lib_getgroups
/* on some systems you can be in several groups */
else
{
static int maxgroups;
gid_t *groups=0;
register int n;
if(maxgroups==0)
{
/* first time */
if((maxgroups=getgroups(0,groups)) < 0)
{
/* pre-POSIX system */
maxgroups=NGROUPS_MAX;
}
}
groups = (gid_t*)malloc((maxgroups+1)*sizeof(gid_t));
n = getgroups(maxgroups,groups);
while(--n >= 0)
{
if(groups[n] == statb.st_gid)
{
mode <<= 3;
break;
}
}
}
#endif /* _lib_getgroups */
if(statb.st_mode & mode)
return(0);
}
return(-1);
}
#ifdef _lib_setreuid
static void setids(int mode,int owner,int group)
{
if(mode & S_ISGID)
setregid(rgroupid,group);
/* set effective uid even if S_ISUID is not set. This is because
* we are *really* executing EUID root at this point. Even if S_ISUID
* is not set, the value for owner that is passsed should be correct.
*/
setreuid(ruserid,owner);
}
#else
/*
* This version of setids creats a /tmp file and copies itself into it.
* The "clone" file is made executable with appropriate suid/sgid bits.
* Finally, the clone is exec'ed. This file is unlinked by a grandchild
* of this program, who waits around until the text is free.
*/
static void setids(int mode,uid_t owner,gid_t group)
{
register int n,m;
int pv[2];
/*
* Create a token to pass to the new program for validation.
* This token can only be procured by someone running with an
* effective userid of root, and hence gives the clone a way to
* certify that it was really invoked by THISPROG. Someone who
* is already root could spoof us, but why would they want to?
*
* Since we are root here, we must be careful: What if someone
* linked a valuable file to tmpname?
*/
unlink(tmpname); /* should normally fail */
#ifdef O_EXCL
if((n = open(tmpname, O_WRONLY | O_CREAT | O_EXCL, SPECIAL)) < 0 ||
unlink(tmpname) < 0)
#else
if((n = open(tmpname, O_WRONLY | O_CREAT ,SPECIAL)) < 0 || unlink(tmpname) < 0)
#endif
error_exit(badexec);
if(n != FDVERIFY)
{
close(FDVERIFY);
if(fcntl(n,F_DUPFD,FDVERIFY) != FDVERIFY)
error_exit(badexec);
}
mode |= S_IEXEC|(S_IEXEC>>3)|(S_IEXEC>>6);
/* create a pipe for synchronization */
if(pipe(pv) < 0)
error_exit(badexec);
if((n=fork()) == 0)
{ /* child */
close(FDVERIFY);
close(pv[1]);
if((n=fork()) == 0)
{ /* grandchild -- cleans up clone file */
signal(SIGHUP, SIG_IGN);
signal(SIGINT, SIG_IGN);
signal(SIGQUIT, SIG_IGN);
signal(SIGTERM, SIG_IGN);
read(pv[0],pv,1); /* wait for clone to close pipe */
while(unlink(tmpname) < 0 && errno == ETXTBSY)
sleep(1);
exit(0);
}
else if(n == -1)
exit(1);
else
{
/* Create a set[ug]id file that will become the clone.
* To make this atomic, without need for chown(), the
* child takes on desired user and group. The only
* downsize of this that I can see is that it may
* screw up some per- * user accounting.
*/
if((m = open(THISPROG, O_RDONLY)) < 0)
exit(1);
if((mode & S_ISGID) && setgid(group) < 0)
exit(1);
if((mode & S_ISUID) && owner && setuid(owner) < 0)
exit(1);
#ifdef O_EXCL
if((n = open(tmpname,O_WRONLY|O_CREAT|O_TRUNC|O_EXCL, mode)) < 0)
#else
unlink(tmpname);
if((n = open(tmpname,O_WRONLY|O_CREAT|O_TRUNC, mode)) < 0)
#endif /* O_EXCL */
exit(1);
/* populate the clone */
m = mycopy(m,n);
if(chmod(tmpname,mode) <0)
exit(1);
exit(m);
}
}
else if(n == -1)
error_exit(badexec);
else
{
arglist[0] = (char*)tmpname;
close(pv[0]);
/* move write end of pipe into FDSYNC */
if(pv[1] != FDSYNC)
{
close(FDSYNC);
if(fcntl(pv[1],F_DUPFD,FDSYNC) != FDSYNC)
error_exit(badexec);
}
/* wait for child to die */
while((m = wait(0)) != n)
if(m == -1 && errno != EINTR)
break;
/* Kill any setuid status at this point. That way, if the
* clone is not setuid, we won't exec it as root. Also, don't
* neglect to consider that someone could have switched the
* clone file on us.
*/
if(setuid(ruserid) < 0)
error_exit(badexec);
execv(tmpname,arglist);
error_exit(badexec);
}
}
/*
* create a unique name into the <template>
*/
static void maketemp(char *template)
{
register char *cp = template;
register pid_t n = getpid();
/* skip to end of string */
while(*++cp);
/* convert process id to string */
while(n > 0)
{
*--cp = (n%10) + '0';
n /= 10;
}
}
/*
* copy THISPROG into the open file number <fdo> and close <fdo>
*/
static int mycopy(int fdi, int fdo)
{
char buffer[BLKSIZE];
register int n;
while((n = read(fdi,buffer,BLKSIZE)) > 0)
if(write(fdo,buffer,n) != n)
break;
close(fdi);
close(fdo);
return n;
}
#endif /* _lib_setreuid */