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TUCoPS :: Unix :: General :: krnl20.htm

Paper: Delivering Signals for Fun and Profit



    Most unices


    Razor  Team  (BindView)  is  proud  to announce their new security
    paper, "Delivering signals for fun and profit".  This paper is  an
    attempt to discuss security aspects of very common signal  handler
    coding   practices,   describing   theoretical   background    and
    demonstrating actual  attack scenarios  against live  code in Unix
    environment.  The paper is available at:

    According to a popular belief, writing signal handlers has  little
    or nothing to do with secure programming, as long as handler  code
    itself looks good.  At the same time, there have been  discussions
    on functions that  shall be invoked  from handlers, and  functions
    that shall never, ever be  used there.  Most Unix  systems provide
    a standarized set of signal-safe library calls.  Few systems  have
    extensive  documentation  of  signal-safe  calls  -  that includes
    OpenBSD, Solaris, etc.:

        "The  following   functions  are   either  reentrant   or  not
        interruptible  by  sig-  nals   and  are  async-signal   safe.
        Therefore applications may  invoke them, without  restriction,
        from signal-catching functions:

        _exit(2), access(2), alarm(3), cfgetispeed(3), cfgetospeed(3),
        cfsetispeed(3), cfsetospeed(3), chdir(2), chmod(2),  chown(2),
        close(2),  creat(2),  dup(2),  dup2(2),  execle(2), execve(2),
        fcntl(2),   fork(2),    fpathconf(2),   fstat(2),    fsync(2),
        getegid(2), geteuid(2),  getgid(2), getgroups(2),  getpgrp(2),
        getpid(2), getppid(2), getuid(2), kill(2), link(2),  lseek(2),
        mkdir(2), mkfifo(2), open(2), pathconf(2), pause(2),  pipe(2),
        raise(3), read(2), rename(2), rmdir(2), setgid(2), setpgid(2),
        setsid(2),     setuid(2),     sigaction(2),      sigaddset(3),
        sigdelset(3),  sigemptyset(3),  sigfillset(3), sigismember(3),
        signal(3),   sigpending(2),   sigprocmask(2),   sigsuspend(2),
        sleep(3),   stat(2),   sysconf(3),   tcdrain(3),    tcflow(3),
        tcflush(3),   tcgetattr(3),   tcgetpgrp(3),    tcsendbreak(3),
        tcsetattr(3),  tcsetpgrp(3),   time(3),  times(3),   umask(2),
        uname(3), unlink(2), utime(3), wait(2), waitpid(2),  write(2),
        sigpause(3), sigset(3).

        All  functions  not  in  the  above  list are considered to be
        unsafe with respect to signals.  That is to say, the behaviour
        of  such  functions  when  called  from  a  signal  handler is
        undefined.   In  general  though,  signal  handlers  should do
        little more than set a flag; most other actions are not safe."

    It  is  suggested  to  take  special  care  when  performing   any
    non-atomic operations while signal delivery is not blocked, and/or
    not  to  rely  on  internal  program  state  in  signal   handler.
    Generally, signal handlers should do not much more than setting  a
    flag, whenever it is acceptable.

    Unfortunately,   there   were   no   known,   practical   security
    considerations of  such bad  coding practices.   And while  signal
    can  be  delivered  _anywhere_  during  the userspace execution of
    given program, most of programmers never take enough care to avoid
    potential implications caused by  this fact.  Approximately  80 to
    90% of signal handlers we  have examined were written in  insecure

    This paper is an attempt  to demonstrate and analyze actual  risks
    caused by  this kind  of coding  practices, and  to discuss threat
    scenarios that  can be  used by  an attacker  in order to escalate
    local privileges, or, sometimes, gain remote access to a  machine.
    This  class  of  vulnerabilities  affects  numerous complex setuid
    programs  (Sendmail,  screen,  pppd,  etc.)  and  several  network
    daemons (ftpd, httpd and so on).

    Thanks to Theo  de Raadt for  bringing this problem  to Zalewski's
    attention; to Przemyslaw Frasunek for remote attack  possibilities
    discussion; Dvorak, Chris Evans  and Pekka Savola for  outstanding
    contribution  to  heap  corruption  attacks  field;  Gregory  Neil
    Shapiro  and  Solar  Designer  for  their  comments  on the issues
    discussed below.  Additional  thanks to Mark Loveless,  Dave Mann,
    Matt Power  and other  RAZOR team  members for  their support  and

    1) Impact: handler re-entry (Sendmail case)
    Before we discuss more generalized attack scenarios, we would like
    to  explain  signal  handler  races  starting with very simple and
    clean example.  We would try to exploit non-atomic signal handler.
    The following code generalizes, in simplified way, very common bad
    coding practice  (which is  present, for  example, in  setuid root
    Sendmail program up to 8.11.3 and 8.12.0.Beta7):

     * This is a generic verbose signal handler - does some  *
     * logging and cleanup, probably calling other routines. *
    void sighndlr(int dummy) {
      // *** Initial cleanup code, calling the following somewhere:
      // *** 1 *** >> Additional clean-up code - unlink tmp files, etc <<
       * This is a signal handler declaration somewhere *
       * at the beginning of main code.                 *
      // *** Other initialization routines, and global pointer
      // *** assignment somewhere in the code (we assume that
      // *** nnn is partially user-dependent, yyy does not have to be):
      // *** 2 *** >> further processing, allocated memory <<
      // *** 2 *** >> is filled with any data, etc...     <<

    This  code  seems  to  be  pretty  immune  to any kind of security
    compromises.  But this is just an illusion.  By delivering one  of
    the signals handled by sighndlr() function somewhere in the middle
    of main code  execution (marked as  '*** 2 ***'  in above example)
    code  execution  would  reach  handler  function.  Let's assume we
    delivered SIGHUP.   Syslog message  is written,  two pointers  are
    freed, and some more clean-up is done before exiting (*** 1 ***).

    Now, by  quickly delivering  another signal  - SIGTERM  (note that
    already delivered signal is masked and would be not delivered,  so
    you cannot deliver SIGHUP, but there is absolutely nothing against
    delivering  SIGTERM)  -  attacker  might cause sighndlr() function
    re-entry.  This is a very common condition - 'shared' handlers are
    declared for SIGQUIT, SIGTERM, SIGINT, and so on.

    Now,  for  the  purpose  of  this  demonstration, we would like to
    target heap structures by exploiting free() and syslog() behavior.
    It is very important to understand how [v]syslog()  implementation
    works.  We would focus on Linux glibc code - this function creates
    a temporary copy of the logged message in so-called  memory-buffer
    stream, which is dynamically allocated using two malloc() calls  -
    the first one allocates general stream description structure,  and
    the other one  creates actual buffer,  which would contain  logged

    Please refer the following URL for vsyslog() function sources:

    Stream management functions (open_memstream, etc.) can be found at

    In  order  for  this  particular  attack  to  be  successful,  two
    conditions have to be met:

        * syslog() data must  be user-dependent (like in  Sendmail log
          messages describing transferred mail traffic),
        * second of these two global memory blocks must be aligned the
          way  that  would  be  re-used  in  second   open_memstream()
          malloc() call.

    The second buffer (global_ptr2) would be free()d during the  first
    sighndlr()  call,  so  if  these  conditions  are  met, the second
    syslog() call would  re-use this memory  and overwrite this  area,
    including heap-management structures, with user-dependent syslog()

    Of course, this situation is  not limited to two global  buffers -
    generally, we need one out of any number of free()d buffers to  be
    aligned  that  way.   Additional  possibilities  are  related   to
    interrupting  free()  chain  by  precise  SIGTERM  delivery and/or
    influencing  buffer  sizes  /  heap  data order by using different
    input data patterns.

    If so,  the attacker  can cause  second free()  pass to  be called
    with a pointer to user-dependent data (syslog buffer), this  leads
    to instant root compromise - see excellent article by Chris  Evans
    (based on observations by Pekka Savola):

    Practical  discussion  and  exploit  code  for  the  vulnerability
    discussed in above article can be found there:

    Below is a sample 'vulnerable program' code:

    #include <signal.h>
    #include <syslog.h>
    #include <string.h>
    #include <stdlib.h>
    void *global1, *global2;
    char *what;
    void sh(int dummy) {
    int main(int argc,char* argv[]) {

    You can exploit it, forcing free() to be called on a memory region
    filled with 0x41414141 (you can see this value in the registers at
    the time of crash -- the bytes represented as 41 in hex are set by
    the 'A'  input characters  in the  variable $LOG  below).   Sample
    command lines for a Bash shell are:

        $ gcc vuln.c -o vuln
        $ PAD=`perl -e '{print "x"x410}'`
        $ LOG=`perl -e '{print "A"x100}'`
        $ ./vuln $LOG $PAD & sleep 1; killall -HUP vuln; sleep 1; killall -TERM vuln

    The result should  be a segmentation  fault followed by  nice core
    dump (for Linux glibc 2.1.9x and 2.0.7).

        (gdb) back
        #0  chunk_free (ar_ptr=0x4013dce0, p=0x80499a0) at malloc.c:3069
        #1  0x4009b334 in __libc_free (mem=0x80499a8) at malloc.c:3043
        #2  0x80485b8 in sh ()
        #4  0x400d5971 in __libc_nanosleep () from /lib/
        #5  0x400d5801 in __sleep (seconds=10) at ../sysdeps/unix/sysv/linux/sleep.c:85
        #6  0x80485d6 in sh ()

    So, as  you can  see, failure  was caused  when signal handler was
    re-entered.  __libf_free function  was called with a  parameter of
    0x080499a8, which points somewhere in the middle of our AAAs:

        (gdb) x/s 0x80499a8
        0x80499a8:       'A' <repeats 94 times>, "\n"

    You can find  0x41414141 in the  registers, as well,  showing this
    data is being processed.   For more analysis, please refer  to the
    paper mentioned above.

    Obviously,  that  is  just  an  example  of this attack.  Whenever
    signal handler execution is  non-atomic, attacks of this  kind are
    possible by re-entering  the handler when  it is in  the middle of
    performing  non-reentrant  operations.   Heap  damage  is the most
    obvious vector of attack, in this case, but not the only one.

    2) Impact: signal in the middle (screen case)
    The attack  described above  usually requires  specific conditions
    to  be  met,  and  takes  advantage  of  non-atomic signal handler
    execution, which can be  easily avoided by using  additional flags
    or blocking signal delivery.

    But, as signal  can be delivered  at any moment  (unless explictly
    blocked), this is obvious that it is possible to perform an attack
    without re-entering the handler itself.  It is enough to deliver a
    signal  in  a  'not  appropriate'  moment.   There  are two attack

    A) re-entering libc functions:

       Every  function  that  is  not  listed  as  reentry-safe  is  a
       potential source of vulnerabilities.  Indeed, numerous  library
       functions  are  operating  on  global  variables, and/or modify
       global state  in non-atomic  way.   Once again, heap-management
       routines are probably the best example.  By delivering a signal
       when malloc(),  free() or  any other  libcall of  this kind  is
       being  called,  all  subsequent  calls  to  the heap management
       routines  made  from  signal  handler  would have unpredictable
       effect,  as  heap  state  is  completely  unpredictable for the

       Other  good  examples  are  functions  working on static/global
       variables and buffers like certain implementations of strtok(),
       inet_ntoa(), gethostbyname() and so  on. In all cases,  results
       will be unpredictable.

    B) interrupting non-atomic modifications:

       This  is  basically  the  same  problem,  but  outside  library
       functions.  For example, the following code:

        dropped_privileges = 1;

    is, technically speaking, using safe library functions only.  But,
    at the same  time, it is  possible to interrupt  execution between
    substitution  and  setuid()  call,  causing  signal  handler to be
    executed  with   dropped_privileges  flag   set,  but    superuser
    privileges not dropped.

    This, very often, might be a source of serious problems.

    First of all, we would like  to come back to Sendmail example,  to
    demonstrate potential consequences of re-entering libc.  Note that
    signal handler is NOT re-entered - signal is delivered only once:

        #0  0x401705bc in chunk_free (ar_ptr=0x40212ce0, p=0x810f900) at malloc.c:3117  #1  0x4016fd12 in chunk_alloc (ar_ptr=0x40212ce0, nb=8200) at malloc.c:2601
        #2  0x4016f7e6 in __libc_malloc (bytes=8192) at malloc.c:2703
        #3  0x40168a27 in open_memstream (bufloc=0xbfff97bc, sizeloc=0xbfff97c0) at memstream.c:112
        #4  0x401cf4fa in vsyslog (pri=6, fmt=0x80a5e03 "%s: %s", ap=0xbfff99ac) at syslog.c:142
        #5  0x401cf447 in syslog (pri=6, fmt=0x80a5e03 "%s: %s") at syslog.c:102
        #6  0x8055f64 in sm_syslog ()
        #7  0x806793c in logsender ()
        #8  0x8063902 in dropenvelope ()
        #9  0x804e717 in finis ()
        #10 0x804e9d8 in intsig ()                 <---- ** SIGINT **
        #11 <signal handler called>
        #12 chunk_alloc (ar_ptr=0x40212ce0, nb=4104) at malloc.c:2968
        #13 0x4016f7e6 in __libc_malloc (bytes=4097) at malloc.c:2703

    Heap corruption is caused by interruped malloc() call and,  later,
    by calling  malloc() once  again from  vsyslog() function  invoked
    from handler.

    There  are  two  another   examples  of  very  interesting   stack
    corruption  caused  by  re-entering  heap  management  routines in
    Sendmail daemon - in both cases, signal was delivered only once:


        #0  0x401705bc in chunk_free (ar_ptr=0xdbdbdbdb, p=0x810b8e8) at malloc.c:3117
        #1  0xdbdbdbdb in ?? ()


        #9  0x79f68510 in ?? ()
        Cannot access memory at address 0xc483c689

    We'd like to leave this one as  an exercise for a reader - try  to
    figure  out  why  this  happens  and  why  this  problem  can   be
    exploitable.  For now,  we would like to  come back to our  second
    scenario, interrupting non-atomic code to show that targeting heap
    is not the only possibility.

    Some  programs  are  temporarily  returning  to  superuser  UID in
    cleanup routines, e.g., in order  to unlink specific files.   Very
    often, by  entering the  handler at  given moment,  is possible to
    perform  all  the  cleanup  file  access operations with superuser

    Here's an  example of  such coding,  that can  be found  mainly in
    interactive setuid software:

    #include <signal.h>
    #include <string.h>
    #include <stdlib.h>
    void sh(int dummy) {
      printf("Running with uid=%d euid=%d\n",getuid(),geteuid());
    int main(int argc,char* argv[]) {
      // this is a temporarily privileged code:
    ---- EOF ----
    # gcc vuln.c -o vuln; chmod 4755 vuln
    # su user
    $ ./vuln & sleep 3; killall -TERM vuln; sleep 3; killall -TERM vuln
    Running with uid=500 euid=500
    Running with uid=500 euid=0

    Such a  coding practice  can be  found, par  example, in  'screen'
    utility developed  by Oliver  Laumann.   One of  the most  obvious
    locations is CoreDump handler [screen.c]:

        static sigret_t
        CoreDump SIGDEFARG

    SIGSEGV can be  delivered in the  middle of user-initiated  screen
    detach routine, for example. To better understand what and why  is
    going on, here's an strace output for detach (Ctrl+A, D) command:

        23534 geteuid()                         = 0
        23534 geteuid()                         = 0
        23534 getuid()                          = 500
        23534 setreuid(0, 500)                  = 0    *** HERE IT HAPPENS ***
        23534 getegid()                         = 500
        23534 chmod("/home/lcamtuf/.screen/23534.tty5.nimue", 0600) = 0
        23534 utime("/home/lcamtuf/.screen/23534.tty5.nimue", NULL) = 0
        23534 geteuid()                         = 500
        23534 getuid()                          = 0

    Marked line sets uid to  zero.  If SIGSEGV is  delivered somewhere
    near  this  point,  CoreDump()  handler  would  run with superuser
    privileges, due to initial setuid(getuid()).

    3) Remote exploitation of signal delivery (WU-FTPD case)
    This is a very interesting issue, directly related to  re-entering
    libc functions and/or interrupting non-atomic code.  Many  complex
    daemons,  like  ftp,  some  http/proxy  services, MTAs, etc., have
    SIGURG handlers declared  - very often  these handlers are  pretty
    verbose,  calling  syslog(),  or  freeing some resources allocated
    for specific  connection.   The trick  is that  SIGURG, obviously,
    can  be  delivered  over  the  network,  using TCP/IP OOB message.
    Thus,  it  is  possible  to  perform  attacks  using network layer
    without any priviledges.

    Below is a SIGURG handler routine, which, with small modifications
    is shared both by BSD ftpd and WU-FTPD daemons:

        static VOIDRET myoob FUNCTION((input), int input)
          if (getline(cp, 7, stdin) == NULL) {
            reply(221, "You could at least say goodbye.");

    As  you  can  see  in  certain  conditions, dologout() function is
    called.  This routine looks this way:

        dologout(int status)
            if (logged_in) {
                delay_signaling(); /* we can't allow any signals while euid==0: kinch */
                (void) seteuid((uid_t) 0);
                wu_logwtmp(ttyline, "", "");
            if (logging)
                syslog(LOG_INFO, "FTP session closed");

    As you can see, the  authors took an additional precaution  not to
    allow signal  delivery in  the "logged_in"  case.   Unfortunately,
    syslog() is a perfect example  of a libc function that  should NOT
    be  called   during  signal   handling,  regardless   of   whether
    "logged_in"  or  any  other  special  condition  happens  to be in

    As mentioned  before, heap  management functions  such as malloc()
    are called within  syslog(), and these  functions are not  atomic.
    The OOB  message might  arrive when  the heap  is in virtually any
    possible state.  Playing with  uids / privileges / internal  state
    is an option, as well.

    In  most  cases  this  is  a  non-issue  for local attacks, as the
    attacker might control the  execution environment (e.g., the  load
    average,  the  number  of  local  files  that  the daemon needs to
    access, etc.)  and try  a virtually  infinite number  of times  by
    invoking  the  same  program  over  and over again, increasing the
    possibility  of  delivering  signal  at  given  point.  For remote
    attacks, this is a major issue,  but as long as the attack  itself
    won't  cause  service  to  stop  responding, thousands of attempts
    might be performed.


    This is a  very complex and  difficult task.   There are at  least
    three aspects of this:

        * Using reentrant-safe libcalls in signal handlers only.  This
          would require major rewrites of numerous programs.   Another
          half-solution  is  to  implement  a  wrapper  around   every
          insecure libcall used, having special global flag checked to
          avoid re-entry,
        * Blocking  signal delivery  during all  non-atomic operations
          and/or constructing  signal handlers  in the  way that would
          not  rely  on  internal  program  state  (e.g. unconditional
          setting of specific flag and nothing else),
        * Blocking signal delivery in signal handlers.

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