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ELF provides two parallel views of a file's contents. The linking view is defined by the section header table, an array of Elf32_Shdr. The execution view is defined by the program header table, an array of Elf32_Phdr.
Theoretically the ELF specification is quite liberal. Position, contents and order of sections and segments are not restricted. But in real life an operating system is to used to just one program loader, one linker and few compilers. This makes the work of virus writers easier. We can reverse engineer the de-facto standard, a tiny subset of what the ELF standard allows. On a typical system only a minority of programs violates this subset, so ignoring them does not lower chances of survival.
All headers necessary to execute a program are stuffed into the first page (0x1000 bytes on i386), probably to simplify the design of an OS' executable format detector. Dynamically linked executables require a few program headers more than static executables, but that's about all the variation there is. In any case the program header of the code segments is followed by the program header of the data segment. Static executables have the code segment at index 0, dynamically linked executables at index 2.
Let's get a bit more serious and examine the assembly program from The language of evil. A standalone executable built from assembler source is probably the most trivial example we can find.
Command: pre/i386-redhat7.3-linux/segments/objdump.sh
#!/bin/bash
/bin/ls -Ll tmp/i386-redhat7.3-linux/evil_magic/intel
/usr/bin/objdump -fp tmp/i386-redhat7.3-linux/evil_magic/intel |
Output: out/i386-redhat7.3-linux/segments/objdump
-rwxrwxr-x 1 alba alba 416 Jan 8 23:08 tmp/i386-redhat7.3-linux/evil_magic/intel
tmp/i386-redhat7.3-linux/evil_magic/intel: file format elf32-i386
architecture: i386, flags 0x00000102:
EXEC_P, D_PAGED
start address 0x08048080
Program Header:
LOAD off 0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12
filesz 0x00000097 memsz 0x00000097 flags r-x
|
objdump's output is butt-ugly. On to readelf. The line starting with "There are 1 program headers" shows the value of e_phnum and e_phoff.
Command: pre/i386-redhat7.3-linux/segments/readelf.sh
#!/bin/bash
/bin/ls -Ll tmp/i386-redhat7.3-linux/evil_magic/intel
/usr/bin/readelf -l tmp/i386-redhat7.3-linux/evil_magic/intel |
Output: out/i386-redhat7.3-linux/segments/readelf
-rwxrwxr-x 1 alba alba 416 Jan 8 23:08 tmp/i386-redhat7.3-linux/evil_magic/intel
Elf file type is EXEC (Executable file)
Entry point 0x8048080
There are 1 program headers, starting at offset 52
Program Header:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
LOAD 0x000000 0x08048000 0x08048000 0x00097 0x00097 R E 0x1000
Section to Segment mapping:
Segment Sections...
00 .text |
Nice to see the entry point (0x8048080) again. Program layout is a simplified variation of Sort of an answer. The value of FileSiz includes ELF header and program header.
Overhead:
overhead = Entry point - VirtAddr =
0x8048080 - 0x8048000 = 0x80 = 128 bytes
Effective code size:
code size = FileSiz - overhead =
0x97 - 0x80 = 0x17 = 23 bytes
This matches with the disassembly listing. However, the ratio of file size to effective code deserves the title "Bloat", with capital B. Only 6 percent of the file actually do something useful!
Bloat factor:
code size / file size = 23 / 416 = 0.055
Anyway, we see that even for trivial examples the code is surrounded by lots of other stuff. Let's zoom in on our target.
Command: pre/i386-redhat7.3-linux/segments/sh/readelf.sh
#!/bin/bash
/bin/ls -Ll /bin/bash
/usr/bin/readelf -l /bin/bash |
Output: out/i386-redhat7.3-linux/segments/sh/readelf
-rwxr-xr-x 1 root root 541096 Apr 12 2002 /bin/bash
Elf file type is EXEC (Executable file)
Entry point 0x8059440
There are 6 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x08048034 0x08048034 0x000c0 0x000c0 R E 0x4
INTERP 0x0000f4 0x080480f4 0x080480f4 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
LOAD 0x000000 0x08048000 0x08048000 0x7e414 0x7e414 R E 0x1000
LOAD 0x07e420 0x080c7420 0x080c7420 0x05934 0x09ad0 RW 0x1000
DYNAMIC 0x083a0c 0x080cca0c 0x080cca0c 0x000d8 0x000d8 RW 0x4
NOTE 0x000108 0x08048108 0x08048108 0x00020 0x00020 R 0x4
Section to Segment mapping:
Segment Sections...
00
01 .interp
02 .interp .note.ABI-tag .hash .dynsym .dynstr .gnu.version .gnu.version_r .rel.dyn .rel.plt .init .plt .text .fini .rodata
03 .data .eh_frame .dynamic .ctors .dtors .got .bss
04 .dynamic
05 .note.ABI-tag |
Looks intimidating. But then the ELF specification says that only segments of type "LOAD" are considered for execution. Since the flags of the first one include "execute" but not "write" it must be the code segment. The other one has the "write" flags set, so it must be the data segment. There is one possible deviation: On sparc-sunos most executables built by Sun feature a data segment with "execute" flag.
MemSiz (0x9ad0) is larger than FileSiz (0x5934) in the data segment. Just like with mmap(2) excessive bytes are defined to be initialized with 0. The linker takes advantages of that by grouping all variables that should be initialized to zero at the end. Note that the last section of segment 3 (counting starts with 0) is called .bss, the traditional name for this kind of area.
The mapping for segment 2 looks even more complex. But I would guess that .rodata means "read-only data" and .text contains productive code, as opposed to the administrative stuff in the other sections.
Some executables of Red Hat 8.0 have an additional program header of type GNU_EH_FRAME.
Previous examples in The language of evil used an __attribute__ clause to put the code into section .text. Without that it would end up in section .rodata. Both are members of the code segment which is executable in it its entireness; in this regard that would make no difference.
But what about putting the code a write enabled data segment? These settings can probably be changed by mprotect(2). But what are the default settings?
Output = Source: out/i386-redhat7.3-linux/evil_magic/func.inc
const unsigned char in_code[]
__attribute__ (( aligned(8), section(".text") )) =
{
0x53, /* 00000000: push ebx */
0x6A,0x04, /* 00000001: push byte +0x4 */
0x58, /* 00000003: pop eax */
0x31,0xDB, /* 00000004: xor ebx,ebx */
0x43, /* 00000006: inc ebx */
0xB9,0x01,0x80,0x04,0x08, /* 00000007: mov ecx,0x8048001 */
0x6A,0x03, /* 0000000C: push byte +0x3 */
0x5A, /* 0000000E: pop edx */
0xCD,0x80, /* 0000000F: int 0x80 */
0x5B, /* 00000011: pop ebx */
0xC3 /* 00000012: ret */
}; /* 19 bytes (0x13) */ |
Source: src/evil_magic/self_modify.c
#include <setjmp.h>
#include <signal.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "func.inc"
typedef void (*PfnVoid)(void);
#define MEMCPY_TEST(where) \
memcpy(in_##where, in_code, sizeof(in_code)); \
test(#where, (PfnVoid)in_##where)
static jmp_buf env;
static int received_sigill = 0;
static void on_sigill(int sig)
{
printf(" on_sigill=%d ", sig);
received_sigill = 1;
longjmp(env, 1);
}
static void test(const char* name, PfnVoid code)
{
printf("%8p is %s ... ", code, name);
fflush(stdout);
received_sigill = 0;
if (0 == setjmp(env))
{
signal(SIGILL, on_sigill);
code();
}
printf(" sigill=%d\n", received_sigill);
}
static char in_data[sizeof(in_code)];
int main()
{
char* in_heap = malloc(sizeof(in_code));
char in_stack[sizeof(in_code)];
test("code", (PfnVoid)in_code);
MEMCPY_TEST(data);
MEMCPY_TEST(heap);
MEMCPY_TEST(stack);
return 0;
} |
Output: out/i386-redhat7.3-linux/evil_magic/self_modify
0x8048530 is code ... ELF sigill=0
0x804991c is data ... ELF sigill=0
0x8049968 is heap ... ELF sigill=0
0xbffff980 is stack ... ELF sigill=0 |