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Linux virus writing tutorial

29a [4full]
December 1999

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In this paper, I'll discuss how to make a linux virus. Of course you won't use this to make one.

Before let's play, lemme tell ya that the enligsh language is not my native one. There are few code here, as many things are theorical (but applicable, don't fear). Some asm skillz is required, and *nx knowleadge is better. Of course this paper wuz written with vi :)

1. Prelude

1.1 Linux ASM - gdb is your freind but you should have several others.

Here i'll describe what you need, and how to use it.

you can use strip to get some stripped ELFs (for testing purposes) for oldschooler like me, vi is a decent asm editor. many values, infos, structures are in your kernel srcs. use grep. The syntax to compil is quiet easy:

	nasm -f elf virus.asm
	cc virus.o -o virus

et voila.

1.2 Syscalls interface - syntax, and such boring things.

Linux assembly is 32 bits, and use syscalls. It's something between win32 and dos - that's why i love it. The syscalls are functions called by int 80h. It's a gate to C libz. eax holds the function number and the others hold the arguments in order: ebx 1st, ecx 2nd, and so on up to ebp, the 7th. You have the list of the function numberz in /usr/include/asm/unistd.h For example, opening a file looks like:

        mov eax,5               ;  eax = function 5 (open a file)
        mov ebx,fil             ;  ebx = pointer to the file to open
        mov ecx,2               ;  opening flags (0=r/o 1=w/o 2=r/w)
        int 80h                 ;  call the stargate

The return is an handle used for subsequent calls, like for dos or win32. Finding syscall usage is not always an easy task. The few docs i found about it are mentionned at the 4.3 section. Return values are stored in eax, or structures pointed by argz. Some syscalls require you to have superuser priviledges.

2. Obscurs ELF Manipulations.

2.1 Thoses damn structures needed to know.

I hope you like brut infos :) I won't describe everything here, as I ripped the stuff from better docs. You'll find where to get them at the 4.3 section.

ELF format is really flexible; the only fixed part is the ELF header : at BOF. In practice, most of the ELFs you'll find looks like:

elf_header (34h bytes)
program_header (6*20h btyes)
sections 1
sections 2
sections x
sections_header (x*28h bytes)

Basically, elf_header is used to give general informations about other headers.

struct elf_header
0e_identholds the magic values 0x7f,'ELF' and some flags
+10e_typethis word contains the file type (core, exe, ...)
+12e_machinegive the machine needed for running (3 = x86)
+14e_versionELF header version. Currently 1.
+18e_entryvirutal address of entry point. I can see you smiling.
+1ce_phoffprogram header offset (see below)
+20e_shoffsections header offset (see below)
+24e_flagssome other flags (processor specific nfos)
+28e_ehsizesize of the ELF header
+2Ae_phentsizesize of one entry in the program header
+2Ce_phnumnumber of entrys in the program header
+2Ee_shentsizesize of one entry in the section header
+30e_shunumnumber of entrys in the section header
+32e_shstrndxgive the entry number of the name string section (if exists)
struct section_header
0sh_namecontains a pointer to the name string section giving the
+4sh_typegive the section type [name of this section
+8sh_flagssome other flags ...
+csh_addrvirtual addr of the section while running
+10sh_offsetoffset of the section in the file
+14sh_sizezara white phone numba
+18sh_linkhis use depends on the section type
+1csh_infodepends on the section type
+24sh_entsizeused when section contains fixed size entrys
struct program_header
0p_typetype of segment
+4p_offsetoffset in file where to start the segment at
+8p_vaddrhis virtual address in memory
+cp_addrphysical address (if relevant, else equ to p_vaddr)
+10p_fileszsize of datas read from offset
+14p_memszsize of the segment in memory
+18p_flagssegment flags (rwx perms)

As i said above, i won't waste your HD space with tons of flag descriptions. You'll find them in the - nice - ELF documention (urlz at 4.3). Everything is document. It's the linux world. So use the source dudez !!!

2.2 The mysterys of execution time

A lotta things happen when executing the file. (don't forget to check you kernel srcs) The kernel reads the program header and compute segments from the file, then the sections are defined and allocated. You'll notice the modifications while debugging with gdb: before and after 'run' the memory is not in the same state, so set a breakpoint at the virutal entry point to read allocated datas. Finally, miscellanous dynamic values are set.

2.3 the mandragore'z way of smashin the file for phun and profit

Sorry for this ego abuse, but after all, that's a nice way to infect ELFs that i'll describe here, and here's the story of how i elaborated it.

act one:

I tryied a lotta things. At first i though that adding a section at EOF and writing my viral code right after should work. The code is no more accessible at run time, so it segmentation faulted :(

act two:

Then i noticed the hole in virutal memory between the code segment and the data one. But i wuzn't able to use it (and believe me, i tryied many things). Overwriting not really used section is not safe, so forget it.

act three:

Finally I decided to look aroung the segments, not the section. Adding a segment wuz possible, but when i tryied to relocate the program header i came in a lot of troubles. So i decided to enlarge one. As i wanted to code a non-destructive virus, i choose to append to the file, not to overwrite a part of it. The only way then wuz to enlarge a segment. As i append to the file, the only enlargable segment was the data one. The new virtual entry point is easy to calculate from the virutal address of the data segment. I wuz able to run code before the host. I used the old virutal entry point to get back to the host, but it segf'd once again.

act four:

The data segment size is not the same in memory and in the file. It now contains the rest of the file in memory, with our viral code appended. I thought the segfs came from that. And i wuz right. (for once) The .bss section problem arised. Normally, this section's flag is SHT_NOBITS. As you're not talking kernelian, i'll tell ya what it means: this section should not contain bytes from the programs. But i know understand why the data segment stops just before it in the file; the program bytes are copied in memory up to our code. And the .bss section has to be filled of zero. So i decided to overwrite it w/ zeros at runtime before returning to the host. Once again it segf'd. Damn.

act five:

With further testing I discovered the role of the .rel sections. Especially the .rel.bss section. At runtime, the beginning of the .bss section containes relocated values. And i did overwrite then. My first attempt to bypass this problem wuz to check the other sections between the .bss file offset and our viral code, and to overwrite it with zeroes. No, i didn't overwrite the section headers. I'm not so dumb :) When the kernel execute the file, the .bss is already zeroed and the recolation won't be overwritten. This should work! The code wuz bigger (computing zeroable section). But once again i got a lotta troubles ; the .bss section if often bigger than the rest of the file. Yes I overwritten my viral code before discovering it.

act six:

Crashing half of the hosts is not a solution (at least for me). So i took the other solution (may be you already got it) : getting back to the runtime patch and overwrite the .bss section before returning, but avoiding the relocations. Hopefully the relocations things are quiet simple. I included a rutine to get the farest relocation in memory and started to zero the .bss section from there up to the viral code. No. This should have been to easy. Once again i overwritten my viral code.

seventh and final act:

May be you're clever than me and already guess why; because of the .bss size. The last thing to do wuz to add a rutine to migrate behind the .bss section in memory before patching it. et voila.

This is a nice story, but it's not yet ended. A bad point is that's only applicable to regular ELFs: compiled C, (6 entry in the program header, a .bss section, regular entrys sizes, ...) And if you strip an infected ELF, it won't work anymore. I also encountered some problemz w/ proggyz using the kdelib.

Directories Operations

Now that we can infect files, let's find some.

3.1 Syscalls needed to seek directories

For linux, a directory is a file like the others. This is a nice string, but that's not really true :) You can use the open function to access it, but you can't read it with the 'read' function. You have to use 'readdir'. Close it with 'close'. You can't open it with r/w flags. Open it r/o. Many syscalls which requires a file descriptor are applicable with a directory descriptor. Thoses descriptorz are called handle in dos/win32. In case it's not allowed, the syscall return EISDIR (-21). This is a polite way to say "get the hell outta here, it's a fuckin directory."

3.2 Searching for files - and find some

As I not yet ended the way to stay TSR, do it yourself or stay a runtimer linux virus writer. Being a RLVW :) obliges you to seek directories. You can seek the current directory by opening '.' . You can of course access the '..' directory, but beware to not loop when reaching the root. Don't waste the user time by seeking the common directories filled of ELFs if you're not suid, so check it before.

Going further

4.1 Implementing hightech functions

Please don't waste this promised land with lame viruses. Respect the linux users and don't fill the world of shit. So add plenty of nice functions. Here are the ten (old) gold rules;

  1. You will use the utime and state syscall to restore the file times.
  2. You will use the fchmod/chmod and state syscall to restore the file flags.
  3. Retro is limited for now.. just avoid infecting goat files.
  4. Always think about using encryption, poly, RDA and such things!
  5. Disguise the file : overwrite the name section and some other useless part of the file to make it harder to work on. (gdb segfs when it opens an ELF w/o a SHT_STRTAB entry in the section header)
  6. You won't infect unless you're sure to not bug everything.
  7. Don't run in the street and scream < i'm writing a linux virus ! >
  8. You will add a nice payloads, nothing destructiv of course.
  9. Don't be full of yourself. Your viruses are the reflect of yourself.
  10. You will impose yourself a 10th rule. the more are there, the better it is.

4.2 Potentially interesting syscalls

As you found a lotta great ideas while reading the ralf's int list, you'll have some nice one while reading syscalls descriptions. It's a pitie that it's not better documented, but i don't think that many ppl thought it could be usefull. "who will code in asm under linux anyway ?" The umask syscall can be funny in a payload. Adding socket operation can be nice too. Adding a security breach to the system is really tempting. I'm currently looking aroung the ptrace syscall to make a kind of TSRing... But it may has many other interesting implementations. Thanx to kernel panic who pointed it. (in xine 2 or 3, i don't remember) I'm sure they are a lotta things to do w/ the various signals.

4.3 Cyber-bibliography

It's still small for now :/

Most of the stuff will be found here: (be sure to get the ELF documentation and the syscall descriptions)

Some other interesting (?) piece of paperz can be found at:

And the unix-virus mailing list that i just discovered (thnx to Quantum):

-=( Experienced by mandragore )=-

Last wurdz:

Greetz to the whole FSA, and in misorder: Doctor L, mist, darkman, yesna, reptile, raid, billy b., SSR, mammoth, T2, buzz, acidbytes, owl, evul, morphine, gigabyte, and all others.. And to you, linux users: forgive me for having written this paper.

A special though to all arrested vxers. past, present, and futur.

A special handshake to a non-coder friend of mine, YOGI.

Linux is not what it used to be anymore.

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