# simplemod

Author: gallileo

Tags: pwn, eldritch magic

Points: 470

After having a blast playing Codegate finals in Seoul the weekend before, we decided it would be fun to play SECCON Quals while on vacation in Osaka. Little did I know, what eldritch horror would await me in such an innocently named challenge.

## Setup

The challenge consists of a very simple main binary called chall and a dynamic library libmod.so which chall links against. The source code for both is provided1 and chall is indeed very simple:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int getint(void);
void modify(void);
__attribute__((noreturn)) void exit_imm(int status);

__attribute__((constructor))
static int init(){
alarm(30);
setbuf(stdin, NULL);
setbuf(stdout, NULL);
return 0;
}

__attribute__((destructor))
static void fini(){
exit_imm(0);
}

"1. Modify\n"
"0. Exit\n"
"> ");

return getint();
}

int main(void){
puts("You can operate 30 times.");
for(int i=0; i<30; i++){
case 0:
goto end;
case 1:
modify();
puts("Done.");
break;
}
}

end:
puts("Bye.");
return 0;
}


You have access to a simple menu allowing you to call modify up to 30 times, after which the binary will simply exit. The three functions getint, modify, exit_imm are provided by libmod and hence dynamically linked. fini immediately pops out as looking a bit suspicious. It is a destructor, meaning it will be called before we exit.

libmod is not much more complex and looks as follows:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>

#define write_str(s) write(STDOUT_FILENO, s, sizeof(s)-1)

char gbuf[0x100];

static int getnline(char *buf, int size){
int len;

if(size <= 0 || (len = read(STDIN_FILENO, buf, size-1)) <= 0)
return -1;

if(buf[len-1]=='\n')
len--;
buf[len] = '\0';

return len;
}

int getint(void){
char buf[0x10] = {0};

getnline(buf, sizeof(buf));
return atoi(buf);
}

void modify(void){
uint64_t ofs;

write_str("offset: ");
if((ofs = getint()) > 0x2000)
return;

write_str("value: ");
gbuf[ofs] = getint();
}

__attribute__((naked))
void exit_imm(int status){
asm(
"xor rax, rax\n"
"mov al, 0x3c\n"
"syscall"
);
__builtin_unreachable();
}


We now know the purpose of modify, it allows us to write a single byte at an offset < 0x2000 from the start of gbuf. However, gbuf only has a size of 0x100 bytes, so we have a trivial out-of-bounds write in modify! We now also see what exit_imm does. It directly calls the exit syscall, which might seem a bit peculiar. Furthermore, both its declaration in chall and its definition here have some uncommon attributes.

The reason for directly calling the exit syscall is likely to force an immediate termination (as the name implies). Calling libc’s exit would instead result in the rest of the destructors etc. to run. The various attributes are likely to cause a similar situation as in a previous CTF challenge, nightmare. Indeed, disassembling chall we see the following:

fini:
endbr64
push    rbp
mov     rbp, rsp
mov     edi, 0
call    _exit_imm

endbr64
push    rbp
mov     rbp, rsp
lea     rdi, s          ; "\nMENU\n1. Modify\n0. Exit\n> "
call    _puts
call    _getint
pop     rbp
retn


Therefore, if somehow _exit_imm would return instead of terminate the process immediately, we would end up back in the menu function, since there is no ret at the end of fini! But there is some more work to be done, before we can get there.

## Finding a Useful Target for our OOB Write

As explained before, we have an OOB write of up to 30 bytes, up to 0x2000 away from gbuf. gbuf is located in the data section of libmod, so what could we overwrite with this primitive? A quick look at vmmap in gdb:

gef➤  vmmap
[ Legend:  Code | Heap | Stack ]
Start              End                Offset             Perm Path
0x0000555555554000 0x0000555555555000 0x0000000000000000 r-- /home/vagrant/CTF/seccon/simplemod/chall
0x0000555555555000 0x0000555555556000 0x0000000000001000 r-x /home/vagrant/CTF/seccon/simplemod/chall
0x0000555555556000 0x0000555555557000 0x0000000000002000 r-- /home/vagrant/CTF/seccon/simplemod/chall
0x0000555555557000 0x0000555555558000 0x0000000000002000 r-- /home/vagrant/CTF/seccon/simplemod/chall
0x0000555555558000 0x0000555555559000 0x0000000000003000 rw- /home/vagrant/CTF/seccon/simplemod/chall
0x00007ffff7d8b000 0x00007ffff7d8e000 0x0000000000000000 rw-
0x00007ffff7d8e000 0x00007ffff7db6000 0x0000000000000000 r-- /home/vagrant/CTF/seccon/simplemod/libc.so.6
0x00007ffff7db6000 0x00007ffff7f4b000 0x0000000000028000 r-x /home/vagrant/CTF/seccon/simplemod/libc.so.6
0x00007ffff7f4b000 0x00007ffff7fa3000 0x00000000001bd000 r-- /home/vagrant/CTF/seccon/simplemod/libc.so.6
0x00007ffff7fa3000 0x00007ffff7fa7000 0x0000000000214000 r-- /home/vagrant/CTF/seccon/simplemod/libc.so.6
0x00007ffff7fa7000 0x00007ffff7fa9000 0x0000000000218000 rw- /home/vagrant/CTF/seccon/simplemod/libc.so.6
0x00007ffff7fa9000 0x00007ffff7fb6000 0x0000000000000000 rw-
0x00007ffff7fb6000 0x00007ffff7fb7000 0x0000000000000000 r-- /home/vagrant/CTF/seccon/simplemod/libmod.so
0x00007ffff7fb7000 0x00007ffff7fb8000 0x0000000000001000 r-x /home/vagrant/CTF/seccon/simplemod/libmod.so
0x00007ffff7fb8000 0x00007ffff7fb9000 0x0000000000002000 r-- /home/vagrant/CTF/seccon/simplemod/libmod.so
0x00007ffff7fb9000 0x00007ffff7fba000 0x0000000000002000 r-- /home/vagrant/CTF/seccon/simplemod/libmod.so
0x00007ffff7fba000 0x00007ffff7fbb000 0x0000000000003000 rw- /home/vagrant/CTF/seccon/simplemod/libmod.so # data section
0x00007ffff7fbb000 0x00007ffff7fbd000 0x0000000000000000 rw- # could likely overwrite into here
0x00007ffff7fbd000 0x00007ffff7fc1000 0x0000000000000000 r-- [vvar]
0x00007ffff7fc1000 0x00007ffff7fc3000 0x0000000000000000 r-x [vdso]
0x00007ffff7fc3000 0x00007ffff7fc5000 0x0000000000000000 r-- /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
0x00007ffff7fc5000 0x00007ffff7fef000 0x0000000000002000 r-x /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
0x00007ffff7fef000 0x00007ffff7ffa000 0x000000000002c000 r-- /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
0x00007ffff7ffb000 0x00007ffff7ffd000 0x0000000000037000 r-- /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
0x00007ffff7ffd000 0x00007ffff7fff000 0x0000000000039000 rw- /usr/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
0x00007ffffffde000 0x00007ffffffff000 0x0000000000000000 rw- [stack]
0xffffffffff600000 0xffffffffff601000 0x0000000000000000 --x [vsyscall]


We only have access to the data section and an unnamed region of memory right afterwards. Since libmod is partial RELRO, its got is placed right in front of the data section, so we cannot overwrite anything in it2. Additionally, there is nothing else of interest in the data section.

To figure out if there could be anything interesting to overwrite, I just overwrite the whole region I had access to with my primitive with 0x41 in gdb:

gef➤ set *(char [0x2001]*)0x00007ffff7fba0c0 = "AAAAA...A"


I then told the binary to exit and was greeted with a nice SIGSEGV:

Clearly I was messing with some linker data structures! I downloaded glibc source code (just assuming for now it was 2.34, more on that later) and started to look at where the crash occurred. It was happening on some access to the passed in l, a link_map structure. Indeed, by printing l in gdb I could see that I completely overwrite it with 0x41:

gef➤  p *l
$2 = { l_addr = 0x4141414141414141, l_name = 0x4141414141414141 <error: Cannot access memory at address 0x4141414141414141>, l_ld = 0x4141414141414141, // ... l_relro_size = 0x4141414141414141, l_serial = 0x4141414141414141 } gef➤  With some more debugging, I figured out that the link_map I was overwriting belonged to libmod and was at an offset of 0x1160 from gbuf. Having previously looked at similar challenges (such as nightmare), it was clear that I had to cleverly overwrite the link_map to mess with the dynamic linker during runtime and cause it to somehow give me a shell (or equivalent to get the flag). This was easier said than done of course and so we first have to understand how some internals of the dynamic linker on linux work. For this writeup, it is not necessary to fully grasp all the details of the dynamic linker and I will try to describe the important bits as best as I can. ### _dl_fixup First, we have to have a basic understanding, of how the got and friends work. Understanding _dl_runtime_resolve() has a great in depth description, but we do not actually need to know some of the details provided, and some of the later steps are not described. We all know that an external dynamic symbol creates an entry in the got and a relocation. When the symbol is called for the first time, this symbol needs to be then resolved. Further invocations use the stored resolved value, found in the .got.plt section. To resolve the symbol during the first invocation, a stub function is called instead. This stub function pushes the index of the relocation (index into the JMPREL table) as well as the address of the link_map of the current binary to the stack, then finally calls into the dynamic linker. At some point, the dynamic linker will call into _dl_fixup, with the link_map and relocation index as arguments (the previous arguments will have been saved, so they can be used once the function has been resolved). _dl_fixup will now do the following, assuming that we called atoi as an example, in libmod: 1. Use link_map to determine the location of a bunch of important tables / sections, such as SYMTAB (table of all symbols in the binary, both external and internal), STRTAB (table of names used by symbols) and JMPREL (containing the aforementioned relocation information) 2. Read the relocation from JMPREL (+0x680) based on the passed in index (4). This contains the following things: • Type of relocation (e.g. ELF_MACHINE_JMP_SLOT for a got relocation) • Index into the symbol table SYMTAB (6) • Offset from binary, where to write the resolved address to3 (+0x4038), the location of atoi in .got.plt in our case. 3. Read the symbol (named ref hereinafter) that should be resolved, by using the index from the relocation. This contains the following things (only relevant ones listed): • Index into the string table STRTAB (0x66) giving the name of the symbol. • Type of the symbol (e.g. STT_FUNC since atoi is a function). • Linkage visibility (e.g. STB_GLOBAL) • Section where the symbol can be found • Value (i.e. offset where the symbol can be found, in most cases) 4. Read the name of the symbol from STRTAB by using ref.st_name. 5. Iterate over all currently loaded libraries, by iterating over all link_maps. For each link_map, do the following: 1. Use some hash table magic to quickly determine if the name of the symbol is exported by this library. 2. Use this to retrieve the symbol (named sym hereinafter) defined in the SYMTAB of the current link_map. 3. Check that the name originally retrieved in 4. matches the name found at STRTAB[sym.st_name] of the current link_map. 4. If yes, then return the found sym. 6. If we found the symbol in the previous step (let it be sym in link_map map), write map.l_addr + sym.st_value where the relocation told us to (atoi@.got.plt in the example), where map.l_addr specifies the base address of the library described by map. 7. Return map.l_addr + sym.st_value, so that the caller can also call into the resolved symbol (remember, originally we wanted to call e.g. atoi and to first resolve it). Some of the referenced structures for the example atoi look as follows: • Relocation for atoi in libmod: Elf64_Rela { r_offset = 0x4038, r_info = ELF64_R_INFO(6, ELF_MACHINE_JMP_SLOT), r_addend = 0 }  • Symbol for atoi in libmod (most fields are empty, since it is an external symbol): Elf64_Sym { st_name = 0x66, st_info = ELF64_ST_INFO(STB_GLOBAL, STT_FUNC), st_other = 0, st_shndx = 0, st_value = 0, st_size = 0, }  • Symbol for atoi in libc (will be used to fill in atoi@.got.plt, when atoi called the first time in libmod): Elf64_Sym { st_name = 0x4f60, st_info = ELF64_ST_INFO(STB_GLOBAL, STT_FUNC), st_other = 0, st_shndx = 0xf, st_value = 0x43640, st_size = 0x19, }  Hopefully, this gives you enough of an understanding to follow the rest of the writeup. However, we are unfortunately not quite done yet with linker internals. ### _dl_fini Fortunately, this part should be a lot simpler to understand. Whenever a dynamically linked binary terminates normally (e.g. calling exit or just returning from main), _dl_fini will be invoked first, to cleanup all the dynamically loaded libraries and call their registered destructors. This looks something like this: 1. Collect all loaded libraries in a list, by iterating through all link_maps. 2. Sort them based on dependencies, but the main binary must always be first. 3. Iterate over the sorted list and for every link_map: 1. Load the array specified by the FINI_ARRAY table from link_map. 2. Call all functions in that array. 3. Call the function specified by the FINI item from link_map, if it exists. The locations of these tables / sections (e.g. SYMTAB, STRTAB, JMPREL, FINI_ARRAY, FINI) are all stored in the l_info member of a link_map. In particular, an entry in l_info is a pointer to a struct Elf64_Dyn, which is defined as follows: typedef struct { Elf64_Sxword d_tag; /* Dynamic entry type */ union { Elf64_Xword d_val; /* Integer value */ Elf64_Addr d_ptr; /* Address value */ } d_un; } Elf64_Dyn;  Since the d_tag is never checked in any of the interesting dl_* functions, we can safely ignore that part and only worry about d_un. d_un then stores a pointer to the relevant section / table in memory4. With all this knowledge about dynamic linker internals out of the way, we can now finally delve into the actual exploit. ## fini is not the End of the Line My hunch was now that I had to mess with my link_map, such that _dl_fixup will incorrectly resolve a symbol. I played around a bit more with overwriting only parts of the link_map, but then I suddenly realized something. I was crashing during the resolution of atoi - which was not resolved yet - since my writes were done entirely in gdb. If my writes were done “properly” using modify, atoi would have been already resolved and I would not be crashing there. I quickly retried, this time first doing one useless write of 0 to offset 0, then overwriting in gdb and finally exiting: It seems that my initial idea would be a bit harder to pull off, since all functions would be resolved already, right? Not entirely, since fini of the main binary calls exit_imm, which of course has not been resolved yet. By setting a breakpoint at _dl_fixup and not modifying the link_map, I indeed observed the call to _dl_fixup from inside fini: This lead me to formulate a plan. ## Stage 1: Achieving Calls of Arbitrary Addresses Relative libmod By modifying l_info of the libmod link_map, we can influence where the dynamic linker thinks SYMTAB and STRTAB are located. In particular, we can make them be after the gbuf and hence arbitrarily attacker controlled. We can therefore construct a fake symbol that is named exit_imm, but has a value (and hence resolved address) of anything we want. Since the resolved address will be calculated as the value of the symbol (controlled) plus the base address of libmod, we can make exit_imm resolve to any address relative to libmod. Because exit_imm is called, the resolved address is called and we hence have a call to an arbitrary address, relative to libmod. ### Home Grown vs. Store Bought libc While we do not control the arguments that are passed to exit_imm (indeed $rsi is 0, so resolving to e.g. system would never work), we “just” need to make exit_imm resolve to a one gadget in libc and we have a shell, right? After all, shared libraries are always loaded at a constant offset from each other and so a relative offset from libmod can also get us addresses to libc gadgets. Even though the constant offset is very dependent on the environment and we are not given a docker setup for this challenge, we can always bruteforce the offset in a pinch.

Unfortunately, the libc used on the server is “home-built glibc-2.3x”. Therefore, we can likely not use an one gadgets directly. We might be able to also bruteforce the one gadget offsets, but it started to sound like this was not the intended way.

### It Works on my Machine!

After some discussion with the author over DMs, it was revealed that the remote had a modified dynamic linker, where the individual dynamic libraries were loaded at a random offset! My current strategy would definitely not pan out then. In my opinion, this crucial detail should have been included in the challenge description. Furthermore, there should have been a docker setup, to test the challenge locally, as the offset between gbuf and link_map is highly dependent on the environment. Therefore, I first had to bruteforce this offset on remote, which felt quite unnecessary.

To bruteforce the offset, I noticed that _dl_fini has some asserts, if triggered could be a good indicator that we hit the right offset. In particular, it makes sure that for all link_maps, link_map.l_real == link_map. Therefore, we could try to overwrite link_map.l_real for every possible offset, if we see the assertion message, we have hit the correct one. However, if we accidentally overwrite link_map.l_next instead, then the assertion might still trigger, since the iteration will then use a bogus link_map next. Hence, our actual target is link_map.l_next and to verify, we make sure that offset+0x10 (i.e. overwriting link_map.l_real) also triggers the assertion.

This gives us an offset of 0x13e0 for the remote.

### Implementing the Attack

As explained before, we change the link_map such that SYMTAB and STRTAB are after gbuf and hence can be controlled by us as well, with the OOB writes. To achieve that, I overwrite the lowest two bytes of the Elf64_Dyn pointer for SYMTAB and STRTAB, such that it now points inside the l_info array. Therefore, d_un would contain pointers that were pointing somewhere relative to libmod and I could then overwrite the lowest two bytes of that pointer again to make SYMTAB and STRTAB point to after gbuf:

// before overwrite
0x7ffff7fb9e98: Elf64_Dyn { d_tag = 5, d_un = 0x7ffff7fb6460 }
0x7ffff7fb9ea8: Elf64_Dyn { d_tag = 6, d_un = 0x7ffff7fb6328 }
// ...
// ...
l_info[5] = 0x7ffff7fb9e98,
l_info[6] = 0x7ffff7fb9ea8,
// ...
0x7ffff7fbb330:
l_info[26] = 0x7ffff7fb9e68,
l_info[27] = 0x7ffff7fb9e58,
// ...
}

// after overwrite
// ...
l_info[5] = 0x7ffff7fbb330,
l_info[6] = 0x7ffff7fbb330,
// ...
0x7ffff7fbb330: Elf64_Dyn { d_tag = 0x7ffff7fb9e68, d_un = 0x7ffff7fba098 } // interpretation of two entries below
l_info[26] = 0x7ffff7fb9e68,
l_info[27] = 0x7ffff7fba098,
// ...
}


Note: Since only the lowest 3 nibbles are not randomized by ASLR, this requires us to bruteforce the 4th nibble. It is only a 4-bit brute force though, so it is fine. The official solution has a much better idea. Instead, we can just make the l_info entires for SYMTAB and STRTAB use the entry for a different section, that is much closer to gbuf, such as the .got.plt section.

Once we have SYMTAB and STRTAB under our control, it becomes pretty simple. Observe the index into SYMTAB used for the exit_imm resolution, then create a fake symbol at that offset:

Elf64_Sym {
st_name = 0, // useful, since this means we need to call modify less often
st_info = ELF64_ST_INFO(STB_GLOBAL, STT_FUNC),
st_other = 0,
st_shndx = 0xe,
st_value = 0x1054, // whatever we want to call, this specific one will be explained later
st_size = 0,
}


We also need to write exit_imm to STRTAB at the fake STRTAB location. With that out of the way, we can proceed with the next stage.

## Stage 2: Achieving Calls to Arbitrary Functions by Name in libc

After realizing we could not directly call into libc using stage 1, I assumed we had to further trick the linker into e.g. calling system for us. Indeed, I then realized I could use stage 1 to call one of the existing got stub functions and (even though everything was already resolved), it would try to resolve things again. However, since the link_map is now under our control, we can change which symbol _dl_fixup will try to resolve. Furthermore, since the stub function will call the resolved symbol, this also means we can call an arbitrary symbol, as long as one of the loaded libraries exports it.

### Implementation

Note: My exploit during the CTF created a fake relocation entry here. While writing this writeup, I realized this is actually not needed at all as becomes evident later!

First we have to modify l_info for JMPREL, such that we can create a fake relocation. This is done the exact same way as before and SYMTAB, STRTAB and JMPREL are all at the same place. Then, we make the fake relocation refering to a fake symbol which we want to resolve:

Elf64_Rela {
r_offset = 0x4038, // explained later why this is necessary
r_info = ELF64_R_INFO(11, ELF_MACHINE_JMP_SLOT), // the symbol index here was necessary, since I ran out of bytes to write and this happens to point to something that can be interpreted as a valid symbol :)
}


As explained in the comment, the fake symbol is not really controlled this time around, but that does not matter. The only important bit of the symbol is that the name index is not too large (so we can control the name). The fake symbol will therefore be:

Elf64_Sym {
st_name = 0x1080,
// ... some other values, we don't actually care
}


## Stage 3: Achieving Calls to Arbitrary Addresses Relative to libc?

At this point I could call arbitrary functions inside libc, but I had no control over the arguments. In particular, rdi was fixed to 0, meaning system was just returning empty. At this point, I was trying to pivot back to using a one_gadget. I noticed that I could also reach libc’s link_map with my OOB write. Therefore, I could change link_map.l_addr for libc and hence mess with the resolved address for any function by adding a constant offset. However, none of the one_gadgets of my local libc would work with the register state. Furthermore, without the remote libc, it would still be very hard to actually exploit this. I had to come up with a new idea.

## Stage 3: Achieving Calls to Arbitrary Addresses Relative to libc? Rewriting the GOT

Suddenly I remembered an important fact. At the end of _dl_fixup, it would write the resolved address to whatever offset the relocation was specifying. Indeed, if I would specify atoi@.got.plt, it would write whatever address it resolved (e.g. system) there and any further calls to atoi from libmod would instead call into system! As explained at the very beginning, if the call to exit_imm returns normally instead of exiting immediately, control will flow into the menu function, which incidentally calls getint, eventually resulting in atoi(getline()), i.e. calling atoi with controlled input! Therefore, I just had to specify atoi@.got.plt as the offset for the fake relocation and make sure exit_imm would return normally. Indeed, since I replaced exit_imm with the call to the got stub function (which in turn calls system), it would return normally.

Although this idea should have only required minimal changes, during the CTF I had to rework quite a bit of my exploit, since it initially used one byte too much.

### No Shell For You

Unfortunately, when I tried this locally, it would crash during the final call to system (i.e. with the controlled argument), due to stack alignment. My teammate jokingly suggested to just throw it against remote and I naivly assumed he was serious. After around 40 attempts (remember it should only be a 4-bit bruteforce), I was just about to kill my tmux panes, when I noticed that one of them spat out the flag! Good thing I thought my teammate was serious :P

### Later Realizations

As touched on before, this exploit does not actually need to have a fake relocation. Indeed, we can just reuse the atoi relocation and everything works the same (besides having to create a fake symbol for system). This alone saves around 5 bytes in the exploit.

## Conclusion

In the end, I learned a lot (actually way too much) about the internals of ld. The challenge was kinda fun and invoking some eldritch linker magic to get the shell felt quite satisfying. Nevertheless, I still think a local environment would have prevented some pain points, especially since I was not the only one who could not get the full exploit to work locally due to stack alignment.

You can find the original exploit I used during the CTF here. You can find a slightly modified version that does not need a fake relocation here.

1. Very much appreciated, I think more CTFs should start providing source for pwn challenges that do not need (IMO) unnecessary rev components.

2. Or at least not without some eldritch magic, as you will see later ;)

3. This is important and took me way too long to use to my advantage.

4. There is a flag in the link_map that turns the absolute addresses d_un.d_ptr into relative offsets from link_map.l_addr, but that ended up not being useful in my exploit.