CTFtime.org / 0CTF/TCTF 2019 Quals / baby

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I want to give a shoutout to ghostly_grey for working with me to solve this chal. It was definitely
a team effort and I don't know if either of us could have done it alone!

Now, the writeup. For this challenge, we are given a binary and libc.
The binary has the following protections:

[*] '/home/a/Desktop/0ctf_2019/aegis/aegis'
Arch: amd64-64-little
RELRO: Full RELRO
Stack: Canary found
NX: NX enabled
PIE: PIE enabled
FORTIFY: Enabled
ASAN: Enabled
UBSAN: Enabled

Okay so besides the normal protections, the binary is protected by Address Sanitizer!!
Getting past this defense is by far the most difficult part of the challenge.

From reversing the binary, we see that this is a standard heap challenge. You can create a note,
update a note, delete a note, show a note, and exit. The notes are stored in a global array, and you
can only allocate a maximum of 10.

___ ____ _____ _____ _______ ____ _____ _____ ____ ___ _ ___
/ _ \ / ___|_ _| ___/ /_ _/ ___|_ _| ___| |___ \ / _ \/ |/ _ \
| | | | | | | | |_ / / | || | | | | |_ __) | | | | | (_) |
| |_| | |___ | | | _/ / | || |___ | | | _| / __/| |_| | |\__, |
\___/ \____| |_| |_|/_/ |_| \____| |_| |_| |_____|\___/|_| /_/

======Protected Notebook======
1. Add note
2. Show note
3. Update note
4. Delete note
5. Exit
Choice:

There is another function, secret, which allows you to write a nullbyte anywhere into memory.
You can only use this function once and it can be accessed by putting the number "666" in the menu.
It will prove essential for our exploit, but more on that later.

The first vulnerability that we found was a simple UAF. In the delete function,
the note pointers are never nulled out like they should be. This allows us
to access these pointers and overwrite data that has already been freed.

==4316==ERROR: AddressSanitizer: heap-use-after-free on address 0x602000000010 at pc 0x555555668553 bp 0x7fffffffe310 sp 0x7fffffffe308
READ of size 8 at 0x602000000010 thread T0
#0 0x555555668552 (/home/a/Desktop/0ctf_2019/aegis/aegis+0x114552)
#1 0x555555668a6c (/home/a/Desktop/0ctf_2019/aegis/aegis+0x114a6c)
#2 0x7ffff6e24b96 (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
#3 0x55555556f8e9 (/home/a/Desktop/0ctf_2019/aegis/aegis+0x1b8e9)

0x602000000010 is located 0 bytes inside of 16-byte region [0x602000000010,0x602000000020)
freed by thread T0 here:
#0 0x55555562f5f0 (/home/a/Desktop/0ctf_2019/aegis/aegis+0xdb5f0)
#1 0x5555556688be (/home/a/Desktop/0ctf_2019/aegis/aegis+0x1148be)
#2 0x555555668a7a (/home/a/Desktop/0ctf_2019/aegis/aegis+0x114a7a)
#3 0x7ffff6e24b96 (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)

previously allocated by thread T0 here:
#0 0x55555562f7c0 (/home/a/Desktop/0ctf_2019/aegis/aegis+0xdb7c0)
#1 0x555555668375 (/home/a/Desktop/0ctf_2019/aegis/aegis+0x114375)
#2 0x555555668a65 (/home/a/Desktop/0ctf_2019/aegis/aegis+0x114a65)
#3 0x7ffff6e24b96 (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)

SUMMARY: AddressSanitizer: heap-use-after-free (/home/a/Desktop/0ctf_2019/aegis/aegis+0x114552)
Shadow bytes around the buggy address:
0x0c047fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c047fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c047fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c047fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c047fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c047fff8000: fa fa[fd]fd fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff8010: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c047fff8050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
==4316==ABORTING

In order to attack this vulnerability, you need to null out the byte that ASAN has poisoned.
In the above example, it is 0x0c047fff8002. ASAN does NOT use ASLR, so we can determine the address
of the poison byte we want to NULL out by either looking at the ASAN output, or doing a simple computation

ShadowAddr = (Addr >> 3) + Offset.

It turns out, this UAF is not very useful to us. Why? Well, let's take a look at ASAN's Heap:

ASAN Heap snapshot 1 (Before Free):
00:0000│ 0x602000000000 ◂— 0x2ffffff00000002
01:0008│ 0x602000000008 ◂— 0x2080000120000010
02:0010│ 0x602000000010 —▸ 0x60b0000000f0 ◂— 0x6161616161616161 ('aaaaaaaa')
03:0018│ 0x602000000018 —▸ 0x555555668ab0 (cfi_check) ◂— jmp 0x555555667d10
04:0020│ 0x602000000020 ◂— 0x0

ASAN Heap snapshot (After Free):
00:0000│ 0x602000000000 ◂— 0x200000000000003
01:0008│ 0x602000000008 ◂— 0x2080000120000010
02:0010│ 0x602000000010 —▸ 0x60b061000001 ◂— 0x0
03:0018│ 0x602000000018 —▸ 0x555555668ab0 (cfi_check) ◂— jmp 0x555555667d10
04:0020│ 0x602000000020 ◂— 0x0

In this example, we've allocated a note of size 100. The pointer in the global Notes array will be
0x602000000010. This address simply contains a pointer to the note's data.
However, you can see that after the free, this pointer become mangled!
Thus, we are unable to reuse this dangling pointer to our advantage as the program will just segfault.

After this revelation, we continued to look for more vulnerabilities. It turns out, there is also a heap
buffer overflow in the update function. The update function uses strlen to read in new content, but it adds a byte
to the amount of data that can be read in. Thus, if we do multiple successive updates, we can gradually overflow an
object.

What can we actually attack with this overflow? Well, if we are clever about the size of our notes,
the content chunks will be place on the same page as the chunks which contain pointers to the note content.
For example, when we allocate a chunk of size 16, ASAN's heap will look like this:

00:0000│ 0x602000000000 ◂— 0x2ffffff00000002
01:0008│ 0x602000000008 ◂— 0x900000120000010
02:0010│ 0x602000000010 —▸ 0xa0061616161 ◂— 0x0
03:0018│ 0x602000000018 ◂— 0xbebebe0000000000
04:0020│ 0x602000000020 ◂— 0x2ffffff00000002
05:0028│ 0x602000000028 ◂— 0x2080000120000010
06:0030│ 0x602000000030 —▸ 0x602000000010 —▸ 0xa0061616161 ◂— 0x0
07:0038│ 0x602000000038 —▸ 0x555555668ab0 (cfi_check) ◂— jmp 0x555555667d10

In this example, the chunk's content begins at 0x602000000010 and the chunk which contains a data
pointer begins at 0x602000000020.

So, by using multiple updates to the same note, we can achieve an overflow of arbitrary length.
However, we run into a problem. The addresses 0x602000000020 and 0x602000000028 have been poisoned
by ASAN. Thus, if we want to write into one of them, we need to use the secret function and it's
one byte null overwrite. We only get to do this once, however, so we have no chance of directly
making it to the data pointer located at 0x602000000030 because there are two poisoned addresses
between our content and the pointer.

So, all we can attack with this overflow is the chunk's metadata. So what to they bytes located at
0x602000000020 and 0x2080000120000010 actually mean? Well, let's take a look at the ASAN allocator's
source code located here:

https://github.com/llvm-mirror/compiler-rt/blob/master/lib/asan/asan_allocator.cc

I've pulled out two of the most important code sections for our purposes.

// The memory chunk allocated from the underlying allocator looks like this:
// L L L L L L H H U U U U U U R R
// L -- left redzone words (0 or more bytes)
// H -- ChunkHeader (16 bytes), which is also a part of the left redzone.
// U -- user memory.
// R -- right redzone (0 or more bytes)
// ChunkBase consists of ChunkHeader and other bytes that overlap with user
// memory.

// If the left redzone is greater than the ChunkHeader size we store a magic
// value in the first uptr word of the memory block and store the address of
// ChunkBase in the next uptr.
// M B L L L L L L L L L H H U U U U U U
// | ^
// ---------------------|
// M -- magic value kAllocBegMagic
// B -- address of ChunkHeader pointing to the first 'H'
static const uptr kAllocBegMagic = 0xCC6E96B9;

struct ChunkHeader {
// 1-st 8 bytes.
u32 chunk_state : 8; // Must be first.
u32 alloc_tid : 24;

u32 free_tid : 24;
u32 from_memalign : 1;
u32 alloc_type : 2;
u32 rz_log : 3;
u32 lsan_tag : 2;
// 2-nd 8 bytes
// This field is used for small sizes. For large sizes it is equal to
// SizeClassMap::kMaxSize and the actual size is stored in the
// SecondaryAllocator's metadata.
u32 user_requested_size : 29;
// align < 8 -> 0
// else -> log2(min(align, 512)) - 2
u32 user_requested_alignment_log : 3;
u32 alloc_context_id;
};

// Every chunk of memory allocated by this allocator can be in one of 3 states:
// CHUNK_AVAILABLE: the chunk is in the free list and ready to be allocated.
// CHUNK_ALLOCATED: the chunk is allocated and not yet freed.
// CHUNK_QUARANTINE: the chunk was freed and put into quarantine zone.
enum {
CHUNK_AVAILABLE = 0, // 0 is the default value even if we didn't set it.
CHUNK_ALLOCATED = 2,
CHUNK_QUARANTINE = 3
};

Okay, so we see that the first 29 bits of the data located at 0x602000000028 represent the
chunks length. Also, the first 8 bits of the metadata determine whether the chunk is
available, allocated, or in quarantine.

Before we continue, we have to go over a very important concept of ASAN's allocator. The allocator
is not designed to be hyper efficient, but to find bugs. Thus, it will try to avoid allocating
valid memory over freed memory at all costs so it can catch bugs like UAF. It does this by
putting free chunks into quarantine. Quarantined chunks are only reused after a certain amount
of memory has been freed. The standard setting is quarantine_size_mb=256M which can be seen
if you set the environment variable export ASAN_OPTIONS=verbosity=1. Thus, we need to free
a HUGE amount of memory before it gets reused.

How do we do this with only 10 chunks with a maximum size of 1024? Answer: overwrite the size
of the victim chunk with a HUGE number. From our experience, 0xffffff should do it.

Now we're cookin'. By overwriting the size of the next chunk with a huge number and
setting the other metadata bytes properly, we can get a huge chunk of memory to be recycled.
Thus, if we allocate some notes in this memory, we should be able to attack the UAF
we discovered before and gain arbitrary read write across the process's address space.

You might be wondering, how can we overwrite the size of the data located at 0x602000000028?
Isn't it also poisoned? Answer: ASAN does not protect against non-qword aligned overwrite. Basically,
we would have to write the address 0x602000000030 before ASAN would notice the overflow, since we nulled
out the previous byte in shadow memory with the secret function.

Once we are able to set the data pointer of the first chunk we allocated, we can read and write the process
address space as we please. However, the challenge isn't over. There is a check in the update
function that ensures your data pointer is located in ASAN's heap

if ( *(_QWORD *)data_pointer >> 44 != 6LL )

Thus, we only get one write for our exploit to work. Another problem is that the stack
if full of junk and the only one_gadgets we have are these:

0x4f2c5 execve("/bin/sh", rsp+0x40, environ)
constraints:
rcx == NULL

0x4f322 execve("/bin/sh", rsp+0x40, environ)
constraints:
[rsp+0x40] == NULL

0x10a38c execve("/bin/sh", rsp+0x70, environ)
constraints:
[rsp+0x70] == NULL

It is also very difficult to find a target which will be executed between our memory write and the call to
error() where $RCX is null.

So, where can we write? Looking through the disassembly of the functions

_asan_handle_no_return
_ubsan_handle_cfi_check_fail_abort

we see there is a call to __sanitizer::Die().

In this function, there is the code snippet:

if ( __sanitizer::UserDieCallback )
__sanitizer::UserDieCallback(this);
v2 = &__sanitizer::InternalDieCallbacks;

Thus, if there are any UserDieCallbacks registered, they will be executed.
If we put one_gadget here, we will get the same issue. The stack is too full
of junk for the rsp+X constraints to be passed.

Looking at the registers when our gadget is called, we have the following state:

────────────────────────────────────────────────────────────────────────────────────[ REGISTERS ]────────────────────────────────────────────────────────────────────────────────────
RAX 0x7ffff6ea0f05 (strsignal+165) ◂— mov rax, rbp
RBX 0x555556504880 —▸ 0x5555556388b0 (__sanitizer::SuppressionContext::GetMatched(__sanitizer::InternalMmapVector<__sanitizer::Suppression*>*)+352) ◂— push rbp
RCX 0x7fffffffe0a0 —▸ 0x7fffffffe150 ◂— 0x3d3d343300000003
RDX 0x7fffffffe0a0 —▸ 0x7fffffffe150 ◂— 0x3d3d343300000003
RDI 0x7fffffffe101 ◂— 0x100007fffffffe3
RSI 0x0
R8 0x6a6cb03abcebc041
R9 0x0
R10 0x0
R11 0x206
R12 0x55555556f8c0 (ptrace+7344) ◂— xor ebp, ebp
R13 0x7fffffffe460 ◂— 0x1
R14 0x0
R15 0x0
RBP 0x555556504878 ◂— 0x0
RSP 0x7fffffffe2e0 ◂— 0x1
RIP 0x555555656148 ◂— call rax

We also control the value at offset 8 in the stack shown below.

00:0000│ rsp 0x7fffffffe2e0 ◂— 0x1
01:0008│ 0x7fffffffe2e8 —▸ 0x7ffff6f0d398 (exec_comm+2520) ◂— lea rdi, [rip + 0xa9afb]
02:0010│ 0x7fffffffe2f0 —▸ 0x7fffffffe320 —▸ 0x7fffffffe360 —▸ 0x7fffffffe380 —▸ 0x555555668ac0 ◂— ...
03:0018│ 0x7fffffffe2f8 —▸ 0x555555667959 ◂— sub rsp, 8
04:0020│ 0x7fffffffe300 ◂— 0x1
05:0028│ 0x7fffffffe308 —▸ 0x55555566877e ◂— mov edi, dword ptr [rbp - 0x14]
06:0030│ 0x7fffffffe310 —▸ 0x7fffffffe320 —▸ 0x7fffffffe360 —▸ 0x7fffffffe380 —▸ 0x555555668ac0 ◂— ...
07:0038│ 0x7fffffffe318 —▸ 0x7fffffffe360 —▸ 0x7fffffffe380 —▸ 0x555555668ac0 ◂— push r15

So, RSI is already NULL and we need to set RDI to "/bin/sh" and RDX to NULL or environ for our exploit to work.
Looking at the following one_gadget code

0x7ffff6f0d393 <exec_comm+2515>: lea rsi,[rsp+0x70]
0x7ffff6f0d398 <exec_comm+2520>: lea rdi,[rip+0xa9afb] # 0x7ffff6fb6e9a
0x7ffff6f0d39f <exec_comm+2527>: mov rdx,QWORD PTR [rax]
0x7ffff6f0d3a2 <exec_comm+2530>: call 0x7ffff6ee7e30 <execve>

we see that it is possible to set rdx and rdi without changing rsi before execve is called.
However, this requires setting the correct value in rax. rax must point to NULL, and the only
other register that does this in the current state is rbp. Looking for a gadget in libc, we find

0x000000000009df05 : mov rax, rbp ; pop rbx ; pop rbp ; ret

This will set rax to rbp which points to NULL, pop twice, and return to the address on the stack which
we control!

Thus, if we call this gadget using the UserDieCallback
and we only use the part of the one_gadget starting at 0x7ffff6f0d39f, we can
call execve("/bin/sh", NULL, NULL)

Success!!! This will give us our shell and our flag!
flag{ASan_and_CFI_absolute_defense}

Overall this challenge was definitely tough. I'd like to thank the CTF
organizers for creating such a great challenge, it was very fun to pwn!

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