                             ==Phrack Inc.==

                Volume 0x10, Issue 0x47, Phile #0x0C of 0x11

|=-----------------------------------------------------------------------=|
|=-[ Finding hidden kernel modules (extrem way reborn): 20 years later ]-=|
|=-----------------------------------------------------------------------=|
|=------------------[ g1inko <g1inko at hotmail com> ]-------------------=|
|=-----------------------------------------------------------------------=|

0 intro
1 Some words on LKM-rootkits
2 On existing tools
    2.1 Some words on the original module_hunter
    2.2 rkspotter and __module_address() problem
    2.3 Other challenges in adopting _that_ extrem way
3 The rebirth
    3.1 Detecting a stray struct module
    3.2 On virtual memory of x86_64 architecture
    3.3 Paging levels while solving the 'problem of unmapped'
4 Other architectures and outro
5 References
6 The code


0 intro
=======

Several years ago, while trying to get a grasp on Linux kernel rootkits, 
I came across an old Phrack Linenoise article [1] that discussed an 
interesting way of finding them. It totally caught my attention, even 
though it was so mysterious and far beyond my knowledge and experience 
back then.

It took me some time to fill in the gaps in my knowledge. Eventually I 
realized I was ready to reimplement the idea of finding hidden kernel 
modules in memory for modern kernels and x86_64 machines. Now, 20 years 
after the original publication, I am somewhat proud to finally share the 
rebirth of this technique, with all the honors to madsys for inspiration.


1 Some words on LKM-rootkits
============================

Since the first publication (known to me, at least) about Linux kernel 
rootkits in Phrack in April 1997 [2], and right up to this day, malicious 
loadable kernel modules (LKM-rootkits) all tend to use the same self-
hiding technique, namely unlinking themselves from the in-kernel linked 
list of loaded modules. This prevents the module from showing up in procfs
and lsmod, and rmmod is unable to find and unload such modules.

Some rootkits also try to mess with the memory afterwards, to make 
forensics even harder. For example, since version 2.5.71, Linux poisons 
stale list pointers of the unlinked struct by setting them to LIST_POISON1
and LIST_POISON2 (0x00100100 and 0x00200200). This is used to help detect 
memory bugs.

Some anti-rootkits use this to detect unlinked LKM descriptors, thus 
detecting a kernel rootkit. However, a rootkit can overwrite these values
after unlinking and evade this check. For instance, the KoviD LKM that 
appeared in 2022 [3] does this.

Also, despite the fact that LKM rootkits do unlink themselves from the 
modules list, they still can be listed via sysfs in /sys/modules. This is
even mentioned in the Volatility documentation [4] and is an established 
method for detecting such rootkits. Although Volatility developers claim 
to never have faced a rootkit that would also remove itself from sysfs, 
KoviD rootkit does it as well. 

For that, KoviD uses sysfs_remove_file() helper, and sets its state to
MODULE_STATE_UNFORMED. This state is for cases when a module is still 
setting up, and the kernel module loader is still running. This trick 
helps to evade anti-rootkits that rely on the kernel __module_address() 
function when enumerating virtual memory, such as rkspotter [5].


2 On existing tools
===================

2.1 Notes on the original module_hunter
---------------------------------------

The original implementation was created during the time of Linux 2.2-2.4
for i386 machines. Now it is Linux ~6.8 and lots of x86_64 systems around.
So many things have changed or removed, and new features were introduced.
The kernel is known to have highly unstable internal API, no surprise that
the 20-years-old module_hunter.c wouldn't build anymore. By understanding 
how it works, it turns out to be possible to reimplement the technique for
a modern kernel.

In short, the logic for finding a malicious LKM was to go through the 
memory region that contained module structs, and dump the info if it 
contained anything that looked like a sane struct module. At the time, 
this struct had way less fields than it does today, as it has been heavily
reworked and extended over the years.

For example, there is no more 'size' field, but many others were added. 
It is interesting that module's name at that time was stored using a 
pointer, while nowadays it is a static array of chars right within the 
struct module.


2.2 rkspotter and __module_address() problem
--------------------------------------------

While looking for a way to look for safe memory addresses via brute force,
I found the aforementioned anti-rootkit named rkspotter. It detects several
hiding techniques which allows it to find rootkits even if one the methods 
fail, but it relies on the __module_address() function in the kernel. 

This function was unexported in 2020 during a series of kernel patches [6],
and became unavailable out-of-kernel since Linux 5.4.118. This means that 
we must avoid using it as well.

The core idea of rkspotter is to go through the so-called module mapping 
space, and check which address belongs to which module using 
__module_address(). According to the doc, it lets one 'get the module which
contains an address'. 

For a given address, __module_address() returns a struct module pointer of 
a corresponding LKM. This function was a convenient way to get module info,
all the work for dereferencing page tables and checking for physical page 
presence was done internally.

Well yes, I know that I could try to copycat this __module_address() in the
module's code, but what I wanted to copycat was module_hunter by madsys, so
forget it :D


2.3 Other challenges in adopting _that_ extrem way
--------------------------------------------------

Based on what has been changed over the years, we will need to solve a few
problems to implement a newer tool (or rather a PoC?) for finding stray LKM
descriptors. Problems like:

  - Fix broken kernel API. As the code of module_hunter is actually very 
    small, and the only API used was for procfs, it didn't take long to 
    find a way to export a proc file to communicate with the kernel module.

  - Choose new fields of struct module that are the most appropriate for 
    detecting a stray struct.

  - Memory management on x86_64 differs from that of i386, so the code that
    checks for the page's presence needs to be fully reimplemented, more on
    that below.

  - Kernel virtual memory layout on x86_64 is also totally different from 
    that of i386. For instance, the kernel part of virtual memory space on 
    x86_64 is very huge compared to vmalloc area of its 32-bit predecessor,
    where struct modules were allocated back then (that is, 128 TB vs. 
    128 MB). 

    Since the modules region is way larger on x86_64, more checks are 
    needed to eliminate false positives on garbage remnants in memory.

With enough time and persistence, we at least get repaid with new cool 
knowledge, so let's go!


3 The rebirth
=============

3.1 Detecting a stray struct module
-----------------------------------

After playing with existing struct module fields I decided to introduce 
more checks on memory contents, beyond just checking for the validity of
a module's name. That alone showed to be insufficient for x86_64 modules 
memory area--more on this in 3.2), such as:

- The state field has one of the sane values: MODULE_STATE_LIVE,
  MODULE_STATE_COMING, MODULE_STATE_GOING, MODULE_STATE_UNFORMED;

- The init and exit fields either point into the module mapping space of 
  x86_64 or are NULL;

- At least one of init, exit, next and prev fields is not NULL and point 
  to either module mapping area or are canonical (e.g. list pointers);

- core_layout.size is not 0, but equals to 0 modulo PAGE_SIZE.

This list may vary in future, especially if i catch some more sophisticated
LKM. The check may become more flexible, but it works as PoC quite well.


3.2 On virtual memory of x86_64 architecture
--------------------------------------------

Now that we've determined fields of interest, we need to know where to 
start and finish testing for struct module similarity. Otherwise it would
be quite tough to bruteforce the whole almost-64-bit virtual address space.

In the virtual memory layout of modern x86_64 Linux, there is a dedicated 
module mapping space [7], with size of 1520 MB, for both 48- and 57-bit 
virtual addresses. The start address is designated with a macro 
MODULES_VADDR and the end address is represented by a MODULES_END macro.

After some more playing, I found out that module mapping space is where 
both module binaries, and their descriptors get allocated. This is just 
fine, as going through many terabytes of the vast virtual address space 
was my biggest concern in terms of execution time.


3.3 Paging levels while solving the 'problem of unmapped'
---------------------------------------------------------

NOTE: If you're not quite familiar with paging mechanism, refer to e.g. the
      OSDev wiki [8] or Linux documentation [9] (or processor's).

Well, now we are a facing the same problem as the one noted in the original
paper:

  ``By far, maybe you think: umm, it's very easy to use brute force to list
    those evil modules". But it is not true because of an important
    reason: it is possible that the address which you are accessing is
    unmapped, thus it can cause a paging fault and the kernel would report:
    "Unable to handle kernel paging request at virtual address".  ''

This would be resolved automatically when using __module_address(), but we
cannot afford it, so we need to check for page presence ourselves. To do 
that, we need to go through tables that contain info about relations of 
virtual and physical pages for a process. The kernel space part is the 
same for each process because it maps to the same physical pages.

For supporting more physical memory, more page table levels were added in 
64-bit x86, and now it can be 4 or 5 of them depending on both hardware 
and kernel support. For each paging level, the PDE/PTE being dereferenced 
must be checked for presence in RAM by using one of the following macros:

- pgd_present();
- p4d_present() (if 5-level paging is used or emulated);
- pud_present();
- pmd_present();
- pte_present().

Without getting too deep into the MMU guts, the check for the top level, 
using currently available kernel macros and functions would look like this:

    ----snip----
    struct mm_struct *mm = current->mm;
    ----snip----
    pgd = pgd_offset(mm, addr);
    if (!pgd || pgd_none(*pgd) || !pgd_present(*pgd) )
        return false;
    ----snip----

You may find this check a bit paranoid and, especially looking at similar 
in-kernel functions, maybe it's okay to remove pgd_none(). When I'm writing
kernel C for a subsystem I am not quite familiar with, I feel much better 
safe than sorry :D 

There was a function called kern_addr_valid() performing similar checks, 
but it was removed from the kernel a year ago [10]. dump_pagetable(),
spurious_kernel_fault(), mm_find_pmd() and some others also do that.

As for the varying number of paging levels, we could find out the proper 
number with ether CONFIG_PGTABLE_LEVELS or CONFIG_X86_5LEVEL. The first 
one seems better if I (or anyone else) decide to port it to some other 
architecture.

Support for 5-level paging was introduced in 2017 with versions 4.11-4.12 
[11, 12]. Interestingly, with CONFIG_PGTABLE_LEVELS=5 the kernel wouldn't 
break on 4-level hardware [13]:

    ``In this case additional page table level - p4d - will be folded at
      runtime.''


4 Other architectures and outro
===============================

The only thing that prevents us from getting the code to work on 
architectures other than x86_64 is (obviously) architecture-dependent
stuff. Virtual address spaces of other architectures (i.e. AArch64) 
is different, not only in its layout and size, but also in the possible
number of paging levels. This may vary from 2 to 4, depending on the page
size and virtual addresses bits [14]. 

Notably, the code did compile for AArch64 running the 6.1.21 kernel, 
until I added a check for CONFIG_X86_64. Due to memory management 
differences, it of course did not report anything found.

As for kernel versions compatibility, I've tested the code on 4.4, 5.14, 
5.15 and 6.5 x86_64 kernels. It still may fail on some intermediate or 
newer versions, most likely somewhere in the page table dereferencing 
code. Feel free to let me know if that happens.

Once all the hardware incompatibility issues are properly taken into 
account, I believe it's possible to run the tool on other architectures 
as well. Hopefully I can add support for at least AArch64 once I'm ready
to dive into its memory management. PRs and suggestions (see [15]) are 
also welcome! :^)


5 References
============

1.  https://phrack.org/issues/61/3.html#article
2.  https://phrack.org/issues/50/5.html#article
3.  https://github.com/carloslack/KoviD
4.  https://github.com/volatilityfoundation/volatility/wiki/Linux-Command-
Reference#linux_check_modules
5.  https://github.com/linuxthor/rkspotter
6.  https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.4.118
7.  https://www.kernel.org/doc/Documentation/x86/x86_64/mm.txt
8.  https://wiki.osdev.org/Paging
9.  https://www.kernel.org/doc/html/v6.5/mm/page_tables.html
10. https://lore.kernel.org/lkml/Y05fQrd4TYaOnks%2F@infradead.org/
11. https://github.com/torvalds/linux/commit/b8504058a06bd19286c8b59539eebfda69d1ecb5
12. https://lwn.net/Articles/716324/
13. https://www.kernel.org/doc/html/v5.9/x86/x86_64/5level-paging.html#enabling-5-level-paging
14. https://www.kernel.org/doc/html/v6.5/arch/arm64/memory.html
15. https://github.com/ksen-lin/nitara2


6 The code (cleared a bit for the paper)
========================================

-----BEGIN NITARA2.C-----

/* 
 * original idea: madsys, "Finding hidden kernel modules (the extrem way)"
 *                https://phrack.org/issues/61/3.html#article
 *
 * usage: cat /proc/nitara2 && dmesg
 */

#include <asm/page.h>
#include <asm/fixmap.h>
#include <linux/errno.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/version.h>
#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 5, 0)
#  include <linux/mm.h>
#elif LINUX_VERSION_CODE >= KERNEL_VERSION(5, 8, 0)
#  include <linux/pgtable.h>
#elif LINUX_VERSION_CODE >= KERNEL_VERSION(4, 11, 0)
#  include <asm-generic/pgtable-nop4d.h>
#else /* < 4.11 */
#  include <asm/pgtable.h>
#endif
#include <linux/string.h>
#include <linux/sched.h>
#include <linux/types.h>
#include <linux/unistd.h>
#include <uapi/linux/stat.h>

#ifndef CONFIG_X86_64
#  error "arch not supported :("
#endif

#define NITARA_PRINTK(fmt, args...) \
        printk("%s: " fmt, module_name(THIS_MODULE), ##args)
#define NITARA_MODSIZE (0x1000 * PAGE_SIZE)

#ifndef sizeof_field
#define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))
#endif

/* NOTE: arch-specific, we don't handle it yet */
#ifndef __canonical_address
static __always_inline u64 __canonical_address(u64 vaddr, u8 vaddr_bits)
{
    return ((s64)vaddr << (64 - vaddr_bits)) >> (64 - vaddr_bits);
}
#endif

#ifndef __is_canonical_address
static __always_inline u64 __is_canonical_address(u64 vaddr, u8 vaddr_bits)
{
    return __canonical_address(vaddr, vaddr_bits) == vaddr;
}
#endif

#define is_canonical_48(p) __is_canonical_address((unsigned long)p, 48)
#define is_canonical_or_zero(p) (p == NULL || is_canonical_48(p))
#define is_canonical_high_or_zero(p) \
        (p == NULL || ((unsigned long)p >= VMALLOC_START && is_canonical_48(p)))

#if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 4, 0)
#define MODSIZE(p)                              \
       (p->mem[MOD_TEXT].size                   \
       + p->mem[MOD_INIT_TEXT].size             \
       + p->mem[MOD_INIT_DATA].size             \
       + p->mem[MOD_INIT_RODATA].size           \
       + p->mem[MOD_RO_AFTER_INIT].size         \
       + p->mem[MOD_RODATA].size                \
       + p->mem[MOD_DATA].size)
#else
#  define MODSIZE(p) (p->core_layout.size)
#endif

/*
 * https://stackoverflow.com/questions/11134813/
 * https://stackoverflow.com/questions/66593710/
 * https://lwn.net/Articles/716324/
 */
static bool valid_addr(unsigned long addr, size_t size)
{
    pgd_t *pgd;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 11, 0)
    p4d_t *p4d;
#endif
    pmd_t *pmd;
    pud_t *pud;
    pte_t *pte;
    struct mm_struct *mm = current->mm;
    unsigned long end_addr;

    pgd = pgd_offset(mm, addr);
    if (unlikely(!pgd) || unlikely(pgd_none(*pgd)) || unlikely(!pgd_present(*pgd)) )
        return false;
        
#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 11, 0)
    p4d = p4d_offset(pgd, addr);
    if (unlikely(!p4d) || unlikely(p4d_none(*p4d)) || unlikely(!p4d_present(*p4d)) )
        return false;
    pud = pud_offset(p4d, addr);
#else
    pud = pud_offset(pgd, addr);
#endif
    if (unlikely(!pud) || unlikely(pud_none(*pud)) || unlikely(!pud_present(*pud)))
        return false;

    pmd = pmd_offset(pud, addr);
    if (unlikely(!pmd) || unlikely(pmd_none(*pmd)) || unlikely(!pmd_present(*pmd)))
        return false;

    if (pmd_trans_huge(*pmd)) {
        end_addr = (((addr >> PMD_SHIFT) + 1) << PMD_SHIFT) - 1;
        goto end;
    }
    // NOTE: pte_offset_map() is unusable out-of-tree on >=6.5.
    //       As pte_offset_kernel() seems to work, use it instead :D
    pte = pte_offset_kernel(pmd, addr);
    if (unlikely(!pte) || unlikely(!pte_present(*pte)))
        return false;

    end_addr = (((addr >> PAGE_SHIFT) + 1) << PAGE_SHIFT) - 1;

end:
    if (end_addr >= addr + size - 1)
        return true;

    return valid_addr(end_addr + 1, size - (end_addr - addr + 1));
}

static bool is_within_modules(void *p)
{
    return (unsigned long)p >= MODULES_VADDR && (unsigned long)p < MODULES_END;
}

__maybe_unused static bool is_within_modules_or_zero(void *p)
{
    return p == NULL || is_within_modules(p);
}

static bool check_name_valid(char *s)
{
    size_t i;
    if (!s)
        return false;

    for (i = 0; i < sizeof_field(struct module, name); i += 1) {
        /* we might fail here if the name is "" */
        if (s[i] == '\0' && i != 0)
            break;
        if (s[i] < 0x20 || s[i] > 0x7e)
            return false;
    }
    return true;
}

ssize_t showmodule_read(
    struct file *unused_file,
    char *buffer, size_t len,
    loff_t *off
) {
    struct module *p;
    unsigned long i;

    NITARA_PRINTK("address                           module size\n");
    for (
        i = 0, p = (struct module *)MODULES_VADDR;
        p <= (struct module*)(MODULES_END - 0x10);
        p = ((struct module*)((unsigned long)p + 0x10)), i += 1
    ) {
        if (
            valid_addr((unsigned long)p, sizeof(struct module))
            && p->state >= MODULE_STATE_LIVE
            && p->state <= MODULE_STATE_UNFORMED
            && check_name_valid(p->name)
            // may be unset for modules that can also be compiled in-kernel
            && is_within_modules_or_zero(p->init)
            && is_within_modules_or_zero(p->exit)
            && (p->init || p->exit || p->list.next || p->list.prev)
            // https://elixir.bootlin.com/linux/v5.19/source/include/linux/list.h#L146
            && (is_canonical_high_or_zero(p->list.next) || p->list.next == LIST_POISON1)
            && (is_canonical_high_or_zero(p->list.prev) || p->list.prev == LIST_POISON2)
            && MODSIZE(p) && (MODSIZE(p) % PAGE_SIZE == 0)
        ) {
            NITARA_PRINTK("0x%lx: %20s %u\n", (unsigned long)p, p->name, MODSIZE(p));
        }
    }
    NITARA_PRINTK("end check (total gone %lu steps)\n", i);
    return 0;
}

#if LINUX_VERSION_CODE > KERNEL_VERSION(5, 5, 19)
static struct proc_ops nitara2_ops = {
    .proc_read = showmodule_read,
    .proc_lseek = default_llseek, // otherwise segfaults
};
#else
// include/linux/fs.h#L1692
static struct file_operations nitara2_ops = {
    .read = showmodule_read,
    .llseek = default_llseek,
};
#endif

struct proc_dir_entry *entry;

int init_module()
{
    NITARA_PRINTK("[creating proc entry]\n");
    entry = proc_create_data("nitara2", S_IRUSR, NULL, &nitara2_ops, NULL);
    return 0;
}

void cleanup_module()
{
    NITARA_PRINTK("[cleanup proc]\n");
    proc_remove(entry);
}

MODULE_LICENSE("GPL");
MODULE_AUTHOR("ksen-lin <g1inko<at>hotmail[.]com>");

-----END NITARA2.C-----

|=[ EOF ]=---------------------------------------------------------------=|
