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Ext - Extended Filesystem

Ext2 is the most common filesystem for not journaling partitions (partitions that don't change much) like the boot partition. Ext3/4 are journaling and are used usually for the rest partitions.

All block groups in the filesystem have the same size and are stored sequentially. This allows the kernel to easily derive the location of a block group in a disk from its integer index.

Every block group contains the following pieces of information:

  • A copy of the filesystem’s superblock
  • A copy of the block group descriptors
  • A data block bitmap which is used to identify the free blocks inside the group
  • An inode bitmap, which is used to identify the free inodes inside the group
  • inode table: it consists of a series of consecutive blocks, each of which contains a predefined Figure 1 Ext2 inode number of inodes. All inodes have the same size: 128 bytes. A 1,024 byte block contains 8 inodes, while a 4,096-byte block contains 32 inodes. Note that in Ext2, there is no need to store on disk a mapping between an inode number and the corresponding block number because the latter value can be derived from the block group number and the relative position inside the inode table. For example, suppose that each block group contains 4,096 inodes and that we want to know the address on the disk of inode 13,021. In this case, the inode belongs to the third block group and its disk address is stored in the 733rd entry of the corresponding inode table. As you can see, the inode number is just a key used by the Ext2 routines to retrieve the proper inode descriptor on the disk quickly
  • data blocks, containing files. Any block which does not contain any meaningful information is said to be free.

Ext Optional Features

Features affect where the data is located, how the data is stored in inodes and some of them might supply additional metadata for analysis, therefore features are important in Ext.

Ext has optional features that your OS may or may not support, there are 3 possibilities:

  • Compatible
  • Incompatible
  • Compatible Read Only: It can be mounted but not for writing

If there are incompatible features you won't be able to mount the filesystem as the OS won't know how the access the data.

{% hint style="info" %} A suspected attacker might have non-standard extensions {% endhint %}

Any utility that reads the superblock will be able to indicate the features of an Ext filesystem, but you could also use file -sL /dev/sd*

Superblock

The superblock is the first 1024 bytes from the start and it's repeated in the first block of each group and contains:

  • Block size
  • Total blocks
  • Blocks per block group
  • Reserved blocks before the first block group
  • Total inodes
  • Inodes per block group
  • Volume name
  • Last write time
  • Last mount time
  • Path where the file system was last mounted
  • Filesystem status (clean?)

It's possible to obtain this information from an Ext filesystem file using:

fsstat -o <offsetstart> /pat/to/filesystem-file.ext
#You can get the <offsetstart> with the "p" command inside fdisk

You can also use the free GUI application: https://www.disk-editor.org/index.html
Or you can also use python to obtain the superblock information: https://pypi.org/project/superblock/

inodes

The inodes contain the list of blocks that contains the actual data of a file.
If the file is big, and inode may contain pointers to other inodes that point to the blocks/more inodes containing the file data.

In Ext2 and Ext3 inodes are of size 128B, Ext4 currently uses 156B but allocates 256B on disk to allow a future expansion.

Inode structure:

Offset Size Name DescriptionF
0x0 2 File Mode File mode and type
0x2 2 UID Lower 16 bits of owner ID
0x4 4 Size Il Lower 32 bits of file size
0x8 4 Atime Access time in seconds since epoch
0xC 4 Ctime Change time in seconds since epoch
0x10 4 Mtime Modify time in seconds since epoch
0x14 4 Dtime Delete time in seconds since epoch
0x18 2 GID Lower 16 bits of group ID
0x1A 2 Hlink count Hard link count
0xC 4 Blocks Io Lower 32 bits of block count
0x20 4 Flags Flags
0x24 4 Union osd1 Linux: I version
0x28 69 Block[15] 15 points to data block
0x64 4 Version File version for NFS
0x68 4 File ACL low Lower 32 bits of extended attributes (ACL, etc)
0x6C 4 File size hi Upper 32 bits of file size (ext4 only)
0x70 4 Obsolete fragment An obsoleted fragment address
0x74 12 Osd 2 Second operating system dependent union
0x74 2 Blocks hi Upper 16 bits of block count
0x76 2 File ACL hi Upper 16 bits of extended attributes (ACL, etc.)
0x78 2 UID hi Upper 16 bits of owner ID
0x7A 2 GID hi Upper 16 bits of group ID
0x7C 2 Checksum Io Lower 16 bits of inode checksum

"Modify" is the timestamp of the last time the file's content has been modified. This is often called "mtime".
"Change" is the timestamp of the last time the file's inode has been changed, like by changing permissions, ownership, file name, and the number of hard links. It's often called "ctime".

Inode structure extended (Ext4):

Offset Size Name Description
0x80 2 Extra size How many bytes beyond standard 128 are used
0x82 2 Checksum hi Upper 16 bits of inode checksum
0x84 4 Ctime extra Change time extra bits
0x88 4 Mtime extra Modify time extra bits
0x8C 4 Atime extra Access time extra bits
0x90 4 Crtime File create time (seconds since epoch)
0x94 4 Crtime extra File create time extra bits
0x98 4 Version hi Upper 32 bits of version
0x9C Unused Reserved space for future expansions

Special inodes:

Inode Special Purpose
0 No such inode, numberings starts at 1
1 Defective block list
2 Root directory
3 User quotas
4 Group quotas
5 Boot loader
6 Undelete directory
7 Reserved group descriptors (for resizing filesystem)
8 Journal
9 Exclude inode (for snapshots)
10 Replica inode
11 First non-reserved inode (often lost + found)

{% hint style="info" %} Not that the creation time only appears in Ext4. {% endhint %}

By knowing the inode number you can easily find its index:

  • Block group where an inode belongs: (Inode number - 1) / (Inodes per group)
  • Index inside it's group: (Inode number - 1) mod(Inodes/groups)
  • Offset into inode table: Inode number * (Inode size)
  • The "-1" is because the inode 0 is undefined (not used)
ls -ali /bin | sort -n #Get all inode numbers and sort by them
stat /bin/ls #Get the inode information of a file
istat -o <start offset> /path/to/image.ext 657103 #Get information of that inode inside the given ext file
icat -o <start offset> /path/to/image.ext 657103 #Cat the file

File Mode

Number Description
15 Reg/Slink-13/Socket-14
14 Directory/Block Bit 13
13 Char Device/Block Bit 14
12 FIFO
11 Set UID
10 Set GID
9 Sticky Bit (without it, anyone with Write & exec perms on a directory can delete and rename files)
8 Owner Read
7 Owner Write
6 Owner Exec
5 Group Read
4 Group Write
3 Group Exec
2 Others Read
1 Others Write
0 Others Exec

The bold bits (12, 13, 14, 15) indicate the type of file the file is (a directory, socket...) only one of the options in bold may exit.

Directories

Offset Size Name Description
0x0 4 Inode
0x4 2 Rec len Record length
0x6 1 Name len Name length
0x7 1 File type

0x00 Unknown
0x01 Regular

0x02 Director

0x03 Char device

0x04 Block device

0x05 FIFO

0x06 Socket

0x07 Sym link
0x8 Name Name string (up to 255 characters)

To increase the performance, Root hash Directory blocks may be used.

Extended Attributes

Can be stored in

  • Extra space between inodes (256 - inode size, usually = 100)
  • A data block pointed to by file_acl in inode

Can be used to store anything as a users attribute if the name starts with "user". So data can be hidden this way.

Extended Attributes Entries

Offset Size Name Description
0x0 1 Name len Length of attribute name
0x1 1 Name index

0x0 = no prefix

0x1 = user. Prefix

0x2 = system.posix_acl_access

0x3 = system.posix_acl_default

0x4 = trusted.

0x6 = security.

0x7 = system.

0x8 = system.richacl
0x2 2 Value offs Offset from first inode entry or start of block
0x4 4 Value blocks Disk block where value stored or zero for this block
0x8 4 Value size Length of value
0xC 4 Hash Hash for attribs in block or zero if in inode
0x10 Name Attribute name w/o trailing NULL 
setfattr -n 'user.secret' -v 'This is a secret' file.txt #Save a secret using extended attributes
getfattr file.txt #Get extended attribute names of a file
getdattr -n 'user.secret' file.txt #Get extended attribute called "user.secret"

Filesystem View

To see the contents of the file system, you can use the free tool: https://www.disk-editor.org/index.html
Or you can mount it in your linux using mount command.

https://piazza.com/class_profile/get_resource/il71xfllx3l16f/inz4wsb2m0w2oz#:~:text=The%20Ext2%20file%20system%20divides,lower%20average%20disk%20seek%20time.

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