/usr/share/doc/samhain/manual.html/kerneldef.html

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><H1
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><A
NAME="KERNELDEF"
>5.10. Detecting Kernel rootkits</A
></H1
><P
>  This option is currently supported for Linux on the
  ix86 and x86_64 architectures, kernel versions 2.6.x (ix86, x86_64), 
  and 2.4.x (ix86), and for
  FreeBSD/x86 (tested on FreeBSD 4.6.2, FreeBSD 5) and OpenBSD/ix86 
  (tested with OpenBSD 3.8).
  </P
><DIV
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><B
>Warning</B
></TH
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><TR
><TD
>&nbsp;</TD
><TD
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VALIGN="TOP"
><P
>  It is incorrect to assume that disabling support for loadable
  kernel modules protects against runtime kernel modifications.
  It is possible to modify the kernel via /dev/kmem as well, if this
  device is present and writeable.
  </P
></TD
></TR
></TABLE
></DIV
><P
>  To use this facility, you need to compile with the option:
  </P
><P
>  <B
CLASS="COMMAND"
>./configure --with-kcheck=<TT
CLASS="REPLACEABLE"
><I
>/path/to/System.map</I
></TT
></B
> (Linux), or
  </P
><P
>  <B
CLASS="COMMAND"
>./configure --with-kcheck</B
> (FreeBSD/OpenBSD).
  </P
><P
>  On Linux, <TT
CLASS="FILENAME"
>System.map</TT
> is a file (sometimes with 
  the kernel version appended
  to its name) that is generated when the kernel is compiled, and is usually 
  installed in the same directory as your kernel 
  (e.g. <TT
CLASS="FILENAME"
>/boot</TT
>), or
  in the root directory. To find it, you can use:
  <B
CLASS="COMMAND"
>locate System.map</B
>
  </P
><DIV
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><P
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><IMG
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><TH
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><B
>Linux distributions without <TT
CLASS="FILENAME"
>/dev/kmem</TT
></B
></TH
></TR
><TR
><TD
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>  Many Linux distributions (including Ubuntu, RedHat, and Fedora)
  compile their kernels without support for the 
  <TT
CLASS="FILENAME"
>/dev/kmem</TT
> device. However, the 
  <SPAN
CLASS="APPLICATION"
>samhain</SPAN
> kernel integrity check relies on
  information obtained from this device. To work around this problem,
  as of version 2.7.0, <SPAN
CLASS="APPLICATION"
>samhain</SPAN
> provides
  a loadable kernel module <TT
CLASS="FILENAME"
>samhain_kmem.ko</TT
> which
  generates a file <TT
CLASS="FILENAME"
>/proc/kmem</TT
> that provides
  exactly the same information as <TT
CLASS="FILENAME"
>/dev/kmem</TT
> would.
  If the kernel module is required, you will be prompted to compile
  and install it - using the following commands - during the 
  compilation of <SPAN
CLASS="APPLICATION"
>samhain</SPAN
>.
  </P
><P
>  <PRE
CLASS="SCREEN"
>  <SAMP
CLASS="PROMPT"
>sh$ </SAMP
><KBD
CLASS="USERINPUT"
>make samhain_kmem.ko</KBD
>
  <SAMP
CLASS="PROMPT"
>sh$ </SAMP
><KBD
CLASS="USERINPUT"
>sudo insmod samhain_kmem.ko</KBD
>
  </PRE
>
  </P
></TD
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><TH
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><B
>Updating the kernel</B
></TH
></TR
><TR
><TD
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>  On Linux, after installing a new kernel, you need to configure
  five (5) addresses (see configuration example below), otherwise
  the kernel check will not work anymore (samhain needs to know the new
  position of some objects within the kernel). As explained below,
  you can easily obtain the required values by grepping them from the
  <TT
CLASS="FILENAME"
>System.map</TT
> of your new kernel, which should 
  normally be installed
  into the <TT
CLASS="FILENAME"
>/boot</TT
> directory, together
  with the kernel.
  </P
><P
>  If you need the <TT
CLASS="FILENAME"
>samhain_kmem.ko</TT
> kernel module
  because your kernel does not support the <TT
CLASS="FILENAME"
>/dev/kmem</TT
> 
  device, you will need to recompile this module for your new kernel as well.
  </P
></TD
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><P
></P
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><TH
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><B
>Using the hiding kernel module</B
></TH
></TR
><TR
><TD
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>  If you also use the
  option <B
CLASS="COMMAND"
>./configure --enable-khide</B
> 
  to use a kernel module to hide the presence
  of <SPAN
CLASS="APPLICATION"
>samhain</SPAN
>, the first detected 
  modification of the sys_getdents syscall (to list directories) 
  will only cause a warning (rather than an error), as it is
  presumed to be caused by the samhain_hide LKM).
  </P
><P
> 
  You should
  <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>NOT</I
></SPAN
> initialize the database with the
  samhain_hide LKM loaded (doing so might result in the
  non-detection of a real rootkit if it also only modifies
  the sys_getdents syscall).
  </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="KERNELDEF-CONFIG"
>5.10.1. Configuration</A
></H2
><P
>  This facility is configured in the <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>Kernel</I
></SPAN
> 
  section of the 
  configuration file.
  <PRE
CLASS="PROGRAMLISTING"
>  [Kernel]  
  # activate (0 for switching off) 
  KernelCheckActive=1
  # interval between checks (in seconds, default 300)
  KernelCheckInterval=20 
  # also check the interrupt descriptor table (linux, default TRUE)
  KernelCheckIDT=TRUE 
  # also check the PCI expansion ROM (linux, default TRUE)
  KernelCheckPCI=TRUE 
  # also check the proc filesystem integrity (linux, default TRUE)
  KernelCheckPROC=TRUE 
  # this is the severity (see section <A
HREF="basic-configuration.html#SEVERITYDEF"
>Section 4.1.1</A
>) 
  SeverityKernel=crit 
  #
  # Only needed for Linux, after installing a new kernel. You need the address
  # (first item in the grepped line), prefixed with '0x' to indicate
  # hexadecimal format.
  #
  # this is the address of system_call (grep system_call System.map) 
  KernelSystemCall = 0xc0106cf8
  #
  # this is the address of sys_call_table (grep ' sys_call_table' System.map) 
  KernelSyscallTable = 0xc01efb98
  #
  # this is the address of proc_root (grep ' proc_root$' System.map) 
  KernelProcRoot = 0xc01efb98
  #
  # this is the address of proc_root_inode_operations 
  # (grep proc_root_inode_operations System.map) 
  KernelProcRootIops = 0xc01efb98
  #
  # this is the address of proc_root_lookup
  # (grep proc_root_lookup System.map) 
  KernelProcRootLookup = 0xc01efb98

  </PRE
>
  </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="KERNELDEF-WHAT"
>5.10.2. What is a kernel rootkit ?</A
></H2
><P
>  A <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>rootkit</I
></SPAN
> is a set of programs 
  installed to "keep a backdoor open"
  after an intruder has obtained root access to a system. Usually such
  rootkits are very easy to install, and provide facilities to hide the
  intrusion (e.g. erase all traces from audit logs, install a modified 'ps'
  that will not list certain programs, etc.).
  </P
><P
>  While "normal" rootkits can be detected with checksums on programs, like
  <SPAN
CLASS="APPLICATION"
>samhain</SPAN
> does (the modified 'ps' would have 
  a different checksum than the
  original one), this method can be subverted by rootkits that modify
  the kernel at runtime, either with a 
  <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>loadable kernel module</I
></SPAN
>
  (LKM), i.e. a module that is loaded into 
  the kernel at runtime, or by writing to 
  <TT
CLASS="FILENAME"
>/dev/kmem</TT
> (this allows
  to 'patch' a kernel on-the-fly even if the kernel has 
  <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>no</I
></SPAN
> LKM support). 
  </P
><P
>  Kernel rootkits
  can modify the action of kernel <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>syscalls</I
></SPAN
>. 
  From a users viewpoint,
  these syscalls are the lowest level of system functions, and provide
  access to filesystems, network connections, and other goodies.
  By modifying kernel syscalls, kernel rootkits can hide files,
  directories, processes, or network connections without modifying any
  system binaries. Obviously, checksums are useless in this situation.
  </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="KERNELDEF-HOW"
>5.10.3. Implemented integrity checks</A
></H2
><P
>  When a system call (e.g. open() to open a file) is made by
  an application, the flow of control looks like this:
  </P
><P
><P
></P
><OL
TYPE="1"
><LI
><P
>    An interrupt is triggered, and execution continues at the interrupt
    handler defined for that interrupt. On Linux, interrupt 80 is used.
   </P
><P
>    A rootkit could replace the kernels interrupt handler by an own
    function.
   </P
><P
>    <SPAN
CLASS="APPLICATION"
>Samhain</SPAN
> checks the Interrupt Descriptor Table
    for modifications.
   </P
></LI
><LI
><P
>    The interrupt handler (named system_call() on Linux) 
    looks up the address of the requested syscall
    in the syscall table, and executes a jump to the respective address.
   </P
><P
>    A rootkit may (a) modify the interrupt handler to use a (rootkit-supplied)
    different syscall table, or (b) modify the entries in the syscall table to
    point to the rootkits replacement functions.
   </P
><P
>    <SPAN
CLASS="APPLICATION"
>Samhain</SPAN
> checks (a) the interrupt handler, and
    (b) the syscall table for modifications.
   </P
></LI
><LI
><P
>    The syscall function is executed, and control returns to the application.
   </P
><P
>    A rootkit may overwrite the syscall function to place a jump to its 
    own replacement function at the start of the syscall function.
   </P
><P
>    <SPAN
CLASS="APPLICATION"
>Samhain</SPAN
> checks the first few bytes of each
    syscall function for modifications.
   </P
></LI
></OL
></P
><P
>  In addition to these checks, <SPAN
CLASS="APPLICATION"
>Samhain</SPAN
> will
  check the <TT
CLASS="FILENAME"
>/proc</TT
> inode to
  detect the <SPAN
CLASS="APPLICATION"
>adore-ng</SPAN
> rootkit, which does
  not modify any syscall execution, but rather the VFS (Virtual File System)
  layer of the kernel.
  </P
><P
>  On FreeBSD/OpenBSD, currently only the syscall table (2b) and 
  the system call (3)
  are checked.
  </P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="KERNELDEF-ERRORS"
>5.10.4. Error messages</A
></H2
><P
>  Error messages start with 'POLICY KERNEL'. There are four types of them:
  (a) 'IDT' signifies modified interrupts: old and new address, 
  segment, privilege level, and type are listed, 
  (b) SYSCALL: modified syscall table/syscall code interrupt handler, and
  (c) SYS_GATE: modified interrupt handler for syscalls.
  (d) PROC: modified <TT
CLASS="FILENAME"
>/proc</TT
> system
  </P
><P
>  If an empty slot in the interrupt descriptor table (old address zero) 
  has been modified, this indicates that a new interrupt has been 
  installed. This cannot modify
  the behaviour of user applications (which would not use that interrupt), 
  but could be used by a dedicated
  (rootkit-supplied) application to perform some action (e.g. elevate
  privileges). 
  </P
><P
>  Likewise, if an empty slot in the syscall table (syscall name 
  sys_ni_syscall/_nosys) 
  has been modified, this cannot modify
  the behaviour of user applications, but again could be used by a dedicated
  (rootkit-supplied) application to perform some action.
  </P
><DIV
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></TH
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><TD
>&nbsp;</TD
><TD
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><P
>  As of version 1.8.4, kernel info is stored in the baseline database
  by (mis-)using fields that normally describe some properties of files.
  You may therefore find that error messages have info appended that
  looks like properties you would normally expect for a file (e.g. mtime,
  ctime, link_path ...). This is required for server-side database update
  (if you use samhain as client/server system).
  </P
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