Tracking Linux Internals with Syscalltracker

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original in de Mulyadi Santosa 

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My name is Mulyadi Santosa. I live in Indonesia and work as freelance writer and consultant. My interest is clustering, system security and networking. I also run small privately own company in clustering field aiming for selling cost-of-the-shelf clustering. OpenMosix and openSSI really drawn my focus. In my spare time, reading and sport are great leisure. You can send me e-mail to a_mulyadi@softhome and discuss about it

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Sometimes we want to watch our Linux system more closely. The are many of logging prograns, instrusion detection tools, integrity checking tools and so on. This time I want to introduce you to a mechanism to watch Linux at kernel level, which offer more reliability and more broader coverage.

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Introduction

One day I hang on mailing list discussing about clustering middleware tool. Accidentally there was a thread discussing system anomaly caused by a kernel patch. And then a person replied that he try to recontruct the problem according to steps reported by first person. This person uses a tool called Syscalltracker to help him pinning the problem that arises. And (for myself) I wonder "What kind of tool is Syscaltracker? What is its capability? " For casual user like me, the name "Syscalltracker" alone left me with mysterious suspect :-)

Syscalltracker

There is a solution for tracing this kind of problem. Not a 100% perfect solution but sufficient enough for handling major cases. It is based on the fact that every action or command issued from a shell, user program or daemon (in other word, ANY process) must be executing one or more internal system procedure widely known as system calls. Are you trying to delete a file? It means you call unlink. You ran a shell script? Then it must be calling exec() or execve(). So virtually every action related to the system is directly interpreted as system call. This is the basic idea why tracking based on system calls can be a tough weapon.

Interested?

# tar xzvf syscalltrack-0.82.tar.gz

And then verify that you have the Linux kernel source code at /usr/src.

# rpm -qa | grep -i kernel-source

OR

# ls -al /usr/src/linux-2.4

If one of them gives a negative result, then you need to install them. It is on Redhat CD (#2):

# rpm -replacepkgs -Uvh /path/to/your/RPM/kernel-source-2.4.18-3.i386.rpm

Pay attention that you HAVE TO compile Syscaltracker based on same kernel version (and other additional patches) that your Linux system is currently running. For example: if you use a standard RedHat 7.3 kernel, then you have to compile with the kernel source from the Redhat CD. Or if you want to use your own Linux kernel, later on the you must compile Syscalltracker based on this kernel source.

Besides kernel source code, to ease Syscalltracker compilation, you also need the kernel configuration file from Redhat. Try to check the content of /boot:

# ls -al config*

If there is an output like 'config-2.4.18-3' then you need to copy this file to /usr/src/linux-2.4. Rename it to '.config'

# cp /boot/config-2.4.18-3 /usr/src /linux-2.4/.config

If this file is somehow missing, the you can copy it from kernel source directory. It is located under the configs directory. You must pick one that matches your currently running kernel. So first find it out with

# uname -a

What you are running. It should display the version of the kernel. You can guess it. Suppose the output contains "kernel-2.4.18-3-i386" then you need yo copy kernel-2.4.18-3-i386.config

# cd /usr/src/linux-2.4
# cp configs/kernel-2.4.18-3-i386.config ./.config

Now you just have to run

#cd /usr/src/linux-2.4.18-3
# make mrproper
# make menuconfig

Do the settings of your need and then save/exit. If you use a self compiled kernel but lost you old kernel config file then you must carefully reconstruct it to avoid future problems ( I hope not :-) )

Compile Syscalltracker

Until now, we have prepared all of the requirements. We can go into Syscalltracker compilation:

# cd /usr/src/syscalltrack-0.82
# ./configure (or ./configure -with-linux=/path/to/your/linux/kernel/source)
# make && make install

if the compilation is successful, you will found two new modules:

1. /lib/modules/2.4.18-3/syscalltrack-0.82/sct_rules.o
   
   
2. /lib/modules/2.4.18-3/syscalltrack-0.82/sct_hijack.o
   
   

Those are the modules that are responsible for you system tracing. The author itself uses the term system call hijacking, means intercepting the sytem call and do the pre scripted work before executing original the procedure. Next you need to load them. There is a built script to do it

# sct_load (as root)

Confirm that the kernel loaded it using lsmod. You should see something like:

Module     Size  Used by    Not tainted
sct_rules  257788   2
sct_hijack 110176   1      [sct_rules]
<…..cut………..>

Rules

Congratulations! You have loaded the module and thus you are running the Syscalltracker. But it is not over. You still need to write rules needed by Syscalltracker to work properly. Let's begin with a simple one

rule 
 { 
   syscall_name=unlink 
   rule_name=unlink_rule1 
   action 
   { 
     type=LOG 
     log_format {%comm : %params delete by %euid --> %suid} 
   } 
   when=before 
}

Every rule declared for Syscalltracker begins with the reserved word "rule" followed by an "{". After this you must declare which system call you need to observe using the parameter "syscall_name". There are many system calls you can from pick. For a complete list try to look into the file '/usr/local/lib/syscalltrack-0.82/syscalls.dat-2.4.18-3. The meaning of some system calls is sometimes hard guess, but there are also easy ones. For now I'll pick unlink(). This system call is excuted every time someone or something trys to delete a file. I think this is a good choice for start, so the idea is to watch the deletions that happen on our system.

On the parameter "rule_name", you must supply the name for the rule. This is a free-to-choose entry, just write an easy-to-understand name. I choose "unlink_rule1". At the section "action", you need to write what action Syscalltraker does whenever there is match for system call's name. Syscalltracker is supporting some actions, but here we use LOG type. This action will write the log onto /dev/log. According to the TODO list on the website, there is plan to do system call rewriting. This means you can manipulate system call parameters and inject your own parameter :-)

For LOG action, you need to define output format. There are built in macros to get detailed output.

%ruleid -> rule name that matches intercepted system call
%sid    -> system call identification number 
%sname  -> system call name
%params -> system call parameter
%pid    -> ID from process that call the system call
%uid    -> user ID which executing the system call
%euid   -> effective user id that executing system call
%suid   -> recorded user id that executing the system call
%gid    -> the group id from user that execute this system call
%egid   -> effective group id from user that run the system call
%sgid   -> recorded group id of the user that ran the system call
%comm   -> name of command that executed the system call
%retval -> return value of system call. Only works 
           for LOG action with type "after"

For this example I wrote

.log_format {%comm : %params delete by %euid --> %suid}

It means "I want to log every command that is executing the system call named unlink, with the effetive user id and recorded user id"

In parameter when, we can pick between "before" or "after". The difference is clear, if we use "before" then the recording will be done before the system call is executed. If we use "after" then the recording will take place after the system call has been executed.

Close, the rule with "}" . This whole rule can be written inside normal text file, for example let's name it "try.conf" and save it on /tmp. Next you need to load this rule into Syscalltracker

# sct_config upload /tmp/try.conf

If the rule is written correctly, you will get the output "Successfully uploaded rules from file '/tmp/try.conf' ".

OK, all things went fine. Now comes the testing phase. Start to console, let say xterm inside Xwindow. On one console, watch the Syscalltracker's log using

# sctlog

Soon you will see the output as the result of intercept mechanism of system call if one matches your rule. On another console, do something like:

# cd /tmp
# touch ./dummy
# rm ./dummy

Using the above rule, you will likely get this output on sctlog

"rm" : "./dummy" delete by 0 --> 0

From this output, you will soon learn that this happens:

The command "rm" with the parameter "./dummy is executing the system call unlink(). Or in other words rm was used to delete a file. This command is using effective user id equal to 0 (or root)"

rule
{
   syscall_name = unlink
   rule_name = prevent_delete
   filter_expression {PARAMS[1]=="/etc/passwd" && UID == 0}
   action {
     type = FAIL
     error_code = -1
   }
   when = before
}

This is similar to our first example, but here we use the FAIL action. Exclusively for FAIL, we must define the return value for the intercepted system call. Here I use "-1" or "operation not permitted". The comlete list of this numbers can be found at /usr/include/asm/errno.h.

On the line containing "filter expression", I define a condition for which the checking is done if the first parameter (of system call) equals to "/etc/passwd". This is why we need the PARAMS variable. Note: every section has its own required parameters. Also, this checking is not perfect because one can use something like "cd /etc && rm -f ./passwd". But this is OK to start. We also check if the UID equals to 0 (root).

Add this rule to your previous rule file and reload :

# sct_config delete
# sct_config upload /tmp/try.conf

Pay attention that rule sequence is important. If you declare rule "prevent_delete" before "unlink_rule1", then if you do :

# rm -f /etc/passwd

you will match "prevent_delete" first and the whole action will fail. The "unlink_rule1" will be ignored. But if you swap it ("unlink_rule1" before "prevent_delete") then you just get the log without stopping the action !

There is another system call that is interesting to be observed. It is called ptrace. From "man ptrace", you can learn that this system call is used to observe and control the execution and control the execution of another program. In good hands, this ptrace can be handy tool for debugging, but in the wrong hand, it can be used for analyzing security holes and exploit them. So let's add the rule to log them.

In order to do so, just use rule like this

rule
{
    syscall_name=ptrace
    rule_name=ptrace_rule1
    action {
        type=LOG
      log_format {%comm : %params issued ptrace by %euid --> %suid}
  }
    when=before
}

Notice that we declare ptrace as system call to be watched. To test this, use the strace program to try the rule. First load above rule into syscalltracker and run sctlog. Then run strace against ls for example:

# strace /bin/ls

In sctlog, you should get several line like these:

"strace" : 3, 2019, 24, -1073748200 issued ptrace   by 0 --> 0
"strace" : 24, 2019, 1, 0 issued ptrace   by 0 --> 0
"strace" : 3, 2019, 44, -1073748200 issued ptrace   by 0 --> 0
"strace" : 3, 2019, 24, -1073748200 issued ptrace   by 0 --> 0
"strace" : 3, 2019, 0, -1073748216 issued ptrace   by 0 --> 0
"strace" : 7, 2019, 1, 0 issued ptrace   by 0 --> 0

For those who did not know strace, this is a tool (a powerful one) to trace system call issued inside a executable file. Strace internally is using ptrace to hook itself into a target program so it can trace it. Actually, strace and Syscalltracker are an ideal combo for system and file auditing, so I think it is worth to mention it. Redhat 7.3/8/9 already has it. Just install the RPM (here Redhat 7.3):

# rpm -Uvh /path/to/your/Redhat/RPM/strace-4.4-4.i386.rpm

Now you are one step further in diagnosing your system. Syscalltracker gives the user flexibility and you can learn about system calls. In addition, if you want to view the rules that have been loaded, just do:

# sct_config download

# sct_config delete

# sct_unload

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