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SubjectTID/PID handling and possible rootkit
Hello! 

This is a message forwarded from Red Hat's Phoebe (8.1) Linux
Beta. It seems that some of us, when running "chkrootkit" rootkit
checker, are getting consistent errors complaining that we have
hidden processes from the "ps" command and "procfs".

Can you please help us disguise what's happening?
If so, keep reading :-)

----- Original Message -----
From: David McKellar <djmcke@yahoo.com>
Date: Fri, 28 Feb 2003 09:33:15 -0800 (PST)
To: phoebe-list@redhat.com
Subject: Re: chkrootkit on phoebe

> It says I have LKM also:
>
> Checking `lkm'... You have 12 process hidden for readdir command
> You have 12 process hidden for ps command
> Warning: Possible LKM Trojan installed
>
> My system is on the net all the time so it could very well be infected.

I have done some research and this is what I've found:

Recompiling (get paranoid) chkrootkit creates a separate binary called
"chkproc" that allows you to search for hidden processes. Running
"chkproc -v" on my system reveals this:

ID 1364: not in readdir output
PID 1364: not in ps output
PID 1365: not in readdir output
PID 1365: not in ps output
PID 1366: not in readdir output
PID 1366: not in ps output
You have 3 process hidden for readdir command
You have 3 process hidden for ps command

My computer is behind a firewall that filters all ports < 1023 and I'm a
very paranoic person, so I really doubt I have got infected by a trojan.

In my case, I've found the culprit to be "Evolution": while running
Evolution, there are 3 or more processes not being displayed by the
"ps" command, or even listed under "/proc., but they are accessible by
"cd"-ing to them. If I quit "Evolution", running "./chkproc -v" again does
not generate those warnings.

I think this is related to changes in PID/TID handling by recent kernels.
If my memory serves me well, each thread of a process is given a
unique ID (the Thread ID) which is assigned from the same pool as the
PID, so, there was a time when you could see threads from the output
of the "ps" command. I think this behaviour has changed, and now, you
can't directly see threads by using "ps" or reading from "/proc". However,
the TID number is still reserved from the same pool as PIDs, although
it won't be listed in "/proc". For whatever reason it be, it seems that "procfs"
still allows one to "cd" to the directory entry of a thread by using the
Thread ID (TID), so this could be the culprit of the problem. Since
"chkproc" tries "cd"-ing the hard way into all possible combinations of
directories from within "/proc", "chkproc" is in fact "seeing" was should be
hidden: the entries for threads.

This problem is reproducible on 2.4.20-2.54 and 2.5.63-mm1.

Can any kernel guru help us here? Am we right? Are we infected?

Thanks!

Felipe Alfaro Solana

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