News Group Posts

ksh exec login script

Subject: Re: only one program per login
References: <fd1496bf.0405251239.1c21d7db@posting.google.com>
<20040526021350.GA23909@jpradley.jpr.com>
<40b40112@dnews.tpgi.com.au>
<kOSdnclUFqRrrSndRVn-iQ@comcast.com> 
From: spcecdt@deeptht.armory.com (John DuBois)
Date: 26 May 2004 22:44:31 GMT
Message-ID: <40b51dcf$0$430$8eec23a@newsreader.tycho.net> 


In article <kOSdnclUFqRrrSndRVn-iQ@comcast.com>,
Brian K. White <brian@aljex.com> wrote:
>>>>> #!/bin/sh
>>>>> . /etc/profile
>>>>> run myprogram
>>>>
>>>> wouldn't it also be advisable to 'exec myprogram' at the end of the
>>>> script, instead of just running it?
>I don't think it's exactly a stupid idea.
>
>If you had many users logging in at once, and if they each had several
>concurrent sessions, then this could be a significantly adviseable step
>because it requires one less slot in the process stack (and maybe 3 file
>descriptors to go with it?) for each login.
>
>I have verified this by testing.

Yes, there should probably be an 'exec' there.
Side note: ksh93 is smart enough to not fork-before-exec for a simple command
on the very last line of a script, as long as you haven't asked it to do
anything that requires continued monitoring.

>It also makes the user slightly less likely to be able to break out of the
>script to the shell.
>They can't stuff the keyboard buffer while still in the binary and cause a
>break in that microsecond between binary exit and the next line in the
>script. Sure you should be able to secure that with trap and stty but this
>is even more sure.

Using the scheme I described (setting the user's login shell to be the script),
there is nothing that the user can do to wrest control over the shell.  The
shell isn't reading its standard input for commands, it's reading the shell
program.  It is exactly as secure (in this sense) as a compiled program.

"Breaking out at just the right moment" is only a concern if you are using the
scheme in which the user's login shell is the shell binary, and you try to keep
control within the user's .profile.  There, the shell is prepared to switch its
input from the .profile to the user's tty as soon as it is finished sourcing
the .profile (which can indeed be skipped by sending an interrupt character at
just the right moment).

        John
-- 
John DuBois  spcecdt@armory.com  KC6QKZ/AE  http://www.armory.com/~spcecdt/

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