Using Sudo

By Tony Lawrence
2005-05-12

Security

You might at this point suddenly think "Oh no- that means a sudo user could overwrite important files". We haven't limited the sudo users command set yet, but even if we do, what stops them from using such commands to pervert system files or other commands?

Well, remember that the shell does the redirection BEFORE sudo runs. If the redirection can't be done because of permissions, the command will fail.

[jim@lnxserve /tmp]$ sudo date &gt; /etc/shadow

bash: /etc/shadow: Permission denied

[jim@lnxserve /tmp]$

So that's one thing you don't need to worry about. Actually, sudo itself makes reasonable efforts to protect you from malicious michief by a sudo user. Running "sudo -V" as root shows sudo's settings; part of that is environment variables that it will not pass on or that it will check for dangerous content:

Sudo version 1.6.4

... (stuff deleted)

Environment variables to check for sanity:

        LANGUAGE

        LANG

        LC_*

Environment variables to remove:

        BASH_ENV

        ENV

        TERMCAP

        TERMPATH

        TERMINFO_DIRS

        TERMINFO

        _RLD*

        LD_*

        PATH_LOCALE

        NLSPATH

        HOSTALIASES

        RES_OPTIONS

        LOCALDOMAIN

        IFS

That's the default list; you can add or subtract from it in /etc/sudoers. Note that if you do add or subtract variables, "sudo -V" doesn't reflect those changes.

Let's try that out with our test user. First, we need a simple shell script that will show us the value of environment variables. I'll call it "showme":

We'll have "jim" try it out before making any changes to sudoers:

[jim@lnxserve jim]$ cat showme

set | grep $1

[jim@lnxserve jim]$ export ENV

[jim@lnxserve jim]$ ./showme ENV

BASH_ENV=/home/jim/.bashrc

[jim@lnxserve jim]$ sudo ./showme ENV

SUDO_COMMAND='./showme ENV'

The ENV variable is not picked up by sudo even though it was marked for export. Ordinarily, environment variables would be passed:

[jim@lnxserve jim]$ export BOOP=betty

[jim@lnxserve jim]$ ./showme BOOP

BOOP=betty

[jim@lnxserve jim]$ sudo ./showme BOOP

BOOP=betty

SUDO_COMMAND='./showme BOOP'

[jim@lnxserve jim]$

But we can add to the list of variables to discard:

# sudoers file.

#

# This file MUST be edited with the 'visudo' command as root.

#

Defaults:jim    timestamp_timeout=-1, env_delete+="BOOP"

Note the "+=" to ADD to the environment list. If we had just used "=", that would have replaced all of sudo's defaults. You can also use "-=" to subtract a default variable and allow it to be passwd.

Now "jim" won't get BOOP in his sudo environment.

[jim@lnxserve jim]$ sudo ./showme BOOP

SUDO_COMMAND='./showme BOOP'

Sudo also rearranges your PATH internally. That can be a little confusing:

[jim@lnxserve jim]$ cat ./showme

echo "I'm in /home/jim"

set | grep $1

[jim@lnxserve jim]$ cat ./bin/showme

echo "I'm in /home/jim/bin"

set | grep $1

[jim@lnxserve jim]$ export PATH=".:$PATH"

[jim@lnxserve jim]$ showme PATH

I'm in /home/jim

PATH=.:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/jim/bin

[jim@lnxserve jim]$ sudo showme PATH

I'm in /home/jim/bin

PATH=.:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/jim/bin

SUDO_COMMAND='/home/jim/bin/showme PATH'

[jim@lnxserve jim]$

Although PATH still shows "." at the beginning, the showme in /bin is what is run by sudo. Internally sudo has ignored the leading "." and moved on to find "showme" in /home/jim/bin. Now let's remove the /home/jim/bin/showme:

[jim@lnxserve jim]$ rm bin/showme

[jim@lnxserve jim]$ sudo showme PATH

sudo: ignoring `showme' found in '.'

Use `sudo ./showme' if this is the `showme' you wish to run.

[jim@lnxserve jim]$ sudo ./showme PATH

I'm in /home/jim

PATH=.:/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/jim/bin

SUDO_COMMAND='./showme PATH'

[jim@lnxserve jim]$

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