CVE-2021-42257: check_smart.pl: unprivileged user can alter hard drive settings

[Wolfgang Frisch <wolfgang.frisch () suse com>]

Hello oss-security,

during a routine audit of scripts in openSUSE, I discovered a
vulnerability in `check_smart.pl` [4], a plugin for systems monitoring
software to monitor the values of SMART attributes of hard and solid
state drives, using smartmontools in the background.

## Brief
`check_smart.pl` [1][2] from version 6.1 through 6.9 contained an
insufficient input validation that allowed an unprivileged user to
modify SMART settings, disable SMART monitoring entirely, shut down a
drive or degrade a drive's performance by disabling its read cache. The
bug was fixed with the release of version 6.9.1 [3].

## Detailed description
`check_smart.pl` needs to run as root in order to execute `smartctl`.
This is achieved with an entry in `/etc/sudoers`, which allows a lesser
privileged user, e.g. the one the monitoring system runs under, to
execute it. User input that is passed to `smartctl` is sufficiently
validated apart from one minor oversight.

The -d parameter is validated as follows:
> if (-b $opt_dl || -c $opt_dl || $opt_dl =~ m/\/dev\/bus\/\d/) {
>   # OK
> } else {
>   # NOT OK
> }

Later on, this parameter is passed verbatim to smartctl:
> my $full_command = "$smart_command -d $interface -Hi $device"

So an acceptable device name would be a block special device, a char
special device or match the regex `/dev/bus/\d`. Critically, this regex
matches even when /dev/bus/\d is just a _substring_ of any arbitrary
directory, for example `/tmp/dev/bus/1/sda`.

This can be exploited to pass arbitrary parameters to smartctl, some of
which affect the drive's behavior negatively:

### Steps to reproduce
> su -l -s /bin/bash nagios
> mkdir -p /tmp/dev/bus/1/
> ln -s /dev/sda /tmp/dev/bus/1/
> ls -l /tmp/dev/bus/1/sda
>
> /usr/lib/nagios/plugins/check_smart --debug -i auto -d "/tmp/dev/bus/1/sda -s off"
>
> SMART Disabled.

The upstream developer Claudio Kuenzler was very responsive and quickly
remediated the problem with the release of check_smart-6.9.1 [3].

[1] https://github.com/Napsty/check_smart
[2] https://www.claudiokuenzler.com/monitoring-plugins/check_smart.php
[3] 
https://www.claudiokuenzler.com/blog/1068/check_smart-6.9.1-security-fix-release-pseudo-device-path
[4] https://bugzilla.suse.com/show_bug.cgi?id=1183057

Best regards,
Wolfgang

--
Wolfgang Frisch <wolfgang.frisch () suse com>
Security Engineer
OpenPGP fingerprint: A2E6 B7D4 53E9 544F BC13  D26B D9B3 56BD 4D4A 2D15
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nuremberg, Germany
(HRB 36809, AG Nürnberg)
Managing Director: Felix Imendörffer

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