avcstat Command in Linux

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The avcstat command in Linux is a powerful tool for monitoring Security Enhanced Linux (SELinux) Access Vector Control (AVC) denials. It provides valuable insights into attempted access violations and helps in troubleshooting security issues.

SELinux implements mandatory access control (MAC), which enforces security policies on processes, files, and other system resources. AVC is a core component of SELinux that mediates access attempts and denies those that violate security policies.

Table of Contents

Here is a comprehensive guide to the options available with the avcstat command −

• Understanding avcstat Command in Linux
• How to use avcstat Command in Linux?
• Examples of avcstat Command in Linux
• Alternatives of avcstat Command in Linux

Understanding avcstat Command in Linux

The avcstat command is usually pre-installed on Linux distributions that use Security-Enhanced Linux (SELinux). SELinux is a mandatory access control (MAC) system that restricts access to system resources.

By default, avcstat displays a summary of AVC denials since the system was booted. The output includes −

How to use avcstat Command in Linux?

The avcstat command provides a way to monitor SELinux Access Vector Cache (AVC) statistics. The syntax for the avcstat command in Linux is −

avcstat [options] [interval]

Here's a breakdown of the elements −

• avcstat − This is the command itself used to display SELinux AVC statistics.
• options (optional) − These are flags that modify the output behavior of avcstat.
• interval (optional) − This argument specifies the interval (in seconds) for monitoring AVC statistics in real-time. When provided, avcstat will continuously display updated statistics at that interval.

Examples of avcstat Command in Linux

Here are some examples of how to use avcstat with various options −

Example 1: View Current AVC Statistics (Displaying Basic Statistics)

The avcstat command in Linux offers a limited set of options for customizing its behavior. By default, avcstat shows a snapshot of the AVC statistics since system boot. This includes −

avcstat # Display basic statistics

In the above output, lookups: Total number of permission checks performed by SELinux. hits: Number of checks where a cached decision was found. misses: Number of checks that required a new decision from the policy.

In addition, allocs: Number of times new entries were added to the cache. reclaims: Number of times entries were removed from the cache due to reaching its capacity. frees: Number of times entries were removed from the cache because the decision is no longer relevant.

Example 2: Monitor AVC Activity Continuously (every 5 seconds)

You can monitor how the AVC statistics change over time by specifying an interval (in seconds) as an argument to avcstat. This will continuously display the updated statistics −

avcstat 5  # Monitoring Statistics in Real-time

Example 3: Displaying Cumulative Values

The default behavior shows the difference in statistics since boot. This option displays the cumulative AVC statistics since system boot. By default, avcstat shows the difference in values since the last time it was run. To see the total accumulated values since system start, use the -c option −

avcstat -c # Display cumulative AVC statistics

Important Note − Running avcstat with sudo is recommended as it might require reading system logs.

Example 4: Redirecting the Output

You can redirect the output of avcstat to a file for further analysis. Users can also use the -f option with a custom file path might require root privileges for accessing system files −

avcstat > avclogs.log

Example 5: Specifying a Different Statistics File

By default, avcstat reads data from /sys/fs/selinux/avc/cache_stats. You can specify a different file using the -f option. This might be useful for analyzing statistics from a specific SELinux context −

sudo nano /sys/fs/selinux/avc/cache_stats

Tip: Filtering AVC Denials

Tools like ausearch and aureport can be used to filter and analyze specific AVC denials based on criteria like process ID (PID), time frame, or specific AVC rules. Refer to the man pages of these tools for detailed information −

man avcstat

Alternatives of avcstat Command in Linux

While there's no direct alternative that offers the exact functionality of avcstat, here are some approaches to achieve similar goals depending on your needs −

Using auditctl Command with SELinux-specific Filters

This auditctl command allows you to control and view the system audit log. You can use filters specific to SELinux AVC events within auditctl. This provides a more detailed view of individual AVC decisions compared to the summary statistics offered by avcstat.

Using ausearch Command

This ausearch is a utility specifically designed for searching and analyzing audit logs. Similar to auditctl, you can use filters to focus on SELinux AVC events within the audit log.

Conclusion

auditctl is a valuable tool for system administrators and security professionals managing SELinux-enabled systems. By understanding its functionality and options, you can effectively monitor AVC activity, identify potential security vulnerabilities, and ensure a secure system environment.

If you simply need a quick overview of AVC statistics, avcstat is the easiest option. If you want more detailed information about individual AVC decisions or need to analyze the data further, parsing the raw data or using auditctl with SELinux filters might be better suited.