How to Fix the Permission Denied Error in Linux

In the world of Linux and Unix-like operating systems, the “Permission Denied” error is a common occurrence that stumps many users, particularly when encountered in situations where file permissions seem to be correctly set.

One less obvious but significant cause of this error is the `noexec` mount option. Understanding how this option works is key to diagnosing and resolving related issues.

What is the noexec Mount Option and why does it Lead to Permission Denied

When a filesystem is mounted with noexec option, it restricts the execution of binaries from that filesystem.

Primarily `noexec` is intended to prevent the execution of potentially harmful or untrusted scripts or binaries, especially from volatile locations like `/tmp` or removable media.

Despite a file having executable permissions (`x`), attempting to run a script or binary on a `noexec` mounted filesystem results in a “Permission Denied” error.

This is somewhat counter-intuitive because the error suggests an issue with file permissions, whereas the actual cause is the mounting configuration of the filesystem.

Example of Running a Script on a noexec Mounted Partition

1. Script Setup

Let’s say you have a simple Bash script named example_script.sh located in a directory /var/scripts, and this directory resides on a partition mounted with the `noexec` option. The script could be as simple as:

#!/bin/bash
echo "Hello, World!"

2. Mounting the Partition with `noexec`

In /etc/fstab, the partition might be mounted with an entry like this:

/dev/sdaX /var ext4 defaults,noexec 0 0

Here, /dev/sdaX is the partition, mounted at /var with ext4 filesystem, and `noexec` among the mount options.

3. Attempting to Run the Script

You navigate to the directory and attempt to run the script:

cd /var/scripts
./example_script.sh

4. Encountering the `noexec` Restriction

Instead of executing as usual, the script fails to run, and you receive an error message similar to this:

bash: ./example_script.sh: Permission denied

Even though the file permissions of the script may allow execution (e.g., `rwx` permissions), the `noexec` mount option overrides this and prevents execution.

Workaround to fix permission denied issue

If you want to execute scripts or binaries that are currently located in /var without changing the mount options (specifically the noexec flag), there are a few workarounds you can consider:

1. Copy or Move the Script/Binary to an Executable Location

One of the simplest methods is to move or copy the script or binary to a directory that allows execution. Common directories where execution is permitted include:

/usr/local/bin: For user-installed software.
/opt: For third-party software packages.

For example, you can use the cp or mv command to move your script:

sudo cp /var/my_script.sh /usr/local/bin/

Or:

sudo mv /var/my_script.sh /usr/local/bin/

Then execute it from the new location.

2. Use a Temporary Filesystem

Another approach is to use a temporary filesystem like tmpfs which is usually mounted with execution permissions. You can copy your script or binary to /tmp and run it from there:

cp /var/my_script.sh /tmp/
/tmp/my_script.sh

3. Bind Mount

You can create a bind mount that mounts a subdirectory of /var to another location where execution is permitted. This is a bit more advanced and should be used with caution:

sudo mount --bind /var/my_directory /home/myuser/my_directory
sudo mount -o remount,exec /home/myuser/my_directory

4. Using a Scripting Language Interpreter Directly

If you’re trying to run a script (like a Python or Bash script), you can invoke the interpreter directly, passing the script as an argument. For example:

For a Bash script:

bash /var/my_script.sh

For a Python script:

python /var/my_script.py

5. Create a Symbolic Link

You can create a symbolic link to the script or binary in a directory that allows execution. However, this method will only work if the symbolic link’s target directory allows execution:

ln -s /usr/local/bin/my_script.sh /var/my_script.sh 
/var/my_script.sh

Important Considerations

Security: Keep in mind that these workarounds might bypass certain security measures put in place for a reason. Always consider the security implications in your specific environment.
File Permissions: Ensure that the script or binary has the appropriate file permissions set to allow execution.
System Stability: Be cautious with system changes and always have backups, especially if you’re working on a critical system.

These methods provide alternatives to directly changing mount options, allowing for execution of scripts and binaries located in areas mounted with noexec, while still maintaining system security and integrity.

Remove noexec option to fix permission denied issue

We can also remove noexec option to fix permission denied issue. We will use /var directory as an example below.

In Linux, the /var directory is often mounted with the noexec option for security reasons. This means you can’t execute scripts or binaries that are located in /var or its subdirectories.

Step 1: Identify the Mount Point

First, you need to identify how /var is mounted. Use the following command to see the current mount options:

mount | grep /var

Step 2: Edit /etc/fstab

Open /etc/fstab: This file controls how file systems are mounted at startup. Open it with a text editor. You might need superuser privileges for this.
```
sudo nano /etc/fstab
```
Locate the Entry for /var: Look for a line that contains /var. It will probably look something like this:
```
UUID=xxxxxx /var ext4 defaults,noexec 0 2
```
Modify the Mount Options: Remove noexec from the list of options. Be careful not to change other parts of the line. After modification, it might look like this:
```
UUID=xxxxxx /var ext4 defaults 0 2
```

Step 3: Remount /var

After saving changes to /etc/fstab, you need to remount /var to apply the changes. Use the following command:

sudo mount -o remount /var

Step 4: Verify the Changes

Verify that the noexec option has been removed:

mount | grep /var

Step 5: Reboot (Optional)

While the remount command should apply the changes immediately, in some cases a reboot may be necessary:

sudo reboot

Important Considerations

Security Implications: Removing noexec from /var can expose your system to additional security risks. Scripts or binaries in /var could potentially be executed with harmful intent.
Alternatives: If you need to execute a script or program that’s located in /var, consider moving it to a location that is intended for executables, like /usr/local/bin or /opt.
Backup /etc/fstab: It’s always a good idea to back up /etc/fstab before making changes. Mistakes in this file can lead to system boot issues.
Use with Caution: Be cautious with any changes you make to mount options and file system configurations, as incorrect settings can cause system instability or security vulnerabilities.

By following these steps, you can remove the noexec option from /var, but always weigh the necessity against the potential security risks.

Conclusion

The “Permission Denied” error caused by the `noexec` mount option serves as a reminder of the intricate balance between usability and security in system administration. While it can be an inconvenience, the `noexec` option plays a crucial role in securing Linux systems, especially in environments susceptible to the execution of untrusted code. Understanding its impact and knowing how to work around it, without compromising system security, is an essential skill for users and administrators in Linux environments.