search

In Linux, s refers to "forced bit permissions", which is located in the third position of the user or group permission group. If the s bit is set in the user permission group, when the file is executed, the file executes the program with the file owner UID instead of the user UID; if the s bit is set in the group permission group, when the file is executed, The file is executed with the file owner GID instead of the user GID. The s permission bit is a sensitive permission bit and can easily cause system security issues.

What permissions does linux s have?

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

The most common file permissions in Linux systems are w, r, and x, which correspond to write, read, and execute permissions respectively. However, in addition, Linux also supports another series of permission settings, such as s permissions. , t permissions. The following introduces the specific meaning of s permissions.

s permissions

s, which means set UID or set GID. Located in the third position of the user or group permission group. If the s bit is set in the user permission group, when the file is executed, the file executes the program with the file owner UID instead of the user UID. If the s bit is set in the group permission group, when the file is executed, the file executes the program with the file owner GID instead of the user GID. The s permission bit is a sensitive permission bit and can easily cause system security issues. Please be careful when setting it up and be aware of SUID or SGID files and directories already on your system.

s permissions include two constants, S_ISUID and S_ISGID, which are called forced bit permissions;

S_ISUID can only be applied to binary executable files*;

S_ISGID can be applied to binary executable files* and directories;

*shell scripts cannot, because shell scripts are not binary files.

S_ISUID and S_ISGID are used On binary files:

  • If S_ISUID is true, set the effective user ID of this process to the user ID of the owner of this file.

  • If S_ISGID is true, set the effective user group ID of this process to the group ID of this file.

Since the process has a user/group ID that affects file access permissions, then Equivalent to temporarily possessing the identity of the file owner. A typical file is passwd. If a general user executes the file, during the execution process, the file can obtain root permissions, so that the user's password can be changed.

S_ISGID When used in a directory:

After the current user executes chdir to switch to the directory, the effective user group ID (effective group) of the user process will be set to the user group of the directory. If the user has w in this directory permissions, then the user group ID of the new file created by the user process is the same as the user group ID of the directory.

ls -al /usr/bin/passwd
-rwsr-xr-x 1 pythontab pythontab 32988 2018-03-16 14:25 /usr/bin/passwd

The values ​​of the two constants are:

oct dec
S_ISUID 4000 2048
S_ISGID 2000 1024

is the mask of the 12th and 11th bits respectively.

how to use?

We can set s permissions through character mode: chmod a s filename, or we can use absolute mode to set:

  • Set suid : Set the bit before the corresponding permission bit to 4;

  • Set guid: Set the bit before the corresponding permission bit to 2;

  • Set both: set the bit before the corresponding permission bit to 4 2=6.

Set a file to run with the file owner UID identity:

chmod 4755 test

Set the identity of a file with the file owner's GID when running:

chmod 2755 test

Set the identity of a file with both of the above when running:

chmod 6755 test

Note: When setting the s permission, the file owner and group must first set the corresponding x permission, otherwise the s permission will not really take effect (the ch m o d command does not perform necessary integrity checks, even if the s permission is set without setting the x permission, chmod will not report an error. When we ls -l, we see rwS. The capital S indicates that the s permission is not in effect)

Summary

Executable files are equipped with this permission , you can gain privileges and arbitrarily access all system resources that the owner of the file can use. Please pay attention to files with SUID permissions. Hackers often use this permission to match the root account owner with SUID to silently open a backdoor in the system for future access.

Related recommendations: "Linux Video Tutorial"

The above is the detailed content of What permissions does linux s have?. For more information, please follow other related articles on the PHP Chinese website!

Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Maintenance Mode in Linux: A System Administrator's GuideMaintenance Mode in Linux: A System Administrator's GuideApr 26, 2025 am 12:20 AM

Maintenance mode plays a key role in Linux system management, helping to repair, upgrade and configuration changes. 1. Enter maintenance mode. You can select it through the GRUB menu or use the command "sudosystemctlisolaterscue.target". 2. In maintenance mode, you can perform file system repair and system update operations. 3. Advanced usage includes tasks such as resetting the root password. 4. Common errors such as not being able to enter maintenance mode or mount the file system, can be fixed by checking the GRUB configuration and using the fsck command.

Maintenance Mode in Linux: When and Why to Use ItMaintenance Mode in Linux: When and Why to Use ItApr 25, 2025 am 12:15 AM

The timing and reasons for using Linux maintenance mode: 1) When the system starts up, 2) When performing major system updates or upgrades, 3) When performing file system maintenance. Maintenance mode provides a safe and controlled environment, ensuring operational safety and efficiency, reducing impact on users, and enhancing system security.

Linux: Essential Commands and OperationsLinux: Essential Commands and OperationsApr 24, 2025 am 12:20 AM

Indispensable commands in Linux include: 1.ls: list directory contents; 2.cd: change working directory; 3.mkdir: create a new directory; 4.rm: delete file or directory; 5.cp: copy file or directory; 6.mv: move or rename file or directory. These commands help users manage files and systems efficiently by interacting with the kernel.

Linux Operations: Managing Files, Directories, and PermissionsLinux Operations: Managing Files, Directories, and PermissionsApr 23, 2025 am 12:19 AM

In Linux, file and directory management uses ls, cd, mkdir, rm, cp, mv commands, and permission management uses chmod, chown, and chgrp commands. 1. File and directory management commands such as ls-l list detailed information, mkdir-p recursively create directories. 2. Permission management commands such as chmod755file set file permissions, chownuserfile changes file owner, and chgrpgroupfile changes file group. These commands are based on file system structure and user and group systems, and operate and control through system calls and metadata.

What is Maintenance Mode in Linux? ExplainedWhat is Maintenance Mode in Linux? ExplainedApr 22, 2025 am 12:06 AM

MaintenanceModeinLinuxisaspecialbootenvironmentforcriticalsystemmaintenancetasks.Itallowsadministratorstoperformtaskslikeresettingpasswords,repairingfilesystems,andrecoveringfrombootfailuresinaminimalenvironment.ToenterMaintenanceMode,interrupttheboo

Linux: A Deep Dive into Its Fundamental PartsLinux: A Deep Dive into Its Fundamental PartsApr 21, 2025 am 12:03 AM

The core components of Linux include kernel, file system, shell, user and kernel space, device drivers, and performance optimization and best practices. 1) The kernel is the core of the system, managing hardware, memory and processes. 2) The file system organizes data and supports multiple types such as ext4, Btrfs and XFS. 3) Shell is the command center for users to interact with the system and supports scripting. 4) Separate user space from kernel space to ensure system stability. 5) The device driver connects the hardware to the operating system. 6) Performance optimization includes tuning system configuration and following best practices.

Linux Architecture: Unveiling the 5 Basic ComponentsLinux Architecture: Unveiling the 5 Basic ComponentsApr 20, 2025 am 12:04 AM

The five basic components of the Linux system are: 1. Kernel, 2. System library, 3. System utilities, 4. Graphical user interface, 5. Applications. The kernel manages hardware resources, the system library provides precompiled functions, system utilities are used for system management, the GUI provides visual interaction, and applications use these components to implement functions.

Linux Operations: Utilizing the Maintenance ModeLinux Operations: Utilizing the Maintenance ModeApr 19, 2025 am 12:08 AM

Linux maintenance mode can be entered through the GRUB menu. The specific steps are: 1) Select the kernel in the GRUB menu and press 'e' to edit, 2) Add 'single' or '1' at the end of the 'linux' line, 3) Press Ctrl X to start. Maintenance mode provides a secure environment for tasks such as system repair, password reset and system upgrade.

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

SAP NetWeaver Server Adapter for Eclipse

SAP NetWeaver Server Adapter for Eclipse

Integrate Eclipse with SAP NetWeaver application server.

Atom editor mac version download

Atom editor mac version download

The most popular open source editor

EditPlus Chinese cracked version

EditPlus Chinese cracked version

Small size, syntax highlighting, does not support code prompt function

SublimeText3 English version

SublimeText3 English version

Recommended: Win version, supports code prompts!

MantisBT

MantisBT

Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.