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setuid, setgid and stick bit

There's actually 4 attribute sets you can work with via chmod.

Special, User/Owner, Group, and Others in that order, when working with the four-number chmods, with that first number being special bits that can be set.

chmod 4555 equates to the following:

  • Set UID bit - Run the file as the owner regardless of which user is running it
  • User/Owner: Read, Execute
  • Group: Read, Execute
  • Others: Read, Execute The s in your 'human readable' string for permissions indicates that the SetUID bit (explained below) is set.

Effectively, we can break down the four-number chmod permissions argument into specific descriptors as follows, and doing the math to determine what a 4 in the first section would be, a 5 in the next section, and so on.

Keep in mind that #### is Special User/Owner Group and Others in that order.

For Special attributes (the first number in a four-number chmod argument):

  • Set UID - Run file as owner regardless of the user running it (shows as s in the human-readable permissions string for User section) = +4 (--s under User/Owner)
  • Set GID - Run file as group regardless of the user/group running it (shows as s in the human-readable permissions string for Group section) = +2 (--s under Group)
  • Sticky Bit - EFFECTIVE ON DIRECTORIES ONLY - If set, only the directory's owner user and root can delete the directory, and only the file owner or root can delete files inside it. (shows as t in the human-readable permissions string for Others section) = +1 (--t under Others)

For User/Owner, Group and Others attributes (the last three numbers in a four-number chmod argument):

  • Read = +4 (r--)
  • Write = +2 (-w-)
  • Execute (for files), or 'Enter Into / List Items' (for directories) = +1 (--x)

Examples​

setuid与setgid讲解​

看一下系统中用到它的地方，以/etc/passwd和/usr/bin/passwd为例：

[root@Salve1 school]# ll /etc/passwd /usr/bin/passwd 
-rw-r--r-- 1 root root 2005 Apr 23 01:25 /etc/passwd 
-rwsr-xr-x 1 root root 23420 Aug 11 2010 /usr/bin/passwd 
[root@Salve1 school]#

分析一下，/etc/passwd的权限为 -rw-r--r-- 也就是说： 该文件的所有者拥有读写的权限，而用户组成员和其它成员只有查看的权限。

我们知道，在系统中我们要修改一个用户的密码，root用户和普通用户均可以用/usr/bin/passwd someuser这个命令来修改这个/etc/passwd这个文件，root用户本身拥有对/etc/passwd的写权限，无可厚非；那普通用户呢，这里就用到了setuid，setuid的作用是“让执行该命令的用户以该命令拥有者的权限去执行”，就是普通用户执行passwd时会拥有root的权限，这样就可以修改/etc/passwd这个文件了。

它的标志为：s，会出现在x的地方，例：-rwsr-xr-x 。而setgid的意思和它是一样的，即让执行文件的用户以该文件所属组的权限去执行。

sticky bit​

看一下系统中用到它的地方，以/tmp为例：

[root@Salve1 /]# ll -d /tmp 
drwxrwxrwt 13 root root 4096 Apr 23 02:06 /tmp 
[root@Salve1 /]#

我们知道/tmp是系统的临时文件目录，所有的用户在该目录下拥有所有的权限， 也就是说在该目录下可以任意创建、修改、删除文件，那如果用户A在该目录下创建了一个文件， 用户B将该文件删除了，这种情况我们是不能允许的。

为了达到该目的，就出现了stick bit（粘滞位）的概念。 它是针对目录来说的，如果该目录设置了stick bit（粘滞位），则该目录下的文件除了该文件的创建者和root用户可以删除和修改/tmp目录下的stuff，别的用户均不能动别人的，这就是粘滞位的作用。

References​

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