Linux下文件设置用户编码setuid详解

    咱们知道，Linux每一个文件的详细属性都存储在一个stat结构中，能够用stat命令查看文件的详细属性，例如文件大小，修改时间，所属用户，文件权限等。

struct stat {
    dev_t     st_dev;     /* ID of device containing file */
    ino_t     st_ino;     /* inode number */
    mode_t    st_mode;    /* protection */
    nlink_t   st_nlink;   /* number of hard links */
    uid_t     st_uid;     /* user ID of owner */
    gid_t     st_gid;     /* group ID of owner */
    dev_t     st_rdev;    /* device ID (if special file) */
    off_t     st_size;    /* total size, in bytes */
    blksize_t st_blksize; /* blocksize for filesystem I/O */
    blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
    time_t    st_atime;   /* time of last access */
    time_t    st_mtime;   /* time of last modification */
    time_t    st_ctime;   /* time of last status change */
};

在stat结构中，st_mode字段表示了文件的模式相关属性。下图是关于该字段的详细描述，能够发现st_mode经过一个8进制数字存储文件模式：

• 后三位分布表示了文件读、写、执行的权限以及文件的掩码mask信息
• 第4位表示文件的粘着位，设置用户组编号，设置用户编号信息
• 其余位表示了文件的类型信息（管道，设备，文件夹，普通文件等）

S_IFMT     0170000   bit mask for the file type bit fields
S_IFSOCK   0140000   socket
S_IFLNK    0120000   symbolic link
S_IFREG    0100000   regular file
S_IFBLK    0060000   block device
S_IFDIR    0040000   directory
S_IFCHR    0020000   character device
S_IFIFO    0010000   FIFO
S_ISUID    0004000   set-user-ID bit
S_ISGID    0002000   set-group-ID bit (see below)
S_ISVTX    0001000   sticky bit (see below)
S_IRWXU      00700   mask for file owner permissions
S_IRUSR      00400   owner has read permission
S_IWUSR      00200   owner has write permission
S_IXUSR      00100   owner has execute permission
S_IRWXG      00070   mask for group permissions
S_IRGRP      00040   group has read permission
S_IWGRP      00020   group has write permission
S_IXGRP      00010   group has execute permission
S_IRWXO      00007   mask for permissions for others (not in group)
S_IROTH      00004   others have read permission
S_IWOTH      00002   others have write permission
S_IXOTH      00001   others have execute permission

下面一段代码将对setuid的使用做出简单说明。父进程经过fork建立一个子进程，而后父进程和子进程将分别打印设置用户ID，最后父进程将会尝试建立一个新的文件。

#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>

int main(int argc, char *argv[])
{
    int fd;
    pid_t pid;

    if ((pid = fork()) < 0) {
        fprintf(stderr, "fork error\n");
        exit(1);
    } else if (pid == 0) {
        printf("child real user id: %d\n", getuid());
        printf("child effective user id: %d\n", geteuid());
    } else {
        sleep(1);
        printf("parrent real user id: %d\n", getuid());
        printf("parrent effective user id: %d\n", geteuid());
        exit(0);
    }
    if ((fd = open(argv[1], O_CREAT | O_RDWR)) < 0) {
        fprintf(stderr, "open error\n");
        fprintf(stderr, "error: %s\n", strerror(errno));
    }

    close(fd);

    return 0;
}

编译并执行程序，结果以下：

roo@roose:~$ ls -l a.out
-rwxrwxr-x 1 roo roo 9174 Jan 29 17:17 a.out*
roo@roose:~$ ./a.out /usr/local/testfile
child real user id: 1000
child effective user id: 1000
parrent real user id: 1000
parrent effective user id: 1000
open error
error: Permission denied

根据打印的信息，能够发现

• 父进程和子进程的实际用户ID和有效用户ID都是1000，即为普通用户
• 尝试建立文件/usr/local/testfile时，程序提示权限错误，说明执行用户权限为普通用户

下面修改a.out的属性，设置文件拥有者为root用户，添加setuid

roo@roose:$ sudo chown root:root a.out
roo@roose:$ sudo chmod 4775 a.out
roo@roose:$ ls -l a.out
-rwsrwxr-x 1 root root 9174 Jan 29 17:17 a.out

而后从新执行程序

roo@roose:~$ ./a.out /usr/local/testfile
child real user id: 1000
child effective user id: 0
parrent real user id: 1000
parrent effective user id: 0

根据打印的信息，能够发现：

• 进程的实际用户编号(uid)为执行用户的用户编号1000，即为普通用户
• 进程的有效用户编号(euid)是文件所属用户的用户编号0，即为root用户
• 子进程也集成了父进程的实际用户编号和有效用户编号
• 父进程成功建立了/usr/local/testfile文件，说明程序是以root权限执行的

尝试执行设置了setuid的shell脚本，建立脚本文件test.sh内容以下：

#!/bin/bash
sleep 10
touch /usr/local/testfile

roo@roose:~$ sudo chown root:root test.sh
roo@roose:~$ sudo chmod 4775 test.sh
roo@roose:~$ ./test.sh &
[1] 54280
roo@roose:~$ ps -eo uid,euid,command | grep test.sh
 1000  1000 /bin/bash ./test.sh
 1000  1000 grep --color=auto test.sh
roo@roose:~$ touch: cannot touch ‘/usr/local/testfile’: Permission denied

一样对test.sh赋予相关权限并执行，结果却和上面C生成的可执行文件不太同样：

• 程序的实际用户编号(uid)和有效用户编号(euid)都是1000，即为普通用户；
• 程序也没有权限建立/usr/local/testfile，即添加的设置用户编号没有效果。

为何已经设置了setuid却没有效果呢？经过查找相关资料，发现出现该问题的缘由是：

    出于安全问题，Linux会忽略任何编译器文件的设置用户位。

详细说明以下(博主太懒，有空再翻译)

http://unix.stackexchange.com/questions/364/allow-setuid-on-shell-scripts

Linux ignores the setuid¹ bit on all interpreted executables (i.e. executables starting with a #! line). The comp.unix.questions FAQ explains the security problems with setuid shell scripts. These problems are of two kinds: shebang-related and shell-related; I go into more details below.

If you don't care about security and want to allow setuid scripts, under Linux, you'll need to patch the kernel. As of 3.x kernels, I think you need to add a call to install_exec_creds in the load_script function, before the call to open_exec, but I haven't tested.
Setuid shebang

There is a race condition inherent to the way shebang (#!) is typically implemented:
    The kernel opens the executable, and finds that it starts with #!.
    The kernel closes the executable and opens the interpreter instead.
    The kernel inserts the path to the script to the argument list (as argv[1]), and executes the interpreter.

If setuid scripts are allowed with this implementation, an attacker can invoke an arbitrary script by creating a symbolic link to an existing setuid script, executing it, and arranging to change the link after the kernel has performed step 1 and before the interpreter gets around to opening its first argument. For this reason, most unices ignore the setuid bit when they detect a shebang.

One way to secure this implementation would be for the kernel to lock the script file until the interpreter has opened it (note that this must prevent not only unlinking or overwriting the file, but also renaming any directory in the path). But unix systems tend to shy away from mandatory locks, and symbolic links would make a correct lock feature especially difficult and invasive. I don't think anyone does it this way.

A few unix systems (mainly OpenBSD, NetBSD and Mac OS X, all of which require a kernel setting to be enabled) implement secure setuid shebang using an additional feature: the path /dev/fd/N refers to the file already opened on file descriptor N (so opening /dev/fd/N is roughly equivalent to dup(N)). Many unix systems (including Linux) have /dev/fd but not setuid scripts.

    The kernel opens the executable, and finds that it starts with #!. Let's say the file descriptor for the executable is 3.
    The kernel opens the interpreter.
    The kernel inserts /dev/fd/3 the argument list (as argv[1]), and executes the interpreter.

Sven Mascheck's shebang page has a lot of information on shebang across unices, including setuid support.

Setuid interpreters

Let's assume you've managed to make your program run as root, either because your OS supports setuid shebang or because you've used a native binary wrapper (such as sudo). Have you opened a security hole? Maybe. The issue here is not about interpreted vs compiled programs. The issue is whether your runtime system behaves safely if executed with privileges.     Any dynamically linked native binary executable is in a way interpreted by the dynamic loader (e.g. /lib/ld.so), which loads the dynamic libraries required by the program. On many unices, you can configure the search path for dynamic libraries through the environment (LD_LIBRARY_PATH is a common name for the environment variable), and even load additional libraries into all executed binaries (LD_PRELOAD). The invoker of the program can execute arbitrary code in that program's context by placing a specially-crafted libc.so in $LD_LIBRARY_PATH (amongst other tactics). All sane systems ignore the LD_* variables in setuid executables.     In shells such as sh, csh and derivatives, environment variables automatically become shell parameters. Through parameters such as PATH, IFS, and many more, the invoker of the script has many opportunities to execute arbitrary code in the shell scripts's context. Some shells set these variables to sane defaults if they detect that the script has been invoked with privileges, but I don't know that there is any particular implementation that I would trust.     Most runtime environments (whether native, bytecode or interpreted) have similar features. Few take special precautions in setuid executables, though the ones that run native code often don't do anything fancier than dynamic linking (which does take precautions).     Perl is a notable exception. It explicitly supports setuid scripts in a secure way. In fact, your script can run setuid even if your OS ignored the setuid bit on scripts. This is because perl ships with a setuid root helper that performs the necessary checks and reinvokes the interpreter on the desired scripts with the desired privileges. This is explained in the perlsec manual. It used to be that setuid perl scripts needed #!/usr/bin/suidperl -wT instead of #!/usr/bin/perl -wT, but on most modern systems, #!/usr/bin/perl -wT is sufficient. Note that using a native binary wrapper does nothing in itself to prevent these problems. In fact, it can make the situation worse, because it might prevent your runtime environment from detecting that it is invoked with privileges and bypassing its runtime configurability. A native binary wrapper can make a shell script safe if the wrapper sanitizes the environment. The script must take care not to make too many assumptions (e.g. about the current directory) but this goes. You can use sudo for this provided that it's set up to sanitize the environment. Blacklisting variables is error-prone, so always whitelist. With sudo, make sure that the env_reset option is turned on, that setenv is off, and that env_file and env_keep only contain innocuous variables. TL,DR:     Setuid shebang is insecure but usually ignored.     If you run a program with privileges (either through sudo or setuid), write native code or perl, or start the program with a wrapper that sanitizes the environment (such as sudo with the env_reset option).