二进制文件上的 SUID 位仍然产生“权限被拒绝”错误

Question 1

我通读了手册页对于该system功能，发现以下段落提供了解释：

Do not use system() from a program with set-user-ID or set-group-ID
privileges, because strange values for some environment variables
might be used to subvert system integrity.  Use the exec(3) family of
functions instead, but not execlp(3) or execvp(3).  system() will
not, in fact, work properly from programs with set-user-ID or set-
group-ID privileges on systems on which /bin/sh is bash version 2,
since bash 2 drops privileges on startup.  (Debian uses a modified
bash which does not do this when invoked as sh.)

以下是我在阅读该段落时阅读的其他一些 StackExchange 帖子：

对于后代，并且因为给定的示例可能有助于在其他情况下诊断此问题或类似问题，所以我在下面保留了原始示例。

您是否可能遇到了错误？我无法在运行 Debian 9 和 gcc 6.3 的 Docker 容器中重现您的问题。以下是我如何尝试重新创建您帖子中描述的场景。

首先创建“alice”和“bob”用户并将他们添加到“staff”组：

useradd -m -G staff alice
useradd -m -G staff bob

接下来创建文件并设置其所有权和权限：

# Create a subdirectory to hold the text file
sudo -u alice mkdir -p /home/alice/share/

# Create the text file
sudo -u alice bash -c 'echo "This is foo.txt" > /home/alice/share/foo.txt'

# Set restrictive permissions on the text file
chmod u=rw,g=,o= /home/alice/share/foo.txt

让我们检查一下结果：

$ ls -l /home/alice/share/foo.txt
-rw------- 1 alice alice 16 Nov  4 15:33 /home/alice/share/foo.txt

现在让我们创建一个display-foo使用该system函数的版本：

$ cat <<HEREDOC > /usr/local/src/display-foo.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>

int main(int argc, char **argv) {

    struct passwd *r_pwd = getpwuid(getuid());
    printf("Real user: %s\n", r_pwd->pw_name);

    struct passwd *e_pwd = getpwuid(geteuid());
    printf("Effective user: %s\n", e_pwd->pw_name);

    system("/bin/cat /home/alice/share/foo.txt");
}
HEREDOC

$ gcc /usr/local/src/display-foo.c -o /usr/local/bin/display-foo

让我们设置的所有权和权限display-foo，包括设置 setuid 位：

chown alice:alice /usr/local/bin/display-foo
chmod u=rwx,g=rx,o=rx /usr/local/bin/display-foo
chmod u+s /usr/local/bin/display-foo

我们还检查一下结果：

$ ls -l /usr/local/bin/display-foo
-rwsr-xr-x 1 alice alice 8640 Nov  4 15:40 /usr/local/bin/display-foo

现在我们运行程序 asalice和 as bob：

$ sudo -u alice display-foo

Real user: alice
Effective user: alice
This is foo.txt

$ sudo -u bob display-foo

Real user: bob
Effective user: alice
This is foo.txt

正如您所看到的，看起来一切都按预期进行。

Answer