如果程序设置了setuid位，为什么/ proc / self中的文件最终以root身份拥有？

Question

我有这个小程序：

#define _GNU_SOURCE
#include <unistd.h>
#include <stdio.h>
#include <sys/prctl.h>

extern char **environ;

int main()
{
   char * const arglist[] = { "/bin/ls", "-l", "/proc/self/maps", NULL };
   uid_t uid, euid, suid;
   gid_t gid, egid, sgid;

   getresuid(&uid, &euid, &suid);
   printf("Before: uid: %u, euid: %u, suid: %u\n", uid, euid, suid);
   uid = euid;
   setresuid(uid, euid, suid);
   getresuid(&uid, &euid, &suid);
   printf(" After: uid: %u, euid: %u, suid: %u\n", uid, euid, suid);

   getresgid(&gid, &egid, &sgid);
   printf("Before: gid: %u, egid: %u, sgid: %u\n", gid, egid, sgid);
   gid = egid;
   setresuid(gid, egid, sgid);
   getresuid(&gid, &egid, &sgid);
   printf(" After: gid: %u, egid: %u, sgid: %u\n", gid, egid, sgid);

   printf("Get result == %d\n", prctl(PR_GET_DUMPABLE, 0, 0, 0, 0));
   printf("Set result == %d\n", prctl(PR_SET_DUMPABLE, 1, 0, 0, 0));
   printf("Get result == %d\n", prctl(PR_GET_DUMPABLE, 0, 0, 0, 0));

   if (fork())
   {
      return 0;
   }
   execve(arglist[0], arglist, environ);
}

我将此程序编译为一个名为small-test的可执行文件，并将其所有权更改为测试用户：

[omnifarious@foohost ~]$ ls -l small-test
-rwxrwxr-x. 1 testing testing 8512 Oct 23 12:55 small-test

然后我运行程序：

[omnifarious@foohost ~]$ ./small-test 
Before: uid: 1001, euid: 1001, suid: 1001
 After: uid: 1001, euid: 1001, suid: 1001
Before: gid: 1001, egid: 1001, sgid: 1001
 After: gid: 1001, egid: 1001, sgid: 1001
Get result == 1
Set result == 0
Get result == 1
-r--r--r--. 1 hopper hopper 0 Oct 23 14:50 /proc/self/maps

到现在为止还挺好。 然后我这样做：

[omnifarious@foohost ~]$ sudo chmod ug+s ./small-test
[omnifarious@foohost ~]$ ls -l ./small-test
-rwsrwsr-x. 1 testing testing 8512 Oct 23 12:55 ./small-test
[omnifarious@foohost ~]$ ./small-test 
Before: uid: 1001, euid: 1002, suid: 1002
 After: uid: 1002, euid: 1002, suid: 1002
Before: gid: 1001, egid: 1002, sgid: 1002
 After: gid: 1002, egid: 1002, sgid: 1002
Get result == 0
Set result == 0
Get result == 1
-r--r--r--. 1 root root 0 Oct 23 12:59 /proc/self/maps

为什么/proc/self/maps最终归root而不是经过testing或omnifarious ？ 请注意，如果我卸下fork ，结果不会改变。

这让我感到烦恼的原因是，我需要创建一个程序，该程序将自己作为用户（而不是执行该程序的用户）放入一个命名空间中。 这样一来，我无权访问cgroup和启动程序的用户所拥有的其他东西。 但是不允许我写入程序的uid_map或gid_map ，因此无法正确设置名称空间。

注意：我编辑了这个问题，以包含对prctl的调用，以设置（和读取） DUMPABLE标志作为答案（和手册），指示重置该问题应将所有者固定在/proc/self/*文件上。 正如您在新程序中看到的那样，没有。

编辑：上面的程序有一个错误，其中它正在调用setresuid而不是setresgid 。 即使将调用添加到prctl之后，这也是导致我的问题的原因。 prctl(PR_SET_DUMPABLE, 1); 如果进程的实际和有效组和用户ID不相同，则调用无效。

Answer 1

出于安全原因，任何suid进程都将默认将其/proc/self目录归root拥有（以防止用户进行核心转储并检查其内存中是否包含有价值的信息）。

您可以通过使用prctl PR_SET_DUMPABLE手动使进程转储为suid来设置所有者。

---

这是proc(5) ，其中包含正在发生的事情以及如何影响它的描述：

   /proc/[pid]
          There is a numerical subdirectory for each running
          process; the subdirectory is named by the  process
          ID.

          Each /proc/[pid] subdirectory contains the pseudo-
          files  and  directories  described  below.   These
          files are normally owned by the effective user and
          effective group ID of the process.  However, as  a
          security  measure, the ownership is made root:root
          if the process's "dumpable" attribute is set to  a
          value other than 1.  This attribute may change for
          the following reasons:

          *  The  attribute  was  explicitly  set  via   the
             prctl(2) PR_SET_DUMPABLE operation.

          *  The  attribute  was  reset  to the value in the
             file   /proc/sys/fs/suid_dumpable    (described
             below), for the reasons described in prctl(2).

          Resetting  the  "dumpable"  attribute to 1 reverts
          the ownership of the /proc/[pid]/*  files  to  the
          process's real UID and real GID.

在下面的内容中， suid_dumpable可以说明默认值为何：

          1 ("debug")
                 All  processes  dump  core  when  possible.
                 (Reasons  why  a process might nevertheless
                 not dump core are  described  in  core(5).)
                 The  core  dump  is owned by the filesystem
                 user ID of the dumping process and no secu‐
                 rity is applied.  This is intended for sys‐
                 tem debugging situations only: this mode is
                 insecure  because  it  allows  unprivileged
                 users to examine  the  memory  contents  of
                 privileged processes.

另外， prctl(2)列出了影响可转储性的非suid情况：

   PR_SET_DUMPABLE (since Linux 2.3.20)
          (...)
          Normally,  this  flag is set to 1.  However, it is
          reset to the current value contained in  the  file
          /proc/sys/fs/suid_dumpable  (which  by default has
          the value 0), in the following circumstances:

          *  The process's effective user  or  group  ID  is
             changed.

          *  The  process's  filesystem  user or group ID is
             changed (see credentials(7)).

          *  The process executes (execve(2)) a  set-user-ID
             or  set-group-ID program, resulting in a change
             of either the effective user ID or  the  effec‐
             tive group ID.
          (...)