Linux汇编 系统调用setreuid

肯定不行啊，否则那不是一个漏洞么。

看man手册

Linux: Unprivileged users may only set the real user ID to the real

user ID or the effective user ID.

【1】setresuid

设置成功的条件：

（1）当前euid = 0 （也即root)

(2) 三个参数，没一个等于原来某个id中的一个

以上两个条件，满足一个，setresuid即可正常调用，执行，将进程的id设置为对应的ID.

举例：

（1) ruid = 100, euid = 0, suid = 300

setresuid(200,300,100) 可以成功执行。因为euid = 0 说明是root

（2）ruid = 100, euid = 300, suid = 200

setresuid(200,300,100) 可以成功执行，因为三个新的ID都是原来ID中的一个。

setresuid要么执行成功，所有的id设置成功，要么失败，所有的id设置失败。不能不分设置成功，部分失败。

【2】seteuid

无论什么情况，都只改变euid，不会改变ruid/suid

（1）如果euid = 0，那么新的euid随便设置，都可以成功改变。

（2）如果原来euid != 0 ，不同系统处理方式不一样：

Solaris和Linux只允许新的euid为原来三个ID中的一个。

FreeBSD只允许新的euid等于ruid和suid中的一个。（新的euid不能等于旧的euid)

【3】setreuid

会修改ruid/euid，某些情况下，也可以改变suid

不同的系统对setereuid有不同的处理方式：

Solaris和Linux中：setreuid(geteuid(), getuid())可以实现ruid和euid的交换。

FreeBSD会失败。

【4】setuid

（1) 如果原来的euid = 0，该函数将所有的id设置为新的id

比如：如果原来的id为: ruid = 300, euid = 0, suid = 100

那么setuid(200)后，ruid = 200, euid = 200, suid = 200

（2） 如果原来的euid != 0，但是新的id为原来ruid/suid中的一个，那么也是可以执行的，否则不能执行。

比如：原来三个id为： ruid = 100, euid = 200, suid = 300

那么setuid(300) 执行后： ruid = 100, euid = 300, suid = 300 也就是只改变了euid

setuid(400) 就不能执行。

结论：

（1）对于Linux如果euid != 0，那么新的id必须等于ruid/suid中的一个

(2) 如果euid = 0，三个id都将被设置为新的id，否则只设置euid

只有可以执行的二进制程序才可以设置setuid权限，并且命令执行者要对该程序拥有x权限。对于设定了setuid权限的命令来说，功能是命令执行者在执行该程序时获得该程序文件所有者的身份。setuid权限只在该程序执行过程中有效，也就是说身份改变只在程序执行过程中有效。

为了执行权限检查，传统的 UNIX 实现区分两种类型的进程：特权进程（其有效用户 ID 为0，称为超级用户或 root），和非特权用户（其有效 UID 非0）。特权进程绕过所有的内核权限检查，而非特权进程受基于进程的认证信息（通常是：有效 UID，有效 GID，和补充组列表）的完整权限检查的支配。

自内核 2.2 版本开始，Linux 将传统上与超级用户关联的特权分为几个单元，称为 capabilities （权能），它们可以被独立的启用或禁用。权能是每个线程的属性。

下面的列表展示了 Linux 上实现的权能，以及每种权能允许的 *** 作或行为：

权能的完整实现需要：

在内核 2.6.24 之前，只有前两个要求能够满足；自内核 2.6.24 开始，所有三个要求都能满足。

每个线程具有三个包含零个或多个上面的权能的权能集合：

A child created via fork(2) inherits copies of its parent's capability sets. See below for a discussion of the treatment of capabilities during execve(2).

Using capset(2), a thread may manipulate its own capability sets (see below).

Since Linux 3.2, the file /proc/sys/kernel/cap_last_cap exposes the numerical value of the highest capability supported by the running kernelthis can be used to determine the highest bit that may be set in a capability set.

Since kernel 2.6.24, the kernel supports associating capability sets with an executable file using setcap(8). The file capability sets are stored in an extended attribute (see setxattr(2)) named security.capability. Writing to this extended attribute requires the CAP_SETFCAP capability. The file capability sets, in conjunction with the capability sets of the thread, determine the capabilities of a thread after an execve(2).

The three file capability sets are:

During an execve(2), the kernel calculates the new capabilities of the process using the following algorithm:

其中：

A privileged file is one that has capabilities or has the set-user-ID or set-group-ID bit set.

In order to provide an all-powerful root using capability sets, during an execve(2):

The upshot of the above rules, combined with the capabilities transformations described above, is that when a process execve(2)s a set-user-ID-root program, or when a process with an effective UID of 0 execve(2)s a program, it gains all capabilities in its permitted and effective capability sets, except those masked out by the capability bounding set. This provides semantics that are the same as those provided by traditional UNIX systems.

The capability bounding set is a security mechanism that can be used to limit the capabilities that can be gained during an execve(2). The bounding set is used in the following ways:

Note that the bounding set masks the file permitted capabilities, but not the inherited capabilities. If a thread maintains a capability in its inherited set that is not in its bounding set, then it can still gain that capability in its permitted set by executing a file that has the capability in its inherited set.

Depending on the kernel version, the capability bounding set is either a system-wide attribute, or a per-process attribute.

In kernels before 2.6.25, the capability bounding set is a system-wide attribute that affects all threads on the system. The bounding set is accessible via the file /proc/sys/kernel/cap-bound. (Confusingly, this bit mask parameter is expressed as a signed decimal number in /proc/sys/kernel/capbound.)

Only the init process may set capabilities in the capability bounding setother than that, the superuser (more precisely: programs with the CAP_SYS_MODULE capability) may only clear capabilities from this set.

On a standard system the capability bounding set always masks out the CAP_SETPCAP capability. To remove this restriction (dangerous!), modify the definition of CAP_INIT_EFF_SET in include/linux/capability.h and rebuild the kernel.

The system-wide capability bounding set feature was added to Linux starting with kernel version 2.2.11.

From Linux 2.6.25, the capability bounding set is a per-thread attribute. (There is no longer a systemwide capability bounding set.)

The bounding set is inherited at fork(2) from the thread's parent, and is preserved across an execve(2).

A thread may remove capabilities from its capability bounding set using the prctl(2) PR_CAPBSET_DROP operation, provided it has the CAP_SETPCAP capability. Once a capability has been dropped from the bounding set, it cannot be restored to that set. A thread can determine if a capability is in its bounding set using the prctl(2) PR_CAPBSET_READ operation.

Removing capabilities from the bounding set is supported only if file capabilities are compiled into the kernel. In kernels before Linux 2.6.33, file capabilities were an optional feature configurable via the CONFIG_SECURITY_FILE_CAPABILITIES option. Since Linux 2.6.33, the configuration option has been removed and file capabilities are always part of the kernel. When file capabilities are compiled into the kernel, the init process (the ancestor of all processes) begins with a full bounding set. If file capabilities are not compiled into the kernel, then init begins with a full bounding set minus CAP_SETPCAP, because this capability has a different meaning when there are no file capabilities.

Removing a capability from the bounding set does not remove it from the thread's inherited set. However it does prevent the capability from being added back into the thread's inherited set in the future.

To preserve the traditional semantics for transitions between 0 and nonzero user IDs, the kernel makes the following changes to a thread's capability sets on changes to the thread's real, effective, saved set, and filesystem user IDs (using setuid(2), setresuid(2), or similar):

If a thread that has a 0 value for one or more of its user IDs wants to prevent its permitted capability set being cleared when it resets all of its user IDs to nonzero values, it can do so using the prctl(2) PR_SET_KEEPCAPS operation or the SECBIT_KEEP_CAPS securebits flag described below.

A thread can retrieve and change its capability sets using the capget(2) and capset(2) system calls. However, the use of cap_get_proc(3) and cap_set_proc(3), both provided in the libcap package, is preferred for this purpose. The following rules govern changes to the thread capability sets:

Starting with kernel 2.6.26, and with a kernel in which file capabilities are enabled, Linux implements a set of per-thread securebits flags that can be used to disable special handling of capabilities for UID 0 (root). These flags are as follows:

Each of the above "base" flags has a companion "locked" flag. Setting any of the "locked" flags is irreversible, and has the effect of preventing further changes to the corresponding "base" flag. The locked flags are: SECBIT_KEEP_CAPS_LOCKED, SECBIT_NO_SETUID_FIXUP_LOCKED, SECBIT_NOROOT_LOCKED, and SECBIT_NO_CAP_AMBIENT_RAISE.

The securebits flags can be modified and retrieved using the prctl(2) PR_SET_SECUREBITS and PR_GET_SECUREBITS operations. The CAP_SETPCAP capability is required to modify the flags.

The securebits flags are inherited by child processes. During an execve(2), all of the flags are preserved, except SECBIT_KEEP_CAPS which is always cleared.

An application can use the following call to lock itself, and all of its descendants, into an environment where the only way of gaining capabilities is by executing a program with associated file capabilities:

For a discussion of the interaction of capabilities and user namespaces, see user_namespaces(7).

No standards govern capabilities, but the Linux capability implementation is based on the withdrawn POSIX.1e draft standardsee ⟨ http://wt.tuxomania.net/publications/posix.1e/ ⟩.

From kernel 2.5.27 to kernel 2.6.26, capabilities were an optional kernel component, and can be enabled/disabled via the CONFIG_SECURITY_CAPABILITIES kernel configuration option.

The /proc/PID/task/TID/status file can be used to view the capability sets of a thread. The /proc/PID/status file shows the capability sets of a process's main thread. Before Linux 3.8, nonexistent capabilities were shown as being enabled (1) in these sets. Since Linux 3.8, all nonexistent capabilities (above CAP_LAST_CAP) are shown as disabled (0).

The libcap package provides a suite of routines for setting and getting capabilities that is more comfortable and less likely to change than the interface provided by capset(2) and capget(2). This package also provides the setcap(8) and getcap(8) programs. It can be found at ⟨ http://www.kernel.org/pub/linux/libs/security/linux-privs ⟩.

Before kernel 2.6.24, and from kernel 2.6.24 to kernel 2.6.32 if file capabilities are not enabled, a thread with the CAP_SETPCAP capability can manipulate the capabilities of threads other than itself. However, this is only theoretically possible, since no thread ever has CAP_SETPCAP in either of these cases:

capsh(1), setpriv(1), prctl(2), setfsuid(2), cap_clear(3), cap_copy_ext(3), cap_from_text(3), cap_get_file(3), cap_get_proc(3), cap_init(3), capgetp(3), capsetp(3), libcap(3), credentials(7), user_namespaces(7), pthreads(7), getcap(8), setcap(8)

include/linux/capability.h in the Linux kernel source tree

This page is part of release 4.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at http://www.kernel.org/doc/man-pages/ .

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