AppArmor 如何进行“环境清理”？
AppArmor 文档提到让应用程序能够在有或没有环境清理的情况下执行其他程序。显然，清理的环境更安全，但文档似乎没有具体说明环境清理是如何发生的。
什么是环境清理以及 AppArmor 如何清理环境？
The AppArmor documentation mentions giving applications the ability to execute other programs with or without enviroment scrubbing. Apparently a scrubbed environment is more secure, but the documentation doesn't seem to specify exactly how environment scrubbing happens.
What is environment scrubbing and what does AppArmor do to scrub the environment?
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“环境清理”是指删除各种“危险”环境变量，这些变量可能会影响二进制文件的行为 - 例如，
LD_PRELOAD可用于使动态链接器拉入代码，从而可以对程序的运行进行实质上任意的更改；可以设置一些变量以导致跟踪输出到具有众所周知名称的文件； 作为一种安全措施，这种清理通常是针对 setuid/setgid 二进制文件执行的，但内核提供了一个挂钩，允许安全模块也为任意其他二进制文件启用它。
内核的 ELF 加载器代码 使用此钩子 设置传递给二进制文件的信息“辅助向量”中的
AT_SECURE条目。 （请参阅此处和此处了解实现AppArmor 代码中此挂钩的一部分。）当在用户空间中开始执行时，动态链接器获取此值并使用它来设置
__libc_en able_secure标志；您将看到同一例程还包含为 setuid/setgid 二进制文件设置此标志的代码。 （有 静态链接的二进制文件在其他地方的等效代码。）__libc_enable_secure影响 动态链接器代码主体，并导致 列表要删除的特定环境变量。"Environment scrubbing" is the removal of various "dangerous" environment variables which may be used to affect the behaviour of a binary - for example,
LD_PRELOADcan be used to make the dynamic linker pull in code which can make essentially arbitrary changes to the running of a program; some variables can be set to cause trace output to files with well-known names; etc.This scrubbing is normally performed for setuid/setgid binaries as a security measure, but the kernel provides a hook to allow security modules to enable it for arbitrary other binaries as well.
The kernel's ELF loader code uses this hook to set the
AT_SECUREentry in the "auxiliary vector" of information which is passed to the binary. (See here and here for the implementation of this hook in the AppArmor code.)As execution starts in userspace, the dynamic linker picks up this value and uses it to set the
__libc_enable_secureflag; you'll see that the same routine also contains the code which sets this flag for setuid/setgid binaries. (There is equivalent code elsewhere for binaries which are statically linked.)__libc_enable_secureaffects a number of places in the main body of the dynamic linker code, and causes a list of specific environment variables to be removed.