Managing Privilege in Java

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122 Managing Privilege
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Mistakes in privilege management are often errors of omission That is, programs that should be managing their privilege fail to do so Even when a program does make an active attempt to adhere to least privilege, confusing APIs and unexpected cross-platform behavior could stymie its efforts In this section, we talk about how programs can alter their privileges and some of the mistakes that occur when they do Putting Least Privilege into Practice Privileges on UNIX and Linux systems are primarily controlled by the user ID model, which governs access control decisions Each UNIX process has three user IDs: The real user ID (real uid, or ruid) corresponds to the user who created the process The effective user ID (effective uid, or euid) is used to make access control decisions The saved user ID (saved uid, or suid) holds an inactive uid that is recoverable
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12 Privileged Programs
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Each process also has three group IDs: The real group ID (real gid, or rgid) The effective group ID (effective gid, or egid) The saved group ID (saved gid, or sgid) Because group IDs impact privileges and are managed in much the same way as user IDs, we focus on user IDs and exclude group IDs from the examples in this chapter in favor of simplicity In addition to the three user IDs and three group IDs mentioned earlier, Linux processes have an fsuid and an fsgid, which are used for access control decisions on filesystem resources and permit processes to distinguish between the privileges used to access filesystem resources and other privileged operations These values typically remain synchronized with the euid and egid, and usually play an important role in privilege management only when they are mishandled Privileges on UNIX systems begins with the user login shell The login program begins running as root After it authenticates the user, its job is to launch the user s login shell with the appropriate user and group IDs Because non-setuid processes inherit the user and group IDs of the process that invokes them, login must alter its own user and group IDs to those of the user before it invokes the user s login shell UNIX systems provide a family of functions suffixed with uid and gid for altering the user ID and group ID a program uses The complex behavior of these functions is not fully standardized across platforms and has led to a variety of bugs related to privilege management [Chen, 2002] The rest of this section deals with the correct use of privilege management functions on UNIX systems Table 121 lists four common privilege management functions, along with a brief description of their semantics from the corresponding Linux man page descriptions [Linux, 2005] Although seteuid(), setuid(), and setreuid() are loosely defined by the POSIX standard and are available on all compliant platforms, much of their functionality is left up to individual implementations This leads to a variety of common errors and misuses that we discuss later in this section Our favorite privilege management function, setresuid(), offers clear semantics and is available on many on modern Linux and UNIX distributions, but is notably lacking on Solaris Because of platform constraints, setresuid() should be avoided in code that needs to be highly portable With all these choices and subtleties in behavior, it's not surprising that problems related to privilege management are so common
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122 Managing Privilege
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Table 121 Function prototypes and descriptions for Linux privilege management functions Function Prototype
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int setuid(uid_t uid)
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Description Sets the effective user ID of the current process If the effective UID of the caller is root, the real UID and saved set-user-ID are also set Sets the effective user ID of the current process Unprivileged user processes may only set the effective user ID to the real user ID, the effective user ID, or the saved set-user-ID Sets the real and effective user IDs of the current process If the real user ID is set or the effective user ID is set to a value not equal to the previous real user ID, the saved user ID will be set to the new effective user ID Supplying a value of -1 for either the real or effective user ID forces the system to leave that ID unchanged Unprivileged processes may only set the effective user ID to the real user ID, the effective user ID, or the saved set-user-ID Sets the real user ID, the effective user ID, and the saved set-user-ID of the current process Supplying a value of -1 for either the real or effective user ID forces the system to leave that ID unchanged Unprivileged user processes may change the real UID, effective UID, and saved set-user-ID, each to one of: the current real UID, the current effective UID or the current saved set-user-ID
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int seteuid(uid_t euid)
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int setreuid(uid_t ruid, uid_t euid)
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int setresuid(uid_t ruid, uid_t euid, uid_t suid)
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On platforms where it is defined, we prefer setresuid() for managing privileges because it provides the simplest and most well-defined behavior The function requires that the programmer explicitly state which of the three individual user IDs should be modified and guarantees that the call will have an all-or-nothing effect: If any of the provided user IDs are changed, all of them are changed On Solaris and other platforms where setresuid() is not available, use seteuid() to make changes that alter only the effective user ID and use setreuid() for changes that affect all three user IDs Example 121 shows how a simple privileged program might be structured to temporarily drop privileges, reacquire them to perform a privileged operation, and then drop them permanently when they are no longer needed
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