Runtime Security

Secure processes have some restrictions applied to the evaluation of their dependencies and runpaths to prevent malicious dependency substitution or symbol interposition.

The runtime linker categorizes a process as secure if the issetugid(2) system call returns true for the process.

For 32-bit objects, the default trusted directories that are known to the runtime linker are /lib/secure and /usr/lib/secure. For 64-bit objects, the default trusted directories that are known to the runtime linker are /lib/secure/64 and /usr/lib/secure/64. The utility crle(1) can be used to specify additional trusted directories that are applicable for secure applications. Administrators who use this technique should ensure that the target directories are suitably protected from malicious intrusion.

If an LD_LIBRARY_PATH family environment variable is in effect for a secure process, only the trusted directories specified by this variable are used to augment the runtime linker search rules. See Directories Searched by the Runtime Linker.

In a secure process, any runpath specifications provided by the application or any of its dependencies are used. However, the runpath must be a full path name, that is, the path name must start with a '/'.

In a secure process, the expansion of the $ORIGIN string is allowed only if the string expands to a trusted directory. See Security in Coded Processes. However, should a $ORIGIN expansion match a directory that has already provided dependencies, then the directory is implicitly secure. This directory can be used to provide additional dependencies.

In a secure process, LD_CONFIG is ignored. However, a configuration file that is recorded in a secure application is used. See the -c option of ld(1). A recorded configuration file must be a full path name, that is, the path name starts with a '/'. A recorded configuration file that employs the $ORIGIN string is restricted to known trusted directories. Developers who record a configuration file within a secure application should ensure that the configuration file directory is suitably protected from malicious intrusion. In the absence of a recorded configuration file, a secure process uses the default configuration file, if the configuration file exists. See crle(1).

In a secure process, LD_SIGNAL is ignored.

Additional objects can be loaded with a secure process using the LD_PRELOAD or LD_AUDIT environment variables. These objects must be specified as full path names or simple file names. Full path names are restricted to known trusted directories. Simple file names, in which no '/' appears in the name, are located subject to the search path restrictions previously described. Simple file names resolve only to known trusted directories.

In a secure process, any dependencies that consist of simple file names are processed using the path name restrictions previously described. Dependencies expressed as full path names or relative path names are used as is. Therefore, the developer of a secure process should ensure that the target directory referenced as one of these dependencies is suitably protected from malicious intrusion.

When creating a secure process, do not use relative path names to express dependencies or to construct dlopen(3C) path names. This restriction applies to the application and to all dependencies.