NAME¶

avc_init - legacy userspace SELinux AVC setup

SYNOPSIS¶

#include <selinux/selinux.h>
#include <selinux/avc.h>

int avc_init(const char *msgprefix,
const struct avc_memory_callback *mem_callbacks,
const struct avc_log_callback *log_callbacks,
const struct avc_thread_callback *thread_callbacks,
const struct avc_lock_callback *lock_callbacks);

DESCRIPTION¶

avc_init() is deprecated; please use avc_open(3) in conjunction with selinux_set_callback(3) in all new code.

avc_init() initializes the userspace AVC and must be called before any other AVC operation can be performed. A non-NULL msgprefix will be prepended to all audit messages produced by the userspace AVC. The default is `uavc'. The remaining arguments, if non-NULL, specify callbacks to be used by the userspace AVC.

CALLBACKS¶

The userspace AVC can be directed how to perform memory allocation, logging, thread creation, and locking via callback functions passed to avc_init(). The purpose of this functionality is to allow the userspace AVC to be smoothly integrated into existing userspace object managers.

Use an avc_memory_callback structure to specify alternate functions for dynamic memory allocation.

struct avc_memory_callback {
	void	*(*func_malloc)(size_t size);
	void	(*func_free)(void *ptr);
};

The two fields of the structure should be pointers to functions which behave as malloc(3) and free(3), which are used by default.

Use an avc_log_callback structure to specify alternate functions for logging.

struct avc_log_callback {
	void	(*func_log)(const char *fmt, ...);
	void	(*func_audit)(void *auditdata,
			security_class_t class,
			char *msgbuf, size_t msgbufsize);
};

The func_log callback should accept a printf(3) style format and arguments and log them as desired. The default behavior prints the message on the standard error. The func_audit callback should interpret the auditdata parameter for the given class, printing a human-readable interpretation to msgbuf using no more than msgbufsize characters. The default behavior is to ignore auditdata.

Use an avc_thread_callback structure to specify functions for starting and manipulating threads.

struct avc_thread_callback {
	void	*(*func_create_thread)(void (*run)(void));
	void	(*func_stop_thread)(void *thread);
};

The func_create_thread callback should create a new thread and return a pointer which references it. The thread should execute the run argument, which does not return under normal conditions. The func_stop_thread callback should cancel the running thread referenced by thread. By default, threading is not used; see KERNEL STATUS PAGE and NETLINK NOTIFICATION below.

Use an avc_lock_callback structure to specify functions to create, obtain, and release locks for use by threads.

struct avc_lock_callback {
	void	*(*func_alloc_lock)(void);
	void	(*func_get_lock)(void *lock);
	void	(*func_release_lock)(void *lock);
	void	(*func_free_lock)(void *lock);
};

The func_alloc_lock callback should create a new lock, returning a pointer which references it. The func_get_lock callback should obtain lock, blocking if necessary. The func_release_lock callback should release lock. The func_free_lock callback should destroy lock, freeing any resources associated with it. The default behavior is not to perform any locking. Note that undefined behavior may result if threading is used without appropriate locking.

KERNEL STATUS PAGE¶

Linux kernel version 2.6.37 supports the SELinux kernel status page, enabling userspace applications to mmap(2) SELinux status state in read-only mode to avoid system calls during the cache hit code path.

avc_init() calls selinux_status_open(3) to initialize the selinux status state. If successfully initialized, the userspace AVC will default to single-threaded mode and ignore the func_create_thread and func_stop_thread callbacks. All callbacks set via selinux_set_callback(3) will still be honored.

avc_has_perm(3) and selinux_check_access(3) both check for status updates through calls to selinux_status_updated(3) at the start of each permission query and take the appropriate action.

Two status types are currently implemented. setenforce events will change the effective enforcing state used within the AVC, and policyload events will result in a cache flush.

NETLINK NOTIFICATION¶

In the event that the kernel status page is not successfully mmap(2)'ed the AVC will default to the netlink fallback mechanism, which opens a netlink socket for receiving status updates. setenforce and policyload events will have the same results as for the status page implementation, but all status update checks will now require a system call.

By default, avc_open(3) does not set threading or locking callbacks. In the fallback case, the userspace AVC checks for new netlink messages at the start of each permission query. If threading and locking callbacks are passed to avc_init(), a dedicated thread will be started to listen on the netlink socket. This may increase performance in the absence of the status page and will ensure that log messages are generated immediately rather than at the time of the next permission query.

RETURN VALUE¶

Functions with a return value return zero on success. On error, -1 is returned and errno is set appropriately.

NOTES¶

The msgprefix argument to avc_init() currently has a length limit of 15 characters and will be truncated if necessary.

If a provided func_malloc callback does not set errno appropriately on error, userspace AVC calls may exhibit the same behavior.

If a netlink thread has been created and an error occurs on the socket (such as an access error), the thread may terminate and cause the userspace AVC to return EINVAL on all further permission checks until avc_destroy is called.

AUTHOR¶

Eamon Walsh <ewalsh@tycho.nsa.gov>

SEE ALSO¶

avc_open(3), selinux_status_open(3), selinux_status_updated(3), selinux_set_callback(3), selinux(8)