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Commit bc2a8f418e ("libselinux: add selinux_status_* interfaces for
/selinux/status") introduced the sestatus mechanism, which allows for
mmap()'ing of the kernel status page as a replacement for avc_netlink.
The mechanism was initially intended for userspace object managers that
were calculating access decisions within their application and did not
rely on the libselinux AVC implementation. In order to properly make use
of sestatus within avc_has_perm(), the status mechanism needs to
properly set avc internals during status events; else, avc_enforcing is
never updated upon sestatus changes.
This commit gets rid of the default avc_netlink_open() in
avc_init_internal(), replacing it with selinux_status_open(). In the
event that the kernel status page cannot be mapped, the netlink fallback
will be used. By default, avc_has_perm_noaudit() and
selinux_check_access() will now attempt to read the kernel status page,
which removes a system call from two critical code paths.
Since the AVC thread create/stop callbacks were intended to avoid a
system call in the critical code path, they no longer need to be created
by default. In the event that the kernel status page is successfully
mapped, threads will not be created. Threads will still be
created/stopped for the sestatus fallback codepaths.
Userspace object managers that still need a netlink socket can call
avc_netlink_acquire_fd() to open and/or obtain one.
Update the manpage to reflect the new avc_netlink_acquire_fd()
functionality.
Signed-off-by: Mike Palmiotto <mike.palmiotto@crunchydata.com>
Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
106 lines
3.7 KiB
Groff
106 lines
3.7 KiB
Groff
.TH "selinux_status_open" "3" "22 January 2011" "kaigai@ak.jp.nec.com" "SELinux API documentation"
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.SH "NAME"
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selinux_status_open, selinux_status_close, selinux_status_updated,
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selinux_status_getenforce, selinux_status_policyload and
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selinux_status_deny_unknown \- reference the SELinux kernel status
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without invocation of system calls
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.
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.SH "SYNOPSIS"
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.B #include <selinux/avc.h>
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.sp
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.BI "int selinux_status_open(int " fallback ");"
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.sp
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.BI "void selinux_status_close(void);"
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.sp
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.BI "int selinux_status_updated(void);"
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.sp
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.BI "int selinux_status_getenforce(void);"
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.sp
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.BI "int selinux_status_policyload(void);"
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.sp
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.BI "int selinux_status_deny_unknown(void);"
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.
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.SH "DESCRIPTION"
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Linux 2.6.37 or later provides a SELinux kernel status page; being mostly
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placed on
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.I /sys/fs/selinux/status
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entry. It enables userspace applications to mmap this page with read-only
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mode, then it informs some status without system call invocations.
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.sp
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In some cases that a userspace application tries to apply heavy frequent
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access control; such as row-level security in databases, it will face
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unignorable cost to communicate with kernel space to check invalidation
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of userspace avc.
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.sp
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These functions provides applications a way to know some kernel events
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without system-call invocation or worker thread for monitoring.
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.sp
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.BR selinux_status_open ()
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tries to
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.BR open (2)
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.I /sys/fs/selinux/status
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and
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.BR mmap (2)
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it in read-only mode. The file-descriptor and pointer to the page shall
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be stored internally; Don't touch them directly.
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Set 1 on the
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.I fallback
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argument to handle a case of older kernels without kernel status page support.
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In this case, this function tries to open a netlink socket using
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.BR avc_netlink_open (3)
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and overwrite corresponding callbacks (setenforce and policyload).
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Thus, we need to pay attention to the interaction with these interfaces,
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when fallback mode is enabled.
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.sp
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.BR selinux_status_close ()
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unmap the kernel status page and close its file descriptor, or close the
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netlink socket if fallbacked.
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.sp
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.BR selinux_status_updated ()
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processes status update events. There are two kinds of status updates.
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.B setenforce
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events will change the effective enforcing state used within the AVC, and
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.B policyload
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events will result in a cache flush.
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This function returns 0 if there have been no updates since the last call,
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1 if there have been updates since the last call, or \-1 on error.
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.sp
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.BR selinux_status_getenforce ()
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returns 0 if SELinux is running in permissive mode, 1 if enforcing mode,
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or \-1 on error.
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Same as
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.BR security_getenforce (3)
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except with or without system call invocation.
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.sp
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.BR selinux_status_policyload ()
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returns times of policy reloaded on the running system, or \-1 on error.
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Note that it is not a reliable value on fallback-mode until it receive
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the first event message via netlink socket.
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Thus, don't use this value to know actual times of policy reloaded.
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.sp
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.BR selinux_status_deny_unknown ()
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returns 0 if SELinux treats policy queries on undefined object classes or
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permissions as being allowed, 1 if such queries are denied, or \-1 on error.
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.sp
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Also note that these interfaces are not thread-safe, so you have to protect
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them from concurrent calls using exclusive locks when multiple threads are
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performing.
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.
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.SH "RETURN VALUE"
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.BR selinux_status_open ()
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returns 0 or 1 on success. 1 means we are ready to use these interfaces,
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but netlink socket was opened as fallback instead of the kernel status page.
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On error, \-1 shall be returned.
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.sp
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Any other functions with a return value shall return its characteristic
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value as described above, or \-1 on errors.
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.
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.SH "SEE ALSO"
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.ad l
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.nh
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.BR mmap (2),
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.BR avc_netlink_open (3),
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.BR security_getenforce (3),
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.BR security_deny_unknown (3)
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