набор правил, загруженных в систему аудита ядра (a set of rules loaded in the kernel audit system)
Имя (Name)
audit.rules - a set of rules loaded in the kernel audit system
Описание (Description)
audit.rules is a file containing audit rules that will be loaded
by the audit daemon's init script whenever the daemon is started.
The auditctl program is used by the initscripts to perform this
operation. The syntax for the rules is essentially the same as
when typing in an auditctl command at a shell prompt except you
do not need to type the auditctl command name since that is
implied. The audit rules come in 3 varieties: control, file, and
syscall.
Control
Control commands generally involve configuring the audit system
rather than telling it what to watch for. These commands
typically include deleting all rules, setting the size of the
kernel's backlog queue, setting the failure mode, setting the
event rate limit, or to tell auditctl to ignore syntax errors in
the rules and continue loading. Generally, these rules are at the
top of the rules file.
File System
File System rules are sometimes called watches. These rules are
used to audit access to particular files or directories that you
may be interested in. If the path given in a watch rule is a
directory, then the rule used is recursive to the bottom of the
directory tree excluding any directories that may be mount
points. The syntax of these watch rules generally follow this
format:
-w path-to-file -p permissions -k keyname
where the permission are any one of the following:
r - read of the file
w - write to the file
x - execute the file
a - change in the file's attribute
Watches can also be created using the syscall format described
below which allow for greater flexibility and options. Using
syscall rules you can choose between path and dir which is
against a specific inode or directory tree respectively. It
should also be noted that the recursive directory watch will stop
if there is a mount point below the parent directory. There is an
option to make the mounted subdirectory equivalent by using a -q
rule.
System Call
The system call rules are loaded into a matching engine that
intercepts each syscall that all programs on the system makes.
Therefore it is very important to only use syscall rules when you
have to since these affect performance. The more rules, the
bigger the performance hit. You can help the performance, though,
by combining syscalls into one rule whenever possible.
The Linux kernel has 4 rule matching lists or filters as they are
sometimes called. They are: task, exit, user, and exclude. The
task list is checked only during the fork or clone syscalls. It
is rarely used in practice.
The exit filter is the place where all syscall and file system
audit requests are evaluated.
The user filter is used to filter (remove) some events that
originate in user space. By default, any event originating in
user space is allowed. So, if there are some events that you do
not want to see, then this is a place where some can be removed.
See auditctl(8) for fields that are valid.
The exclude filter is used to exclude certain events from being
emitted. The msgtype and a number of subject attribute fields can
be used to tell the kernel which message types you do not want to
record. This filter can remove the event as a whole and is not
selective about any other attribute. The user and exit filters
are better suited to selectively auditing events. The action is
ignored for this filter, defaulting to "never".
Syscall rules take the general form of:
-a action,list -S syscall -F field=value -k keyname
The -a option tells the kernel's rule matching engine that we
want to append a rule at the end of the rule list. But we need to
specify which rule list it goes on and what action to take when
it triggers. Valid actions are:
always - always create an event
never - never create an event
The action and list are separated by a comma but no space in
between. Valid lists are: task, exit, user, and exclude. Their
meaning was explained earlier.
Next in the rule would normally be the -S option. This field can
either be the syscall name or number. For readability, the name
is almost always used. You may give more than one syscall in a
rule by specifying another -S option. When sent into the kernel,
all syscall fields are put into a mask so that one compare can
determine if the syscall is of interest. So, adding multiple
syscalls in one rule is very efficient. When you specify a
syscall name, auditctl will look up the name and get its syscall
number. This leads to some problems on bi-arch machines. The 32
and 64 bit syscall numbers sometimes, but not always, line up.
So, to solve this problem, you would generally need to break the
rule into 2 with one specifying -F arch=b32 and the other
specifying -F arch=b64. This needs to go in front of the -S
option so that auditctl looks at the right lookup table when
returning the number.
After the syscall is specified, you would normally have one or
more -F options that fine tune what to match against. Rather than
list all the valid field types here, the reader should look at
the auditctl man page which has a full listing of each field and
what it means. But it's worth mentioning a couple things.
The audit system considers uids to be unsigned numbers. The audit
system uses the number -1 to indicate that a loginuid is not set.
This means that when it's printed out, it looks like 4294967295.
But when you write rules, you can use either "unset" which is
easy to remember, or -1, or 4294967295. They are all equivalent.
If you write a rule that you wanted try to get the valid users of
the system, you need to look in /etc/login.defs to see where user
accounts start. For example, if UID_MIN is 1000, then you would
also need to take into account that the unsigned representation
of -1 is higher than 500. So you would address this with the
following piece of a rule:
-F auid>=1000 -F auid!=unset
These individual checks are "anded" and both have to be true.
The last thing to know about syscall rules is that you can add a
key field which is a free form text string that you want inserted
into the event to help identify its meaning. This is discussed in
more detail in the NOTES section.
Примечание (Note)
The purpose of auditing is to be able to do an investigation
periodically or whenever an incident occurs. A few simple steps
in planning up front will make this job easier. The best advice
is to use keys in both the watches and system call rules to give
the rule a meaning. If rules are related or together meet a
specific requirement, then give them a common key name. You can
use this during your investigation to select only results with a
specific meaning.
When doing an investigation, you would normally start off with
the main aureport output to just get an idea about what is
happening on the system. This report mostly tells you about
events that are hard coded by the audit system such as login/out,
uses of authentication, system anomalies, how many users have
been on the machine, and if SE Linux has detected any AVCs.
aureport --start this-week
After looking at the report, you probably want to get a second
view about what rules you loaded that have been triggering. This
is where keys become important. You would generally run the key
summary report like this:
aureport --start this-week --key --summary
This will give an ordered listing of the keys associated with
rules that have been triggering. If, for example, you had a
syscall audit rule that triggered on the failure to open files
with EPERM that had a key field of access like this:
-a always,exit -F arch=b64 -S open -S openat -F exit=-EPERM -k access
Then you can isolate these failures with ausearch and pipe the
results to aureport for display. Suppose your investigation
noticed a lot of the access denied events. If you wanted to see
the files that unauthorized access has been attempted, you could
run the following command:
ausearch --start this-week -k access --raw | aureport --file --summary
This will give an ordered list showing which files are being
accessed with the EPERM failure. Suppose you wanted to see which
users might be having failed access, you would run the following
command:
ausearch --start this-week -k access --raw | aureport --user --summary
If your investigation showed a lot of failed accesses to a
particular file, you could run the following report to see who is
doing it:
ausearch --start this-week -k access -f /path-to/file --raw |
aureport --user -i
This report will give you the individual access attempts by
person. If you needed to see the actual audit event that is being
reported, you would look at the date, time, and event columns.
Assuming the event was 822 and it occurred at 2:30 on 09/01/2009
and you use the en_US.utf8 locale, the command would look
something like this:
ausearch --start 09/01/2009 02:30 -a 822 -i --just-one
This will select the first event from that day and time with the
matching event id and interpret the numeric values into human
readable values.
The most important step in being able to do this kind of analysis
is setting up key fields when the rules were originally written.
It should also be pointed out that you can have more than one key
field associated with any given rule.
Исправление проблем (Troubleshooting)
If you are not getting events on syscall rules that you think you
should, try running a test program under strace so that you can
see the syscalls. There is a chance that you might have
identified the wrong syscall.
If you get a warning from auditctl saying, "32/64 bit syscall
mismatch in line XX, you should specify an arch". This means that
you specified a syscall rule on a bi-arch system where the
syscall has a different syscall number for the 32 and 64 bit
interfaces. This means that on one of those interfaces you are
likely auditing the wrong syscall. To solve the problem, re-write
the rule as two rules specifying the intended arch for each rule.
For example,
-a always,exit -S openat -k access
would be rewritten as
-a always,exit -F arch=b32 -S openat -k access
-a always,exit -F arch=b64 -S openat -k access
If you get a warning that says, "entry rules deprecated, changing
to exit rule". This means that you have a rule intended for the
entry filter, but that filter is no longer available. Auditctl
moved your rule to the exit filter so that it's not lost. But to
solve this so that you do not get the warning any more, you need
to change the offending rule from entry to exit.
Примеры (Examples)
The following rule shows how to audit failed access to files due
to permission problems. Note that it takes two rules for each
arch ABI to audit this since file access can fail with two
different failure codes indicating permission problems.
-a always,exit -F arch=b32 -S open -S openat -F exit=-EACCES -k access
-a always,exit -F arch=b32 -S open -S openat -F exit=-EPERM -k access
-a always,exit -F arch=b64 -S open -S openat -F exit=-EACCES -k access
-a always,exit -F arch=b64 -S open -S openat -F exit=-EPERM -k access
HARD WIRED EVENTS
If auditing is enabled, then you can get any event that is not
caused by syscall or file watch rules (because you don't have any
rules loaded). So, that means, any event from 1100-1299, 1326,
1328, 1331 and higher can be emitted. The reason that there are a
number of events that are hardwired is because they are required
by regulatory compliance and are sent automatically as a
convenience. (For example, logon/logoff is a mandatory event in
all security guidance.) If you don't want this, you can use the
exclude filter to drop events that you do not want.
-a always,exclude -F msgtype=CRED_REFR
Смотри также (See also)
auditctl(8), auditd(8).
