Implementing audit rules (/etc/audit/audit.rules) effectively

[Jan Lieskovsky]

Hello folks,

within the effort to provide an implementation for some task
implying from my daily job recently I started to face the following
question related with auditd - how to write audit rules in most
effective way. I am mainly interested if there's some comparison / research
wrt to if there's is some performance penalty when (syscall, but
in general case doesn't need to be limited to syscall calls) audit
rules are created in the way having just one syscall rule (one -S argument
is provided per audit rule) versus the case when there are more
(compatible) -S arguments provided simultaneously in the particular
audit.rules row?

To provide an example, let's suppose the *chown category of rules:
* the "all-in-one" case:

-a always,exit -F arch=b32 -S chown -S fchown -S fchownat -S lchown -F auid>=500 -F auid!=4294967295 -k perm_mod

vs

* the "one-rule-per-one-row" case:

-a always,exit -F arch=b32 -S chown -F auid>=500 -F auid!=4294967295 -k perm_mod
-a always,exit -F arch=b32 -S fchown -F auid>=500 -F auid!=4294967295 -k perm_mod
-a always,exit -F arch=b32 -S fchownat -F auid>=500 -F auid!=4294967295 -k perm_mod
-a always,exit -F arch=b32 -S lchown -F auid>=500 -F auid!=4294967295 -k perm_mod

Does the fact how the -S arguments are layered across the /etc/audit/audit.rules
file (IOW if being provided within one row or spread within multiple rows) have
some (negative) impact on the audit system's efficiency? [*] If so, is there some
way how to measure the performance penalty in the second case?

Thank you for your time & possible hints in advance.

Regards, Jan.
--
Jan iankko Lieskovsky / Red Hat Security Technologies Team

[*] Not familiar with audit internals, but translating the request to log / audit
the event of particular system call occurrence from the Linux kernel PoV's
it looks this might correspond to the problem of searching for a key / value
in the hash table (having particular system call occurred, insert new entry
under particular hash table's field taking -k keyname argument as the hash
table key). If this analogy is at least a bit appropriate, the all -S's arguments
case above would correspond to a hash table having just one value for each key,
while separating the desired -S arguments into multiple rows would mean to
have a hash table where one key (-k keyname) would have bucket containing
multiple values (e.g. array of them). In this case to locate the particular
value would mean to locate the bucket in the hash table & then subsequently
yet locate the proper value in that array of items (which seems to be more
time / operation expensive than the case of one rule having multiple -S arguments).
Thus could audit upstream clarify, if there's some performance penalty in
the case of multiple -S being split / spread across multiple rules?

[Steve Grubb]

Post by Jan Lieskovsky
Hello folks,
within the effort to provide an implementation for some task
implying from my daily job recently I started to face the following
question related with auditd - how to write audit rules in most
effective way. I am mainly interested if there's some comparison / research
wrt to if there's is some performance penalty when (syscall, but
in general case doesn't need to be limited to syscall calls) audit
rules are created in the way having just one syscall rule (one -S argument
is provided per audit rule) versus the case when there are more
(compatible) -S arguments provided simultaneously in the particular
audit.rules row?

Yes there has. The answer is combine as many syscalls as possible into each
rule. To see why, look at this code:

http://lxr.free-electrons.com/source/kernel/auditsc.c#L747

Basically you have a for loop over each rule and on line 765 it checks by
"anding" the syscalls in the rule. If no match iterate again. So, by
increasing the rules, you increase the iterating for each and every syscall
made whether its of interest or not.

Post by Jan Lieskovsky
-a always,exit -F arch=b32 -S chown -S fchown -S fchownat -S lchown -F
auid>=500 -F auid!=4294967295 -k perm_mod
vs

The first is the recommended format and that is also the way that all sample
rules, such as the stig.rules, is written.

Post by Jan Lieskovsky
-a always,exit -F arch=b32 -S chown -F auid>=500 -F auid!=4294967295 -k
perm_mod -a always,exit -F arch=b32 -S fchown -F auid>=500 -F
auid!=4294967295 -k perm_mod -a always,exit -F arch=b32 -S fchownat -F
auid>=500 -F auid!=4294967295 -k perm_mod -a always,exit -F arch=b32 -S
lchown -F auid>=500 -F auid!=4294967295 -k perm_mod
Does the fact how the -S arguments are layered across the
/etc/audit/audit.rules file (IOW if being provided within one row or spread
within multiple rows) have some (negative) impact on the audit system's
efficiency? [*] If so, is there some way how to measure the performance
penalty in the second case?

Yes. We have done it in the past to come up with the current recommendation.

-Steve