Re: Linux Audit Syscall Monitoring Error
Based on the performance enhancements it would be preferable to use syscall monitors to
monitor the directory.
But the limitation of having the rule not load, as well as any following rules not load
when the directory is not present is effectively a non-start.
The downside is -w style rules are deprecated and while they appear to support -F style
options they aren’t explicitly supported.
So we are left with a situation where we either try and pursue -s style, which can lead to
rules breaking, or -w style, which is legacy, and we are using -w style in a way that it
was not intended to be used, per the docs. I haven’t had time to read the code on this
specific implementation.
Ideally, -s style does a dir check instead of failing to load the rule+ rules following.
- Michael
On Tue, July 23 2024 at 4:15 PM Steve Grubb <sgrubb(a)redhat.com> wrote:
Hello,
On Tuesday, July 23, 2024 9:57:39 AM EDT Michael McKinley wrote:
Auditctl version 3.0.7
Kernel 6.5.0-44-generic.
ok
That extra check on if the directory exists may be the core
difference.
What is interesting is the -w version seems to honor the rule filter given
(-F exe!=/usr/bin/example).
I looked at the watch code. It appears to go back to at least 2008. You can
use whichever notation works for your problem. But the -w rule syntax is
deprecated. All watches in the sample rules have been switched over quite
some time ago.
That said, a watch is set for all syscalls. Starting in 3.1.3, the -F style
rules target specific syscalls based on the perm option. The impact of an
'all' rule is small but measurable across the whole system and this is why
perms are now used to select the syscalls. It makes the whole system run
faster.
For example, with a -w rule, if any program makes a call to read, the kernel
rule matching engine will have to check this rule to see if it matches.
Whereas with the -F style of rule, read is not chosen so this rule is
skipped.
-Steve
On Mon, July 22 2024 at 6:06 PM Steve Grubb
<sgrubb(a)redhat.com>
wrote:
Hello,
On Monday, July 22, 2024 4:00:37 PM EDT Michael McKinley via Linux-audit
wrote:
> I believe I’ve come across a bug in Linux audit when writing syscall
> monitors for a directory.
>
> File watchers are suggested to be syscall rules under the hood. I don’t
> believe this is true, based on the different behavior of syscall rules
> and
> file watcher rules when monitoring directories that don’t exist.
> Suggested
> to be equivalent per auditctl(8):-w /tmp/fakedir -p warx -k test1 -s
> always, exit -F dir=/tmp/fakedir -F perm=warx -k test2
It might be good to know what version of the audit package (auditctl -v)
you are using and the version of the kernel (uname -r). They were
different once upon a time.
> What will happen if the dir doesn’t exist in case 1 is the rule loads and
> continues. In case 2, the rule will fail to load, thus failing to load
> all
> rules below it.
In the current code, when it find a 'w' option, it calls stat on the
directory to ensure it is there. If it is, it converts the call to syscall
auditing on all syscalls. This gets refined when it finds the -perm field.
When it finds a 'S', it adds the syscall without any checks. When it finds
'-F dir=' it verifies the filter that it's being placed on but no other
path checks are done.
That means the -w version is more strict and needs a directory to exist
when the rule loads. The code in this area is probably at least 15 years
old without much reworking.
> The auditctl(8)
> Per the auditctl(8) man page -F (rule fields) are not supported by
> watchers. This doesn’t appear to be true any longer, as watchers do seem
> to honor -F (extensive testing not performed).
My man page differs. It says:
The -w form of writing watches is for backwards compatibility and is
deprecated due to poor system performance. Convert watches of this
form to the syscall based form.
> Any insight or suggestions? I am considering using a watcher with rule
> fields despite it not being officially supported due to the loading error
> with syscalls.
I'd be curious what versions you are using.
-Steve
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