Sycall Rules vs Watch Rules

[Amjad Gabbar]

Hi,

> The perm fields select the right system calls
> that should be reported on.

That is accurate from a functional perspective. There is no change in the
events logged. But there is a difference in performance. This is most
evident for syscalls not part of the perm fields.

Futex is a syscall that I see called fairly often in my system, which is
not part of the perm fields.
As an example, I selected the ospp rules file to measure  performance via a
synthetic test-
https://github.com/linux-audit/audit-userspace/blob/master/rules/30-ospp-v42.rules

stress-ng —futex 1 —futex-ops 1000000

If we look at the performance numbers for the file rules as is, the
auditing percentage is about 14%.

Now if we were to just add the specific syscalls that the perm fields
filter on in the rules file, the auditing percentage would drop to around
2%.

Again this synthetic test is just for demonstration purposes but helps
explain the point. Basically for syscalls not part of the perm fields we
filter them at a much later stage in the AUDIT_PERM case(due to -S all)
whereas if we use specific syscalls within the rule itself, we would exit
the processing in audit_filter_syscall itself for uninteresting syscalls,
hence improving the performance.

>I see a 1 line change that I am testing.
Let me know if you need any help. I did have a partial PR ready for
submission but wanted to get your opinions before submitting anything.

Regards
Ali Adnan

On Tue, Sep 19, 2023 at 6:33 PM Steve Grubb <sgrubb at redhat.com> wrote:

> Hello,
>
> On Tuesday, September 12, 2023 5:20:54 PM EDT Amjad Gabbar wrote:
> > Based on this and some experiments I have been performing, I would
> suggest
> > changing how a lot of the FileSystem rules are written and illustrated.
> > Ex -
> >
> https://github.com/linux-audit/audit-userspace/blob/master/rules/30-pci-dss
> > -v31.rules#L34-L35
> >
> > The rule in the repository is
> > -a always,exit -F path=/etc/sudoers -F perm=wa -F
> > key=10.2.2-priv-config-changes
> >
> > My suggestion is to instead change the rule based on the permissions
> > defined. The above rule would change to the following based on the kernel
> > being used.
> > -a always,exit -S <list of syscalls in audit_write.h and audit_read.h
> > +open,openat> -F path=/etc/sudoers -F perm=wa -F
> > key=10.2.2-priv-config-changes
>
> That should be exactly what the kernel does with the perm fields. The perm
> fields select the right system calls that should be reported on.
>
> > This is higher performance because we are limiting the syscalls instead
> of
> > making use of -S all which has more paths of evaluation for each and
> every
> > syscall.
> >
> > Same thing for watches. Watches are inherently -S all rules which are
> very
> > performance intensive.
> >
> https://github.com/linux-audit/audit-userspace/blob/1482cec74f2d9472f81dd4f
> > 0533484bd0c26decd/lib/libaudit.c#L805
>
> There should be no difference in performance between watches and syscall
> based file auditing.
>
> > Ideally we should limit the syscalls based on the permissions being used.
> >
> > I have implemented the same in my environment rules and have noticed a
> > massive performance difference with no difference in the events being
> > logged since we anyways filter eventually based on the permissions.
> >
> > Let me know what you all think.
>
> I'm looking into this more. I see a 1 line change that I am testing.
>
> -Steve
>
> > On Wed, Sep 6, 2023 at 2:58 PM Richard Guy Briggs <rgb at redhat.com>
> wrote:
> > > On 2023-09-06 10:56, Amjad Gabbar wrote:
> > > > Hi,
> > > >
> > > > I have done some analysis and digging into how both the watch rules
> and
> > > > syscall rules are translated.
> > > >
> > > > From my understanding, in terms of logging, both the below rules are
> > > > similar. There is no difference in either of the rules.
> > > >
> > > > 1. -w /etc -p wa -k ETC_WATCH
> > >
> > > They are similar in this case.
> > > -w behaves differently depending on the existance of the watched entity
> > > and the presence of a trailing "/".  This is why the form above is
> > > deprecated.
> > >
> > > > 2. -a always,exit -F arch=b64 -S <all syscalls part of the write and
> > > > attr
> > > > classes> -F dir=/etc  -F perm=wa -k ETC_WATCH
> > > >
> > > > The write and attr classes consist of syscalls in
> > > > “include/asm-generic/audit_*.h“.
> > > >
> > > >  The perm flag is needed in the second case for including open/openat
> > > >
> > > > syscalls which are not a part of the write and attr syscall list.
> > > >
> > > > I'd like to verify if what I mentioned earlier is accurate, and I
> have
> > > > an
> > > > additional point but depends on whether this is accurate.
> > > >
> > > > Ali
> > >
> > > - RGB
> > >
> > > --
> > > Richard Guy Briggs <rgb at redhat.com>
> > > Sr. S/W Engineer, Kernel Security, Base Operating Systems
> > > Remote, Ottawa, Red Hat Canada
> > > Upstream IRC: SunRaycer
> > > Voice: +1.613.860 2354 SMS: +1.613.518.6570
>
>
>
>
>
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