Sycall Rules vs Watch Rules

[Steve Grubb]

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