Clarification Around File System Auditing

Wed Mar 15 20:29:59 UTC 2023

Yup....I was able to find the dummy check you are referring to and the
audit_reset_context() that is called next(which immediately exits in case
of dummy).
Thanks for the help folks....have a much better understanding of how the
audit context is allocated on enabling syscall auditing and the whole flow
post that.

Had just 1 question wrt watches. IIUC, for watch rules we evaluate all
syscalls (Snippet from audit-userspace:
https://github.com/linux-audit/audit-userspace/blob/1482cec74f2d9472f81dd4f0533484bd0c26decd/lib/libaudit.c#L805
).
But based on the permissions in the rule, we evaluate if the syscall
belongs to a specific Audit Class using audit_match_class() and only log if
the syscalls match/ are part of the class. This also explains why I see
audit_filter_rules() called for watches even if the syscall being performed
is not at all related to file system auditing.

   1. I was wondering why do we not automatically identify if the syscall
   is of interest or not in audit_in_mask() itself based on the rule
   permissions of the watch? In this way we would avoid the additional
   overhead of each syscall going into audit_filter_rules() and then
   evaluating on the AUDIT_PERM case as well.

Currently a watch rule for "wa" permissions for /etc is similar to :
-a always,exit -F arch=b64 -S all -F dir=/etc -F perm=wa -k ETC_WATCH

We only log if the syscall is part of the WRITE and ATTR permissions set.
Instead what I was suggesting was something like this:

-a always,exit -F arch=b64 -S <all syscalls part of the write and attr
classes> -F dir=/etc  -k ETC_WATCH

Please correct me if my understanding in any of the above is incorrect.

On Fri, Mar 10, 2023 at 3:54 PM Richard Guy Briggs <rgb at redhat.com> wrote:

> On 2023-02-17 16:50, Steve Grubb wrote:
> > Hello,
> >
> > On Tuesday, February 14, 2023 3:55:58 PM EST Amjad Gabbar wrote:
> > > Thanks for the reply.
> > > I was trying to evaluate the same via Flamegraphs and what I noticed
> was
> > > that :
> > >
> > > 1. Despite deleting all rules (auditctl -D), there were still calls to
> > > audit_filter_syscall() on each syscall. I assume this is because
> syscall
> > > auditing is enabled and despite no rules, there still will be some
> > > performance impact and calls to syscall filtering functions on each
> > > syscall.
> >
> > Yes.
> >
> > > 2. For a single watch rule as well without any syscall rules, I could
> see
> > > calls to audit_filter_syscall() followed by audit_filter_rules() for
> > > unrelated syscalls such as futex() and recvmsg() - not present in
> > > include/asm-generic/audit_*.h
> > > Why would these functions be called for a single watch rule for
> syscalls
> > > unrelated to the permissions?
> >
> > If auditing is enabled, it will go into the syscall filter for *any*
> syscall.
> > It will go into __audit_syscall_exit for every syscall. If there is an
> audit
> > context, it will go into audit_filter_syscall. The documentation in the
> > comments above these functions is informative.
> >
> > My guess is that this code path might benefit from adding a list_empty
> check.
> > A long time ago, I think we kept a variable that denoted if there were
> any
> > rules and short-circuited if none.
>
> There is essentially an empty list check in __audit_syscall_exit() with
> the dummy check, based on the number of syscall (or io_uring) rules in
> place tracked in audit_n_rules.  Unfortunately, we can't bail from
> __audit_syscall_entry() right after setting dummy because other
> hardwired records can cancel the dummy flag.
>
> > -Steve
> >
> > > On Tue, Feb 14, 2023 at 8:29 AM Steve Grubb <sgrubb at redhat.com> wrote:
> > > > Hello,
> > > >
> > > > On Monday, February 13, 2023 4:24:02 PM EST Amjad Gabbar wrote:
> > > > > I wanted some help in better understanding the workflow of file
> system
> > > > > auditing(watch rules) vs Syscall Auditing(syscall rules). I know in
> > > >
> > > > general
> > > >
> > > > > file system auditing does not have the same performance impact as
> > > > > syscall
> > > > > auditing, even though both make use of syscall exits for their
> > > >
> > > > evaluation.
> > > >
> > > > > From the manpage - "Unlike most syscall auditing rules, watches do
> not
> > > > > impact performance based on the number of rules sent to the
> kernel."
> > > > >
> > > > > From a previous thread, I found this excerpt regarding file watch
> rules
> > > >
> > > > vs
> > > >
> > > > > sycall rules -
> > > > >
> > > > > "The reason it doesn't have performance impact like normal syscall
> > > > > rules
> > > >
> > > > is
> > > >
> > > > > because it gets moved to a list that is not evaluated every
> syscall. A
> > > > > normal syscall rule will get evaluated for every syscall because
> it has
> > > >
> > > > to
> > > >
> > > > > see if the syscall number is of interest and then it checks the
> next
> > > > > rule."
> > > > >
> > > > > Based on this I had a couple of questions:
> > > > >
> > > > > For normal syscall rules, the evaluation happens as
> > > > > __audit_syscall_exit
> > > > > <
> https://elixir.bootlin.com/linux/v6.1.10/C/ident/__audit_syscall_exit>
> > > > > calls audit_filter_syscall
> > > > > (
> https://elixir.bootlin.com/linux/v6.1.10/source/kernel/auditsc.c#L841)
> > > > >
> > > > > Here, we check if the syscall is of interest or not in the
> > > > > audit_in_mask
> > > > > <https://elixir.bootlin.com/linux/v6.1.10/C/ident/audit_in_mask>
> > > >
> > > > function.
> > > >
> > > > > Only if the syscall is of interest do we proceed with examining the
> > > > > task
> > > > > and return on the first rule match.
> > > > >
> > > > > 1. What is the process or code path for watch rules?
> > > > > audit_filter_syscall
> > > > > <
> https://elixir.bootlin.com/linux/v6.1.10/C/ident/audit_filter_syscall>
> > > >
> > > > is
> > > >
> > > > > called for watch rules as well. Then how is it that these are not
> > > > > called
> > > > > for every syscall? Could you point me to the code where the
> evaluation
> > > > > happens only once?
> > > >
> > > > There is a file, kernel/audit_watch.c, that implements the interface
> > > > between
> > > > audit and fsnotify. You would want to learn how fsnotify works to
> > > > understand
> > > > how it avoids the syscall filter.
> > > >
> > > > > 2. Also, do file watches only involve the open system call family
> > > > > (open,
> > > > > openat etc). The man page implies the same, so just wanted to
> confirm.
> > > > >
> > > > > I assume -w /etc -p wa is the same as -a always,exit -S open -S
> openat
> > > > > -F
> > > > > dir=/etc?
> > > >
> > > > It depends on the flag passed for perm as to what syscall it wants.
> See:
> > > >
> > > > include/asm-generic/audit_*.h
> > > >
> > > > -Steve
> >
> >
> >
> >
> > --
> > Linux-audit mailing list
> > Linux-audit at redhat.com
> > https://listman.redhat.com/mailman/listinfo/linux-audit
> >
>
> - RGB
>
> --
> Richard Guy Briggs <rgb at redhat.com>
> Sr. S/W Engineer, Kernel Security, Base Operating Systems
> Remote, Ottawa, Red Hat Canada
> IRC: rgb, SunRaycer
> Voice: +1.647.777.2635, Internal: (81) 32635
>
>
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