Clarification Around File System Auditing

[Steve Grubb]

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.

-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