Re: Clarification Around File System Auditing

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/1482cec74f2d9472f81dd... ). 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(a)redhat.com&gt; 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(a)redhat.com&gt; > 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(a)redhat.com > > https://listman.redhat.com/mailman/listinfo/linux-audit > > > > - RGB > > -- > Richard Guy Briggs <rgb(a)redhat.com&gt; > 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 > >