Re: [PATCH ghak90 V8 07/16] audit: add contid support for signalling the audit daemon

[Paul Moore <paul@paul-moore.com>]

On Mon, Jun 8, 2020 at 2:04 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> On 2020-04-22 13:24, Paul Moore wrote:
> > On Fri, Apr 17, 2020 at 6:26 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > > Paul Moore <paul@paul-moore.com> writes:
> > > > On Thu, Apr 16, 2020 at 4:36 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> > > >> Paul Moore <paul@paul-moore.com> writes:
> > > >> > On Mon, Mar 30, 2020 at 1:49 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> > > >> >> On 2020-03-30 13:34, Paul Moore wrote:
> > > >> >> > On Mon, Mar 30, 2020 at 12:22 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> > > >> >> > > On 2020-03-30 10:26, Paul Moore wrote:
> > > >> >> > > > On Mon, Mar 30, 2020 at 9:47 AM Richard Guy Briggs <rgb@redhat.com> wrote:
> > > >> >> > > > > On 2020-03-28 23:11, Paul Moore wrote:
> > > >> >> > > > > > On Tue, Mar 24, 2020 at 5:02 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> > > >> >> > > > > > > On 2020-03-23 20:16, Paul Moore wrote:
> > > >> >> > > > > > > > On Thu, Mar 19, 2020 at 6:03 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> > > >> >> > > > > > > > > On 2020-03-18 18:06, Paul Moore wrote:
> > > >> >
> > > >> > ...
> > > >> >
> > > >> >> > > Well, every time a record gets generated, *any* record gets generated,
> > > >> >> > > we'll need to check for which audit daemons this record is in scope and
> > > >> >> > > generate a different one for each depending on the content and whether
> > > >> >> > > or not the content is influenced by the scope.
> > > >> >> >
> > > >> >> > That's the problem right there - we don't want to have to generate a
> > > >> >> > unique record for *each* auditd on *every* record.  That is a recipe
> > > >> >> > for disaster.
> > > >> >> >
> > > >> >> > Solving this for all of the known audit records is not something we
> > > >> >> > need to worry about in depth at the moment (although giving it some
> > > >> >> > casual thought is not a bad thing), but solving this for the audit
> > > >> >> > container ID information *is* something we need to worry about right
> > > >> >> > now.
> > > >> >>
> > > >> >> If you think that a different nested contid value string per daemon is
> > > >> >> not acceptable, then we are back to issuing a record that has only *one*
> > > >> >> contid listed without any nesting information.  This brings us back to
> > > >> >> the original problem of keeping *all* audit log history since the boot
> > > >> >> of the machine to be able to track the nesting of any particular contid.
> > > >> >
> > > >> > I'm not ruling anything out, except for the "let's just completely
> > > >> > regenerate every record for each auditd instance".
> > > >>
> > > >> Paul I am a bit confused about what you are referring to when you say
> > > >> regenerate every record.
> > > >>
> > > >> Are you saying that you don't want to repeat the sequence:
> > > >>         audit_log_start(...);
> > > >>         audit_log_format(...);
> > > >>         audit_log_end(...);
> > > >> for every nested audit daemon?
> > > >
> > > > If it can be avoided yes.  Audit performance is already not-awesome,
> > > > this would make it even worse.
> > >
> > > As far as I can see not repeating sequences like that is fundamental
> > > for making this work at all.  Just because only the audit subsystem
> > > should know about one or multiple audit daemons.  Nothing else should
> > > care.
> >
> > Yes, exactly, this has been mentioned in the past.  Both the
> > performance hit and the code complication in the caller are things we
> > must avoid.
> >
> > > >> Or are you saying that you would like to literraly want to send the same
> > > >> skb to each of the nested audit daemons?
> > > >
> > > > Ideally we would reuse the generated audit messages as much as
> > > > possible.  Less work is better.  That's really my main concern here,
> > > > let's make sure we aren't going to totally tank performance when we
> > > > have a bunch of nested audit daemons.
> > >
> > > So I think there are two parts of this answer.  Assuming we are talking
> > > about nesting audit daemons in containers we will have different
> > > rulesets and I expect most of the events for a nested audit daemon won't
> > > be of interest to the outer audit daemon.
> >
> > Yes, this is another thing that Richard and I have discussed in the
> > past.  We will basically need to create per-daemon queues, rules,
> > tracking state, etc.; that is easy enough.  What will be slightly more
> > tricky is the part where we apply the filters to the individual
> > records and decide if that record is valid/desired for a given daemon.
> > I think it can be done without too much pain, and any changes to the
> > callers, but it will require a bit of work to make sure it is done
> > well and that records are needlessly duplicated in the kernel.
> >
> > > Beyond that it should be very straight forward to keep a pointer and
> > > leave the buffer as a scatter gather list until audit_log_end
> > > and translate pids, and rewrite ACIDs attributes in audit_log_end
> > > when we build the final packet.  Either through collaboration with
> > > audit_log_format or a special audit_log command that carefully sets
> > > up the handful of things that need that information.
> >
> > In order to maximize record re-use I think we will want to hold off on
> > assembling the final packet until it is sent to the daemons in the
> > kauditd thread.  We'll also likely need to create special
> > audit_log_XXX functions to capture fields which we know will need
> > translation, e.g. ACID information.  (the reason for the new
> > audit_log_XXX functions would be to mark the new sg element and ensure
> > the buffer is handled correctly)
> >
> > Regardless of the details, I think the scatter gather approach is the
> > key here - that seems like the best design idea I've seen thus far.
> > It enables us to replace portions of the record as needed ... and
> > possibly use the existing skb cow stuff ... it has been a while, but
> > does the skb cow functions handle scatter gather skbs or do they need
> > to be linear?
>
> How does the selection of this data management technique affect our
> choice of field format?

I'm not sure it affects the record string, but it might affect the
in-kernel API as we would likely want to have a special function for
logging the audit container ID that does the scatter-gather management
for the record.  There might also need to be some changes to how we
allocate the records.

However, since you're the one working on these patches I would expect
you to be the one to look into how this would work and what the
impacts might be to the code, record format, etc.

> Does this lock the field value to a fixed length?

I wouldn't think so.  In fact if it did it wouldn't really be a good solution.

Once again, this is something I would expect you to look into.

> Does the use of scatter/gather techniques or structures allow
> the use of different lengths of data for each destination (auditd)?

This is related to the above ... but yes, the reason why Eric and I
were discussing a scatter/gather approach is that it would presumably
allow one to break the single record string into pieces which could be
managed and manipulated much easier than the monolithic record string.

> I could see different target audit daemons triggering or switching to a
> different chunk of data and length.  This does raise a concern related
> to the previous sig_info2 discussion that the struct contobj that exists
> at the time of audit_log_exit called could have been reaped by the time
> the buffer is pulled from the queue for transmission to auditd, but we
> could hold a reference to it as is done for sig_info2.

Yes.

> Looking through the kernel scatter/gather possibilities, I see struct
> iovec which is used by the readv/writev/preadv/pwritev syscalls, but I'm
> understanding that this is a kernel implementation that will be not
> visible to user space.  So would the struct scatterlist be the right
> choice?

It has been so long since I've looked at the scatter-gather code that
I can't really say with any confidence at this point.  All I can say
is that the scatter-gather code really should just be an
implementation detail in the kernel and should not be visible to
userspace; userspace should get the same awful, improperly generated
netlink message it always has received from the kernel ;)

-- 
paul moore
www.paul-moore.com