Re: [PATCH ghak105 V2] audit: remove audit_context when CONFIG_ AUDIT and not AUDITSYSCALL

[Paul Moore]

On Tue, Jan 29, 2019 at 9:54 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> On 2019-01-29 18:26, Paul Moore wrote:
> > On Tue, Jan 29, 2019 at 6:18 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> > > On 2019-01-29 18:07, Paul Moore wrote:
> > > > On Mon, Jan 28, 2019 at 1:33 PM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> > > > > Remove audit_context from struct task_struct and struct audit_buffer
> > > > > when CONFIG_AUDIT is enabled but CONFIG_AUDITSYSCALL is not.
> > > > >
> > > > > Also, audit_log_name() (and supporting inode and fcaps functions) should
> > > > > have been put back in auditsc.c when soft and hard link logging was
> > > > > normalized since it is only used by syscall auditing.
> > > > >
> > > > > See github issue https://github.com/linux-audit/audit-kernel/issues/105
> > > > >
> > > > > Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> > > > > ---
> > > > > Changelog:
> > > > > v2:
> > > > > - resolve merge conflicts from rebase on upstreamed ghak103 patch
> > > > > - wrap task_struct audit_context in CONFIG_AUDITSYSCALL
> > > > >
> > > > >  include/linux/sched.h |   4 +-
> > > > >  kernel/audit.c        | 157 +++-----------------------------------------------
> > > > >  kernel/audit.h        |   9 ---
> > > > >  kernel/auditsc.c      | 150 +++++++++++++++++++++++++++++++++++++++++++++++
> > > > >  4 files changed, 161 insertions(+), 159 deletions(-)
> > > >
> > > > ...
> > > >
> > > > > diff --git a/kernel/audit.c b/kernel/audit.c
> > > > > index 3f3f1888cac7..15e41603fd34 100644
> > > > > --- a/kernel/audit.c
> > > > > +++ b/kernel/audit.c
> > > > > @@ -205,7 +205,9 @@ struct audit_net {
> > > > >   * use simultaneously. */
> > > > >  struct audit_buffer {
> > > > >         struct sk_buff       *skb;      /* formatted skb ready to send */
> > > > > +#ifdef CONFIG_AUDITSYSCALL
> > > > >         struct audit_context *ctx;      /* NULL or associated context */
> > > > > +#endif
> > > > >         gfp_t                gfp_mask;
> > > > >  };
> > > > >
> > > > > @@ -1696,7 +1698,9 @@ static struct audit_buffer *audit_buffer_alloc(struct audit_context *ctx,
> > > > >         if (!nlmsg_put(ab->skb, 0, 0, type, 0, 0))
> > > > >                 goto err;
> > > > >
> > > > > +#ifdef CONFIG_AUDITSYSCALL
> > > > >         ab->ctx = ctx;
> > > > > +#endif
> > > >
> > > > I vaguely remember reading/hearing something in the past about
> > > > kmem_cache_alloc() not returning a zero'd out buffer in all cases, can
> > > > you say for certain that "ab" in this case is always going to be
> > > > zero'd out?  This is an honest question.
> > >
> > > Ok, then maybe we should be using kmem_cache_zalloc() instead of
> > > kmem_cache_alloc() in audit_buffer_alloc()?  (as I've done in
> > > the last patch of ghak81/first patch of ghak90)
> > >
> > > If this is too much overhead, then we can initialize ctx = NULL;
> >
> > We don't need zalloc() since we're setting all the fields, although
> > more on this below ...
>
> Ok...
>
> > > > If we can't guarantee that "ab" is zero'd out, we should manually set
> > > > ab->ctx to NULL when !CONFIG_AUDITSYSCALL.
> > >
> > > But ctx isn't part of struct audit_buffer when !CONFIG_AUDITSYSCALL.  It
> > > is #ifdef-ed out.  What am I missing?
> >
> > You're not, I am.  I saw the obvious bit where you removed it from the
> > task_struct, but completely glossed over the bit where you also
> > removed it from the audit_buffer struct.  My mistake.
> >
> > Once the audit container ID stuff lands we are going to need to have
> > the audit_context pointer in the audit_buffer regardless of the
> > CONFIG_AUDITSYSCALL setting, right?  Assuming the answer is yes, I
> > think I'd just assume leave the pointer in the audit_buffer (setting
> > it to NULL when !CONFIG_AUDITSYSCALL) so we don't have to have those
> > #ifdef's in the middle of the functions (I generally like to avoid
> > those if possible).  I think it's still worth making the changes to
> > task_struct, as that is the right thing to do and doesn't have the
> > same level of impact.
>
> I like to avoid #ifdef compiler directives out when I can too, creating
> stubs in the header file to do the job.
>
> Why do we need an audit_context pointer in struct audit_buffer?  I'll
> take a stab at answering this...  I was thinking it wasn't necessary,
> but now I think I see what I was missing.  I think the only reason is to
> connect records to one event through the timestamp and serial number
> when syscall is disabled.  Up until now it wasn't needed unless full
> syscall functionality was present, but once we have an audit container
> identifier aux record we will need to join them, at minimum with a local
> context for user and netfilter_pkt records.

I also expect us the significance to grow over time as we start to
deal with the event routing problem; one solution would be to track
the audit container ID as a field in the audit_context.

Basically I see the audit_context as the audit "event" data structure
where the audit_buffer is the audit "record" data structure.  Their
use doesn't always line up perfectly with those definitions at
present, but I tend to think of the deviations as problems to correct
over time.

> So I have to ask, does it make sense to restructure things so that the
> struct audit_buffer has a serial and ctime field so that it isn't needed
> in the struct audit_context?  I'm not sure if this is possible.  I'll
> have to go back and look at the code to see if this is the case...

I would say "no" if for no other reason than what I said about about
the audit_context being the "event" data structure.

-- 
paul moore
www.paul-moore.com