Re: [PATCH ghak90 V9 06/13] audit: add contid support for signalling the audit daemon

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

On Wed, Jul 29, 2020 at 3:00 PM Richard Guy Briggs <rgb@redhat.com> wrote:
> On 2020-07-05 11:10, Paul Moore wrote:
> > On Sat, Jun 27, 2020 at 9:22 AM Richard Guy Briggs <rgb@redhat.com> wrote:
> > >
> > > Add audit container identifier support to the action of signalling the
> > > audit daemon.
> > >
> > > Since this would need to add an element to the audit_sig_info struct,
> > > a new record type AUDIT_SIGNAL_INFO2 was created with a new
> > > audit_sig_info2 struct.  Corresponding support is required in the
> > > userspace code to reflect the new record request and reply type.
> > > An older userspace won't break since it won't know to request this
> > > record type.
> > >
> > > Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
> > > ---
> > >  include/linux/audit.h       |  8 ++++
> > >  include/uapi/linux/audit.h  |  1 +
> > >  kernel/audit.c              | 95 ++++++++++++++++++++++++++++++++++++++++++++-
> > >  security/selinux/nlmsgtab.c |  1 +
> > >  4 files changed, 104 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > > index 5eeba0efffc2..89cf7c66abe6 100644
> > > --- a/include/linux/audit.h
> > > +++ b/include/linux/audit.h
> > > @@ -22,6 +22,13 @@ struct audit_sig_info {
> > >         char            ctx[];
> > >  };
> > >
> > > +struct audit_sig_info2 {
> > > +       uid_t           uid;
> > > +       pid_t           pid;
> > > +       u32             cid_len;
> > > +       char            data[];
> > > +};
> > > +
> > >  struct audit_buffer;
> > >  struct audit_context;
> > >  struct inode;
> > > @@ -105,6 +112,7 @@ struct audit_contobj {
> > >         u64                     id;
> > >         struct task_struct      *owner;
> > >         refcount_t              refcount;
> > > +       refcount_t              sigflag;
> > >         struct rcu_head         rcu;
> > >  };
> >
> > It seems like we need some protection in audit_set_contid() so that we
> > don't allow reuse of an audit container ID when "refcount == 0 &&
> > sigflag != 0", yes?
>
> We have it, see -ESHUTDOWN below.

That check in audit_set_contid() is checking ->refcount and not
->sigflag; ->sigflag is more important in this context, yes?

> > > diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
> > > index fd98460c983f..a56ad77069b9 100644
> > > --- a/include/uapi/linux/audit.h
> > > +++ b/include/uapi/linux/audit.h
> > > @@ -72,6 +72,7 @@
> > >  #define AUDIT_SET_FEATURE      1018    /* Turn an audit feature on or off */
> > >  #define AUDIT_GET_FEATURE      1019    /* Get which features are enabled */
> > >  #define AUDIT_CONTAINER_OP     1020    /* Define the container id and info */
> > > +#define AUDIT_SIGNAL_INFO2     1021    /* Get info auditd signal sender */
> > >
> > >  #define AUDIT_FIRST_USER_MSG   1100    /* Userspace messages mostly uninteresting to kernel */
> > >  #define AUDIT_USER_AVC         1107    /* We filter this differently */
> > > diff --git a/kernel/audit.c b/kernel/audit.c
> > > index a09f8f661234..54dd2cb69402 100644
> > > --- a/kernel/audit.c
> > > +++ b/kernel/audit.c
> > > @@ -126,6 +126,8 @@ struct auditd_connection {
> > >  kuid_t         audit_sig_uid = INVALID_UID;
> > >  pid_t          audit_sig_pid = -1;
> > >  u32            audit_sig_sid = 0;
> > > +static struct audit_contobj *audit_sig_cid;
> > > +static struct task_struct *audit_sig_atsk;
> >
> > This looks like a typo, or did you mean "atsk" for some reason?
>
> No, I meant atsk to refer specifically to the audit daemon task and not
> any other random one that is doing the signalling.  I can change it is
> there is a strong objection.

Esh, yeah, "atsk" looks too much like a typo ;)  At the very leask add
a 'd' in there, e.g. "adtsk", but something better than that would be
welcome.

> > > @@ -2532,6 +2620,11 @@ int audit_set_contid(struct task_struct *task, u64 contid)
> > >                         if (cont->id == contid) {
> > >                                 /* task injection to existing container */
> > >                                 if (current == cont->owner) {
> > > +                                       if (!refcount_read(&cont->refcount)) {
> > > +                                               rc = -ESHUTDOWN;
> >
> > Reuse -ENOTUNIQ; I'm not overly excited about providing a lot of
> > detail here as these are global system objects.  If you must have a
> > different errno (and I would prefer you didn't), use something like
> > -EBUSY.
>
> I don't understand the issue of "global system objects" since the only
> time this error would be issued is if its own contid were being reused
> but it hadn't cleaned up its own references yet by either issuing an
> AUDIT_SIGNAL_INFO* request or the targetted audit daemon hadn't cleaned
> up yet.  EBUSY could be confused with already having spawned threads or
> children, and ENOTUNIQ could indicate that another orchestrator/engine
> had stolen its desired contid after we released it and wanted to reuse
> it.

All the more reason for ENOTUNIQ.  The point is that the audit
container ID is not available for use, and since the IDs are shared
across the entire system I think we are better off having some
ambiquity here with errnos.

> This gets me thinking about making reservations for preferred
> contids that are otherwise unavailable and making callbacks to indicate
> when they become available, but that seems undesirably complex right
> now.

That is definitely beyond the scope of this work, or rather *should*
be beyond the scope of this work.

-- 
paul moore
www.paul-moore.com