Re: [PATCH v35 25/29] Audit: Allow multiple records in an audit_buffer

Paul Moore
2022-04-26T18:13:17Z

On Mon, Apr 25, 2022 at 9:06 PM John Johansen
<john.johansen@canonical.com> wrote:
> On 4/18/22 07:59, Casey Schaufler wrote:
> > Replace the single skb pointer in an audit_buffer with
> > a list of skb pointers. Add the audit_stamp information
> > to the audit_buffer as there's no guarantee that there
> > will be an audit_context containing the stamp associated
> > with the event. At audit_log_end() time create auxiliary
> > records (none are currently defined) as have been added
> > to the list.
> >
> > Suggested-by: Paul Moore <paul@paul-moore.com>
> > Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
>
> I agree with Paul that audit_buffer_aux_new() and
> audit_buffer_aux_end() belong in this patch
>
>
> > ---
> > kernel/audit.c | 62 +++++++++++++++++++++++++++++++-------------------
> > 1 file changed, 39 insertions(+), 23 deletions(-)
> >
> > diff --git a/kernel/audit.c b/kernel/audit.c
> > index 6b6c089512f7..4d44c05053b0 100644
> > --- a/kernel/audit.c
> > +++ b/kernel/audit.c
> > @@ -197,8 +197,10 @@ static struct audit_ctl_mutex {
> > * to place it on a transmit queue. Multiple audit_buffers can be in
> > * use simultaneously. */
> > struct audit_buffer {
> > - struct sk_buff *skb; /* formatted skb ready to send */
> > + struct sk_buff *skb; /* the skb for audit_log functions */
> > + struct sk_buff_head skb_list; /* formatted skbs, ready to send */
> > struct audit_context *ctx; /* NULL or associated context */
> > + struct audit_stamp stamp; /* audit stamp for these records */
> > gfp_t gfp_mask;
> > };
> >
> > @@ -1765,10 +1767,13 @@ __setup("audit_backlog_limit=", audit_backlog_limit_set);
> >
> > static void audit_buffer_free(struct audit_buffer *ab)
> > {
> > + struct sk_buff *skb;
> > +
> > if (!ab)
> > return;
> >
> > - kfree_skb(ab->skb);
> > + while((skb = skb_dequeue(&ab->skb_list)))
> > + kfree_skb(skb);
>
> we still have and ab->skb can we have a debug check that its freed by walking the queue?

By definition ab->skb is always going to point at something on the
list, if it doesn't we are likely to have failures elsewhere. The
structure definition is private to kernel/audit.c and the
allocation/creation is handled by an allocator function which always
adds the new skb to the list so I think we're okay.

We could add additional checks, but with audit performance already a
hot topic I would prefer to draw the debug-check line at input coming
from outside the audit subsystem.

--
paul-moore.com