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

[John Johansen <john.johansen@canonical.com>]

On 4/26/22 11:12, Paul Moore wrote:
> 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.
> 
yeah I got that eventually, though it wasn't immediately obvious

> 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.
> 
and that is why I asked for a debug check. But its not a hard requirement
just a nice to have because I have been bitten by internal consistency
issues all to often.