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

John Johansen
2022-04-26T19:01:21Z

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.