Re: [RFC PATCH 2/9] audit,io_uring,io-wq: add some basic audit support to io_uring

Paul Moore
2021-05-24T19:59:28Z

On Sun, May 23, 2021 at 4:26 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
> On 5/22/21 3:36 AM, Paul Moore wrote:
> > On Fri, May 21, 2021 at 8:22 PM Pavel Begunkov <asml.silence@gmail.com> wrote:
> >> On 5/21/21 10:49 PM, Paul Moore wrote:
> [...]
> >>>
> >>> + if (req->opcode < IORING_OP_LAST)
> >>
> >> always true at this point
> >
> > I placed the opcode check before the audit call because the switch
> > statement below which handles the operation dispatching has a 'ret =
> > -EINVAL' for the default case, implying that there are some paths
> > where an invalid opcode could be passed into the function. Obviously
> > if that is not the case and you can guarantee that req->opcode will
> > always be valid we can easily drop the check prior to the audit call.
>
> It is always true at this point, would be completely broken
> otherwise

Understood, I was just pointing out an oddity in the code. I just
dropped the checks from my local tree, you'll see it in the next draft
of the patchset.

> >> So, it adds two if's with memory loads (i.e. current->audit_context)
> >> per request in one of the hottest functions here... No way, nack
> >>
> >> Maybe, if it's dynamically compiled into like kprobes if it's
> >> _really_ used.
> >
> > I'm open to suggestions on how to tweak the io_uring/audit
> > integration, if you don't like what I've proposed in this patchset,
> > lets try to come up with a solution that is more palatable. If you
> > were going to add audit support for these io_uring operations, how
> > would you propose we do it? Not being able to properly audit io_uring
> > operations is going to be a significant issue for a chunk of users, if
> > it isn't already, we need to work to find a solution to this problem.
>
> Who knows. First of all, seems CONFIG_AUDIT is enabled by default
> for many popular distributions, so I assume that is not compiled out.
>
> What are use cases for audit? Always running I guess?

Audit has been around for quite some time now, and it's goal is to
provide a mechanism for logging "security relevant" information in
such a way that it meets the needs of various security certification
efforts. Traditional Linux event logging, e.g. syslog and the like,
does not meet these requirements and changing them would likely affect
the usability for those who are not required to be compliant with
these security certifications. The Linux audit subsystem allows Linux
to be used in places it couldn't be used otherwise (or rather makes it
a *lot* easier).

That said, audit is not for everyone, and we have build time and
runtime options to help make life easier. Beyond simply disabling
audit at compile time a number of Linux distributions effectively
shortcut audit at runtime by adding a "never" rule to the audit
filter, for example:

% auditctl -a task,never

> Putting aside compatibility problems, it sounds that with the amount of overhead
> it adds there is no much profit in using io_uring in the first place.
> Is that so?

Well, if audit alone erased all of the io_uring advantages we should
just rip io_uring out of the kernel and people can just disable audit
instead ;)

I believe there are people who would like to use io_uring and are also
required to use a kernel with audit, either due to the need to run a
distribution kernel or the need to capture security information in the
audit stream. I'm hoping that we can find a solution for these users;
if we don't we are asking this group to choose either io_uring or
audit, and that is something I would like to avoid.

--
paul moore