linux-kernel - Re: audit: io_uring openat triggers audit reference count underflow in worker thread

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Message-ID: <CAHC9VhQ0z4wuH7R=KRcUTyZuRs7adYTiH5JjohJSz4d2-Jd9EQ@mail.gmail.com>
Date:   Sat, 7 Oct 2023 11:13:11 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Dan Clash <Dan.Clash@...rosoft.com>,
        "audit@...r.kernel.org" <audit@...r.kernel.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        brauner@...nel.org
Subject: Re: audit: io_uring openat triggers audit reference count underflow
 in worker thread

On Sat, Oct 7, 2023 at 9:11 AM Jens Axboe <axboe@...nel.dk> wrote:
> On 10/6/23 8:32 PM, Jens Axboe wrote:
> > On 10/6/23 2:09 PM, Dan Clash wrote:

...

> > I'm not fully aware of what audit is doing with struct filename outside
> > of needing it for the audit log. Rather than impose the atomic
> > references for everyone, would it be doable to simply dupe the struct
> > instead of grabbing the (non-atomic) reference to the existing one?
> >
> > If not, since there's no over/underflow handling right now, it'd
> > certainly be cheaper to use an atomic_t here rather than a full
> > refcount.
>
> After taking a closer look at this, I think the best course of action
> would be to make the struct filename refcnt and atomic_t. With audit in
> the picture, it's quite possible to have multiple threads manipulating
> the filename refcnt at the same time, which is obviously not currently
> safe.

Thanks Jens.

I personally would feel a bit better with the additional safety
provided by refount_t, but I agree that there is little chance of an
overflow/underflow in this case so the additional refcount_t checking
is not likely to be needed here.

For the record, this should only be an issue when audit is combined
with io_uring, prior to io_uring there wasn't an issue with multiple
threads attempting to cleanup the filename objects so we didn't have
to worry about racing on filename::refcnt updates.  However, for those
systems where both audit and io_uring are in use we definitely have a
problem and will need the upcoming fix from Dan to ensure the safety
of the system.

Thanks for spotting this Dan and doing the initial investigation into
the problem, if you run into any problems with the patch or need a
hand let me know, I'm happy to help.

> Dan, would you mind sending that as a patch? Include a link to your
> original email:
>
> Link: https://lore.kernel.org/lkml/MW2PR2101MB1033FFF044A258F84AEAA584F1C9A@MW2PR2101MB1033.namprd21.prod.outlook.com/
>
> and a Fixes tag as well:
>
> Fixes: 5bd2182d58e9 ("audit,io_uring,io-wq: add some basic audit support to io_uring")
>
> and CC linux-fsdevel@...r.kernel.org and
> Christian Brauner <brauner@...nel.org> as well.

I'm going to CC Christian on this email just so he has a heads-up
about the problem and knows to expect a patch.

--
paul-moore.com