netdev - Re: [PATCH] nfc: Allow to create multiple virtual nci devices

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Date:   Mon, 31 Oct 2022 08:36:57 -0700
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Leon Romanovsky <leon@...nel.org>
Cc:     bongsu.jeon@...sung.com, krzysztof.kozlowski@...aro.org,
        netdev@...r.kernel.org, syzkaller@...glegroups.com
Subject: Re: [PATCH] nfc: Allow to create multiple virtual nci devices

On Mon, 31 Oct 2022 at 02:23, Leon Romanovsky <leon@...nel.org> wrote:
>
> On Sun, Oct 30, 2022 at 03:29:19PM +0100, Dmitry Vyukov wrote:
> > The current virtual nci driver is great for testing and fuzzing.
> > But it allows to create at most one "global" device which does not allow
> > to run parallel tests and harms fuzzing isolation and reproducibility.
> > Restructure the driver to allow creation of multiple independent devices.
> > This should be backwards compatible for existing tests.
> >
> > Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Cc: Bongsu Jeon <bongsu.jeon@...sung.com>
> > Cc: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> > Cc: netdev@...r.kernel.org
> > ---
> >  drivers/nfc/virtual_ncidev.c | 143 ++++++++++++++++-------------------
> >  1 file changed, 66 insertions(+), 77 deletions(-)
>
> <...>
>
> >  static int virtual_nci_send(struct nci_dev *ndev, struct sk_buff *skb)
> >  {
> > -     mutex_lock(&nci_mutex);
> > -     if (state != virtual_ncidev_enabled) {
> > -             mutex_unlock(&nci_mutex);
> > -             kfree_skb(skb);
> > -             return 0;
> > -     }
> > +     struct virtual_nci_dev *vdev = nci_get_drvdata(ndev);
> >
> > -     if (send_buff) {
> > -             mutex_unlock(&nci_mutex);
> > +     mutex_lock(&vdev->mtx);
> > +     if (vdev->send_buff) {
> > +             mutex_unlock(&vdev->mtx);
> >               kfree_skb(skb);
>
> You probably need to set vdev->send_buff to NULL here.

Hi Leon,

Thanks for looking at this.

Are you sure about setting vdev->send_buff to NULL?
We already have a "cached" skb in vdev->send_buff, we received a new
one in 'skb' and freed it.
I assumed the intention is to keep vdev->send_buff intact.

> >               return -1;
> >       }
> > -     send_buff = skb_copy(skb, GFP_KERNEL);
> > -     mutex_unlock(&nci_mutex);
> > -     wake_up_interruptible(&wq);
> > +     vdev->send_buff = skb_copy(skb, GFP_KERNEL);
>
> You don't check return value of skb_copy(), it can fail, but
> this function will return 0 (success). Do you do it deliberately?
>
> If yes, please add a comment to the code, as it is not clear.

Good question. I just kept all of this logic as it is now and only
removed the global vars.

I guess we need something like this, right?

vdev->send_buff = skb_copy(skb, GFP_KERNEL);
if (!vdev->send_buff) {
    mutex_unlock(&vdev->mtx);
    return -1;
}

Though, it's called only from nci_send_frame() and its return value is
never checked :)

$ git grep nci_send_frame
include/net/nfc/nci_core.h:int nci_send_frame(struct nci_dev *ndev,
struct sk_buff *skb);
net/nfc/nci/core.c:int nci_send_frame(struct nci_dev *ndev, struct sk_buff *skb)
net/nfc/nci/core.c:EXPORT_SYMBOL(nci_send_frame);
drivers/nfc/nfcmrvl/fw_dnld.c:
nci_send_frame(priv->ndev, out_skb);
drivers/nfc/nfcmrvl/fw_dnld.c:          nci_send_frame(priv->ndev, out_skb);
drivers/nfc/nfcmrvl/fw_dnld.c:
nci_send_frame(priv->ndev, out_skb);
net/nfc/nci/core.c:             nci_send_frame(ndev, skb);
net/nfc/nci/core.c:             nci_send_frame(ndev, skb);


> Thanks
>
> > +     mutex_unlock(&vdev->mtx);
> > +     wake_up_interruptible(&vdev->wq);
> >       consume_skb(skb);
> >
> >       return 0;