linux-kernel - Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm

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Date:   Fri, 5 Feb 2021 11:15:11 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Lino Sanfilippo <l.sanfilippo@...bus.com>
Cc:     Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
        jarkko@...nel.org, stefanb@...ux.vnet.ibm.com,
        James.Bottomley@...senpartnership.com, stable@...r.kernel.org,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] tpm: fix reference counting for struct tpm_chip

On Fri, Feb 05, 2021 at 03:55:09PM +0100, Lino Sanfilippo wrote:
> Hi,
> 
> On 05.02.21 14:05, Jason Gunthorpe wrote:
> 
> >>
> >> Commit fdc915f7f719 ("tpm: expose spaces via a device link /dev/tpmrm<n>")
> >> already introduced function tpm_devs_release() to release the extra
> >> reference but did not implement the required put on chip->devs that results
> >> in the call of this function.
> > 
> > Seems wonky, the devs is just supposed to be a side thing, nothing
> > should be using it as a primary reference count for a tpm.
> > 
> > The bug here is only that tpm_common_open() did not get a kref on the
> > chip before putting it in priv and linking it to the fd. See the
> > comment before tpm_try_get_ops() indicating the caller must already
> > have taken care to ensure the chip is valid.
> > 
> > This should be all you need to fix the oops:
> > 
> > diff --git a/drivers/char/tpm/tpm-dev-common.c b/drivers/char/tpm/tpm-dev-common.c
> > index 1784530b8387bb..1b738dca7fffb5 100644
> > +++ b/drivers/char/tpm/tpm-dev-common.c
> > @@ -105,6 +105,7 @@ static void tpm_timeout_work(struct work_struct *work)
> >  void tpm_common_open(struct file *file, struct tpm_chip *chip,
> >                      struct file_priv *priv, struct tpm_space *space)
> >  {
> > +       get_device(&priv->chip.dev);
> >         priv->chip = chip;
> >         priv->space = space;
> >         priv->response_read = true;
> 
> This is racy, isnt it? The time between we open the file and we want to grab the
> reference in common_open() the chip can already be unregistered and freed.

No, the cdev layer holds the refcount on the device while open is
being called.

Jason