linux-kernel - Re: [PATCH v2] tpm: Rework open/close/shutdown to avoid races

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Date:   Mon, 7 Mar 2022 18:45:22 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     Sergey Temerkhanov <s.temerkhanov@...il.com>,
        Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tpm: Rework open/close/shutdown to avoid races

On Mon, Mar 07, 2022 at 06:44:17PM +0200, Jarkko Sakkinen wrote:
> On Mon, Mar 07, 2022 at 11:22:27AM -0500, James Bottomley wrote:
> > On Mon, 2022-03-07 at 17:41 +0200, Jarkko Sakkinen wrote:
> > [...]
> > > James, would it possible for you to construct a proper patch from
> > > this and send it so we could include it to the next PR (use my tree
> > > as baseline)?
> > 
> > Sure,  There's not really any identifiable patch for a fixes tag, since
> > the race really always existed.
> > 
> > How about the below.
> > 
> > James
> > 
> > -------------------8>8>8><8<8<8------------------
> > 
> > From 3e0e640e47e5728b68693c5d45bf3cdd0bff48ca Mon Sep 17 00:00:00 2001
> > From: James Bottomley <James.Bottomley@...senPartnership.com>
> > Date: Mon, 7 Mar 2022 11:16:55 -0500
> > Subject: [PATCH] tpm: use try_get_ops() in tpm-space.c
> > 
> > As part of the series conversion to remove nested TPM operations:
> > 
> > https://lore.kernel.org/all/20190205224723.19671-1-jarkko.sakkinen@linux.intel.com/
> > 
> > exposure of the chip->tpm_mutex was removed from much of the upper
> > level code.  In this conversion, tpm2_del_space() was missed.  This
> > didn't matter much because it's usually called closely after a
> > converted operation, so there's only a very tiny race window where the
> > chip can be removed before the space flushing is done which causes a
> > NULL deref on the mutex.  However, there are reports of this window
> > being hit in practice, so fix this by converting tpm2_del_space() to
> > use tpm_try_get_ops(), which performs all the teardown checks before
> > acquring the mutex.
> > 
> > Signed-off-by: James Bottomley <James.Bottomley@...senPartnership.com>
> > ---
> >  drivers/char/tpm/tpm2-space.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2-space.c
> > index 265ec72b1d81..ffb35f0154c1 100644
> > --- a/drivers/char/tpm/tpm2-space.c
> > +++ b/drivers/char/tpm/tpm2-space.c
> > @@ -58,12 +58,12 @@ int tpm2_init_space(struct tpm_space *space, unsigned int buf_size)
> >  
> >  void tpm2_del_space(struct tpm_chip *chip, struct tpm_space *space)
> >  {
> > -	mutex_lock(&chip->tpm_mutex);
> > -	if (!tpm_chip_start(chip)) {
> > +
> > +	if (tpm_try_get_ops(chip) == 0) {
> >  		tpm2_flush_sessions(chip, space);
> > -		tpm_chip_stop(chip);
> > +		tpm_put_ops(chip);
> >  	}
> > -	mutex_unlock(&chip->tpm_mutex);
> > +
> >  	kfree(space->context_buf);
> >  	kfree(space->session_buf);
> >  }
> > -- 
> > 2.34.1
> > 
> > 
> 
> Thank you! LGTM. 
> 
> Reviewed-by: Jarkko Sakkinen <jarkko@...nel.org>
> 
> BR, Jarkko

If possible, can you send this an identifiable patch (separate email)
with my reviewed-by applied, and I'll take it from there (i.e. it's
then available all trackers such patchwork etc.)?

BR, Jarkko