[PATCH] tpm: fix crash in tpm_tis

[PATCH] tpm: fix crash in tpm_tis

[Jarkko Sakkinen]

rmmod crashes the driver because tpm_chip_unregister() already sets ops
to NULL. Release ops in tpm_dev_release() so that tpm2_shutdown() can be
cleanly executed and also because it is symmetrical where they are
allocated (in tpmm_chip_alloc()).

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Fixes: 4d3eac5e156a ("tpm: Provide strong locking for device removal")
---
 drivers/char/tpm/tpm-chip.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index f62c851..5241bc4 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -127,6 +127,11 @@ static void tpm_dev_release(struct device *dev)
 	idr_remove(&dev_nums_idr, chip->dev_num);
 	mutex_unlock(&idr_lock);
 
+	/* Make the driver uncallable. */
+	down_write(&chip->ops_sem);
+	chip->ops = NULL;
+	up_write(&chip->ops_sem);
+
 	kfree(chip);
 }
 
@@ -266,11 +271,6 @@ static void tpm_del_char_device(struct tpm_chip *chip)
 	mutex_lock(&idr_lock);
 	idr_replace(&dev_nums_idr, NULL, chip->dev_num);
 	mutex_unlock(&idr_lock);
-
-	/* Make the driver uncallable. */
-	down_write(&chip->ops_sem);
-	chip->ops = NULL;
-	up_write(&chip->ops_sem);
 }
 
 static int tpm1_chip_register(struct tpm_chip *chip)
-- 
1.9.1

Re: [PATCH] tpm: fix crash in tpm_tis

[Jarkko Sakkinen]

On Thu, Apr 07, 2016 at 07:36:54AM -0700, Jason Gunthorpe wrote:
>    I will have to look closer after the conference, but this does not look
>    right.
> 
>    I vaguely recall commenting on this before. Move the shutdown into the
>    core code to fix it.

This fix that I sent is not the right way to do it.

One example scenario:

1. TIS driver gets detached, which causes tpm_tis_remove() to be called.
2. Some in-kernel subsystem uses TPM, which should not be done since the
   hardware is already unitialized.
3. The devres subsystem sets ops to NULL.

Even though the fix is wrong I feel that it might put the rwsem into
question.

I'm just thinking that maybe there could be a release callback in
tpm_class_ops that could be called by tpm_del_char_device(). There can't
be clients for the chip at that point so no synchronization mechanism
is needed.

>    Jason

/Jarkko

Re: [PATCH] tpm: fix crash in tpm_tis

[Jarkko Sakkinen]

On Mon, Apr 11, 2016 at 11:41:24AM +0300, Jarkko Sakkinen wrote:
> On Thu, Apr 07, 2016 at 07:36:54AM -0700, Jason Gunthorpe wrote:
> >    I will have to look closer after the conference, but this does not look
> >    right.
> > 
> >    I vaguely recall commenting on this before. Move the shutdown into the
> >    core code to fix it.
> 
> This fix that I sent is not the right way to do it.
> 
> One example scenario:
> 
> 1. TIS driver gets detached, which causes tpm_tis_remove() to be called.
> 2. Some in-kernel subsystem uses TPM, which should not be done since the
>    hardware is already unitialized.
> 3. The devres subsystem sets ops to NULL.
> 
> Even though the fix is wrong I feel that it might put the rwsem into
> question.
> 
> I'm just thinking that maybe there could be a release callback in
> tpm_class_ops that could be called by tpm_del_char_device(). There can't
> be clients for the chip at that point so no synchronization mechanism
> is needed.

As a fix for this regression moving shutdown to tmp_chip_unregister() does
make more sense since the patch is already merged to next. Lets not get
stuck into locking discussion...

/Jarkko