Re: [PATCH] tpm: fix a race condition in tpm2_unseal_trusted()

Jarkko Sakkinen
2016-08-29T15:25:29Z

On Sun, Aug 28, 2016 at 12:51:49PM -0600, Jason Gunthorpe wrote:
> On Sun, Aug 28, 2016 at 08:36:52AM +0200, Jarkko Sakkinen wrote:
> >
> > @@ -576,7 +576,8 @@ static int tpm2_load(struct tpm_chip *chip,
> > goto out;
> > }
> >
> > - rc = tpm_transmit_cmd(chip, buf.data, PAGE_SIZE, "loading blob");
> > + rc = tpm_transmit_cmd(chip, buf.data, PAGE_SIZE, TPM_TRANSMIT_UNLOCKED,
> > + "loading blob");
>
> I still don't like this, required mutex's should not be split outside the
> function that needs them without more a more obvious indication:
>
> > + mutex_lock(&chip->tpm_mutex);
> > rc = tpm2_load(chip, payload, options, &blob_handle);
> > if (rc)
> > - return rc;
>
> I recommend you stick with the idiom and do this:
>
> mutex_lock(&chip->tpm_mutex);
> rc = tpm2_load(chip, payload, options, &blob_handle, TPM_TRANSMIT_UNLOCKED);
>
> Which makes it easy to see we are doing it right everywhere.

Why consume stack for unnecessary stuff? This is a static function. For
me this sounds like cutting hairs really.

One thing that would improve readability would be to rename internal
functions tpm2_load and tpm2_unseal to tpm2_load_cmd and tpm2_unseal_cmd
in order to underline that they are command wrappers and not to mix with
tpm2_unseal_trusted().

I've been thinking to move that kind of convetion at least with TPM2
specific stuff when a function is clearly a wrapper. It kind of
documents the call path. I.e. if function satifies a constraint that
it prepares a command blob, calls tpm_transmit_cmd once and then
returns results to the caller this would be the naming convention.

> Jason

/Jarkko