Re: [PATCH v2 1/5] tpm: introduce tpm_pcr_algorithms()

[Jarkko Sakkinen]

On Tue, May 30, 2017 at 10:44:50AM +0200, Roberto Sassu wrote:
> On 5/24/2017 7:35 PM, Jarkko Sakkinen wrote:
> > On Mon, May 22, 2017 at 11:07:54AM +0200, Roberto Sassu wrote:
> > > On 5/20/2017 3:18 PM, Jarkko Sakkinen wrote:
> > > > On Wed, May 17, 2017 at 10:42:35AM +0200, Roberto Sassu wrote:
> > > > > On 5/15/2017 3:18 PM, Roberto Sassu wrote:
> > > > > > 
> > > > > > 
> > > > > > On 5/15/2017 12:36 PM, Jarkko Sakkinen wrote:
> > > > > > > On Fri, May 05, 2017 at 04:21:48PM +0200, Roberto Sassu wrote:
> > > > > > > > This function allows TPM users to know which algorithms the TPM
> > > > > > > > supports.
> > > > > > > > It stores the algorithms in a static array of 'enum tpm2_algorithms',
> > > > > > > > allocated by the caller. If the array is not large enough, the function
> > > > > > > > returns an error. Otherwise, it returns the number of algorithms written
> > > > > > > > to the array. If the TPM version is 1.2, the function writes
> > > > > > > > TPM2_ALG_SHA1
> > > > > > > > to first element of the array.
> > > > > > > > 
> > > > > > > > Writing the algorithm also for TPM 1.2 has the advantage that callers
> > > > > > > > can use the API, tpm_pcr_algorithms() and tpm_pcr_extend(), regardless
> > > > > > > > of the TPM version.
> > > > > > > > 
> > > > > > > > A minor change added to this patch was to make available the size of
> > > > > > > > the active_banks array, member of the tpm_chip structure, outside
> > > > > > > > the TPM driver. The array size (TPM_ACTIVE_BANKS_MAX) has been exported
> > > > > > > > to include/linux/tpm.h.
> > > > > > > > 
> > > > > > > > With this information, callers of tpm_pcr_algorithms() can provide
> > > > > > > > a static array with enough room for all the algorithms, instead
> > > > > > > > of receiving the pointer of a dynamic array that they have to free
> > > > > > > > later.
> > > > > > > > 
> > > > > > > > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > > > > > > ---
> > > > > > > > v2
> > > > > > > > 
> > > > > > > > - tpm_pcr_algorithms() returns supported algorithms also for TPM 1.2
> > > > > > > > 
> > > > > > > >  drivers/char/tpm/tpm-interface.c | 46
> > > > > > > > ++++++++++++++++++++++++++++++++++++++++
> > > > > > > >  drivers/char/tpm/tpm.h           | 13 +-----------
> > > > > > > >  include/linux/tpm.h              | 19 +++++++++++++++++
> > > > > > > >  3 files changed, 66 insertions(+), 12 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/drivers/char/tpm/tpm-interface.c
> > > > > > > > b/drivers/char/tpm/tpm-interface.c
> > > > > > > > index 4ed08ab..b90de3d 100644
> > > > > > > > --- a/drivers/char/tpm/tpm-interface.c
> > > > > > > > +++ b/drivers/char/tpm/tpm-interface.c
> > > > > > > > @@ -911,6 +911,52 @@ int tpm_pcr_extend(u32 chip_num, int pcr_idx,
> > > > > > > > const u8 *hash)
> > > > > > > >  EXPORT_SYMBOL_GPL(tpm_pcr_extend);
> > > > > > > > 
> > > > > > > >  /**
> > > > > > > > + * tpm_pcr_algorithms - get TPM IDs of active PCR banks algorithms
> > > > > > > 
> > > > > > > The grammar is incorrect here I believe. Should rather be
> > > > > > > 
> > > > > > >   "algorithms of the active PCR banks"
> > > > > > > 
> > > > > > > And there is no such thing as "TPM ID".
> > > > > > > 
> > > > > > > > + * @chip_num:    tpm idx # or ANY
> > > > > > > > + * @count:    # of items in algorithms
> > > > > > > > + * @algorithms:    array of TPM IDs
> > > > > > > > + *
> > > > > > > > + * Returns < 0 on error, and the number of active PCR banks on success.
> > > > > > > > + *
> > > > > > > > + * TPM 1.2 has always one SHA1 bank.
> > > > > > > > + */
> > > > > > > > +int tpm_pcr_algorithms(u32 chip_num, int count,
> > > > > > > > +               enum tpm2_algorithms *algorithms)
> > > > > > >                        unsigned int
> > > > > > > 
> > > > > > > In addition the function name is not too greatg,
> > > > > > > 
> > > > > > > Your syntax for return value is not correct. In addition after
> > > > > > > describing the return value there should not be anything. You should
> > > > > > > study
> > > > > > > 
> > > > > > > https://www.kernel.org/doc/Documentation/kernel-doc-nano-HOWTO.txt
> > > > > > > 
> > > > > > > Better name for the function would be tpm_get_pcr_algorithms().
> > > > > > > 
> > > > > > > > +{
> > > > > > > > +    struct tpm_chip *chip;
> > > > > > > > +    int i;
> > > > > > > > +
> > > > > > > > +    if (count <= 0 || algorithms == NULL)
> > > > > > > > +        return -EINVAL;
> > > > > > > 
> > > > > > > Is there a legal case where caller would pass these values? Now it
> > > > > > > looks like that there is.
> > > > > > > 
> > > > > > > 'count' should unsigned int and zero should be a legal value for
> > > > > > > count.
> > > > > > 
> > > > > > I wanted to avoid that a negative value returned by tpm_pcr_algorithms()
> > > > > > is passed to tpm_pcr_extend() as unsigned int.
> > > > > > 
> > > > > > 
> > > > > > > That said I think the whole design is wrong as you could calculate
> > > > > > > array for algs only one time and pass a const reference to it on
> > > > > > > request.
> > > > > > 
> > > > > > Ok. If I understood it correctly, you are saying to pass a const
> > > > > > reference of chip->active_banks. Then, I should also:
> > > > > 
> > > > > This is not a viable option. chip could be freed and the reference
> > > > > becomes invalid, without increasing the reference count.
> > > > > 
> > > > > Did you think about something different?
> > > > > 
> > > > > Thanks
> > > > > 
> > > > > Roberto
> > > > 
> > > > Two alternatives come in mind.
> > > > 
> > > > 1. add tpm_put to linclude/linux/tpm.h
> > > > 2. take put_ops away from the implementation
> > > > 3. add documentation that tpm_put needs to be called
> > > > 
> > > > or acceptable alternative would memcpy in the implementation
> > > > 
> > > > In any case, you should probably drop completely 'count' and have
> > > > function for getting the number of active banks.
> > > 
> > > Having a variable number, makes things more complicated.
> > > In the IMA patch set for multiple template digests,
> > > the array of algorithms is stored inside a structure
> > > called digest descriptor. The array size must be fixed,
> > > to avoid VLAIS (which is not supported by clang).
> > 
> > What is VLAI?
> 
> Variable-length arrays in structures.
> 
> 
> > You can always allocate it from heap.
> 
> The algorithms to be used by IMA are set in a __setup function.
> 
> 
> > > Since the size of tpm_chip->active_banks is a small number,
> > > we can drop count, and assume that the size of the array
> > > passed to tpm_pcr_algorithms() always matches the size
> > > of the active_banks array.
> > > 
> > > Roberto
> > 
> > I don't understand what you mean.
> 
> I meant that callers of tpm_pcr_algorithms() can always pass
> an array whose size is the same of that of tpm_chip->active_banks.
> Since the array size is known, passing count is not necessary.
> 
> Roberto

OK, got it.

/Jarkko