Re: [PATCH 1/3] integrity: TPM internel kernel interface

Serge E. Hallyn
2008-10-22T16:09:17Z

Quoting Rajiv Andrade (srajiv@linux.vnet.ibm.com):
> On Tue, 2008-10-14 at 17:23 -0500, Serge E. Hallyn wrote:
> > Quoting Mimi Zohar (zohar@linux.vnet.ibm.com):
> > > The internal TPM kernel interface did not protect itself from
> > > the removal of the TPM driver, while being used. We continue
> > > to protect the tpm_chip_list using the driver_lock as before,
> > > and are using an rcu lock to protect readers. The internal TPM
> >
> > I still would like to see this spelled out somewhere - correct me
> > if I'm wrong but none of the patches sent so far have this spelled
> > out in in-line comments, do they?
> >
> > It does look sane:
> >
> > 1. writes to tpm_chip_list are protected by driver_lock
> > 2. readers of the list are protected by rcu
> > 3. chips which are read from the tpm_chip_list, if they
> > are used outside of the rcu_read_lock(), are pinned
> > using get_device(chip->dev) before releasing the
> > rcu_read_lock.
> >
> > Like I say it looks sane, but something like the above summary
> > could stand to be in a comment on top of tpm.c or something.
> >
> No problem, I'll submit a patch containing a proper comment section to
> be applied on top of these, maybe after they get accepted.

Great, thanks.

> > > kernel interface now protects itself from the driver being
> > > removed by incrementing the module reference count.
> > >
> > > Resubmitting integrity-tpm-internal-kernel-interface.patch, which
> > > was previously Signed-off-by Kylene Hall.
> > > Updated per feedback:
> > >
> > > Adds the following support:
> > > - make internal kernel interface to transmit TPM commands global
> > > - adds reading a pcr value
> > > - adds extending a pcr value
> > > - adds lookup the tpm_chip for given chip number and type
> > >
> > > Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
> > > Signed-off-by: Rajiv Andrade <srajiv@linux.vnet.ibm.com>
> >
> > Now there are other, existing callers of tpm_transmit. Are they
> > all protected by sysfs pinning the kobject and thereby the device,
> > for the duration of the call?
> >
>
> They aren't called through sysfs, but are still protected. These new
> functions get chip data consistently by using rcu_read. Then, after
> computing what's intended to be written back to the chip, tpm_transmit
> sends the new data while using tpm_mutex, so both operations are
> performed without the risk of a race condition.

Can you show me where the refcount for dev is incremented (under the
rcu_read_lock), either in sysfs code or tpm code? I'm not finding
it, but it may just be done in some subtle way that I'm glossing over.

thanks,
-serge