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SubjectRe: [tpmdd-devel] [PATCH 2/3] TPM: Close data_pending and data_buffer races
On Tue, 27 Dec 2011, Mimi Zohar wrote:

> On Fri, 2011-12-23 at 07:25 -0700, Tim Gardner wrote:
> > On 12/22/2011 01:02 PM, Rajiv Andrade wrote:
>
> <snip>
>
> > > It's inside the mutex region.
> > >
> >
> > Actually, the patch you sent (https://lkml.org/lkml/2011/12/22/257) is
> > _outside_ the mutex area, but I got your drift.
>
> Yes, thanks for pointing it out in your original comments. I haven't
> tested the following patch, but perhaps it will help clarify the updated
> version, that I think Rajiv was describing ...
>

That's exactly it Mimi, thanks for writing down the patch.

> diff --git a/drivers/char/tpm/tpm.c b/drivers/char/tpm/tpm.c
> index 6a8771f..7dafd95 100644
> --- a/drivers/char/tpm/tpm.c
> +++ b/drivers/char/tpm/tpm.c
> @@ -1210,7 +1210,6 @@ ssize_t tpm_read(struct file *file, char __user *buf,
> del_singleshot_timer_sync(&chip->user_read_timer);
> flush_work_sync(&chip->work);
> ret_size = atomic_read(&chip->data_pending);
> - atomic_set(&chip->data_pending, 0);
> if (ret_size > 0) { /* relay data */
> if (size < ret_size)
> ret_size = size;
> @@ -1221,8 +1220,10 @@ ssize_t tpm_read(struct file *file, char __user *buf,
> if (rc)
> ret_size = -EFAULT;
>
> + atomic_set(&chip->data_pending, 0);
> mutex_unlock(&chip->buffer_mutex);
> - }
> + } else if (ret_size < 0)
> + atomic_set(&chip->data_pending, 0);
>
> return ret_size;
> }

Tim, do you still see a race with the changes posted above?

Rajiv

>
> > Even clearing chip->data_pending _inside_ the mutex area, I'm not sure
> > it closes all of the race windows. I think its still possible to race
> > with mod_timer() and del_singleshot_timer_sync(). Therein lies my point,
> > if the exclusion code is not _obviously_ correct for something this
> > simple, then its probably not. I think my patch is the correct approach.
>
> With the above patch, tpm_read() resets the data_pending flag only after
> copying the data to userspace, resolving the original race condition
> described.
>
> > > This would require another fix though. tpm_write() doesn't check
> > > tpm_transmit return code (and it should).
> > > In case it returns an error (< 0), chip->data_pending would remain the
> > > same forever with that change.
> > >
> >
> > This observation is also correct, but not relevant to the exclusion
> > races. It deserves a separate patch.
>
> With the above patch, tpm_read() resets the data_pending flag only if
> set, resolving the other race condition(s).
>
> thanks,
>
> Mimi
>



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