Messages in this thread | | | Date | Wed, 11 Jan 2012 17:43:31 -0200 | From | Rajiv Andrade <> | Subject | Re: [tpmdd-devel] [PATCH 2/3] TPM: Close data_pending and data_buffer races |
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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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