Re: [PATCH 2/3] TPM: Close data_pending and data_buffer races

[Tim Gardner]

On 12/20/2011 09:38 AM, Rajiv Andrade wrote:
> On 06/12/11 16:29, Tim Gardner wrote:
> > There is a race betwen tpm_read() and tpm_write where both
> > chip->data_pending
> > and chip->data_buffer can be changed by tpm_write() when tpm_read()
> > clears chip->data_pending, but before tpm_read() grabs the mutex.
> >
> > Protect changes to chip->data_pending and chip->data_buffer by expanding
> > the scope of chip->buffer_mutex.
> >
> > Reported-by: Seth Forshee<seth.forshee@xxxxxxxxxxxxx>
> > Cc: Debora Velarde<debora@xxxxxxxxxxxxxxxxxx>
> > Cc: Rajiv Andrade<srajiv@xxxxxxxxxxxxxxxxxx>
> > Cc: Marcel Selhorst<m.selhorst@xxxxxxxxxx>
> > Cc: tpmdd-devel@xxxxxxxxxxxxxxxxxxxxx
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Tim Gardner<tim.gardner@xxxxxxxxxxxxx>
> > ---
> > drivers/char/tpm/tpm.c | 17 +++++++++--------
> > 1 files changed, 9 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/char/tpm/tpm.c b/drivers/char/tpm/tpm.c
> > index b366b34..70bf9e5 100644
> > --- a/drivers/char/tpm/tpm.c
> > +++ b/drivers/char/tpm/tpm.c
> > @@ -1074,12 +1074,15 @@ ssize_t tpm_write(struct file *file, const
> > char __user *buf,
> > struct tpm_chip *chip = file->private_data;
> > size_t in_size = size, out_size;
> >
> > + mutex_lock(&chip->buffer_mutex);
> > +
> > /* cannot perform a write until the read has cleared
> > either via tpm_read or a user_read_timer timeout */
> > - while (atomic_read(&chip->data_pending) != 0)
> > + while (atomic_read(&chip->data_pending) != 0) {
> > + mutex_unlock(&chip->buffer_mutex);
> > msleep(TPM_TIMEOUT);
> > -
> > - mutex_lock(&chip->buffer_mutex);
> > + mutex_lock(&chip->buffer_mutex);
> > + }
> >
> > if (in_size> TPM_BUFSIZE)
> > in_size = TPM_BUFSIZE;
> > @@ -1112,22 +1115,20 @@ 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);
> > + mutex_lock(&chip->buffer_mutex);
> > + ret_size = atomic_xchg(&chip->data_pending, 0);
> > if (ret_size> 0) { /* relay data */
> > ssize_t orig_ret_size = ret_size;
> > if (size< ret_size)
> > ret_size = size;
> >
> > - mutex_lock(&chip->buffer_mutex);
> > rc = copy_to_user(buf, chip->data_buffer, ret_size);
> > memset(chip->data_buffer, 0, orig_ret_size);
> > if (rc)
> > ret_size = -EFAULT;
>
> What about just moving atomic_set(&chip->data_pending, 0); to here?
> If I'm not missing anything, this would be cleaner.
>
> Rajiv

I'm not sure I agree. Moving just that statement doesn't close the race. 
Perhaps you could send me your version of this patch so that its clear 
what you are suggesting.

rtg
--
Tim Gardner tim.gardner@xxxxxxxxxxxxx
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