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

[Rajiv Andrade]

On 20-12-2011 17:39, Tim Gardner wrote:
> 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
diff --git a/drivers/char/tpm/tpm.c b/drivers/char/tpm/tpm.c
index 6a8771f..6a37212b 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;
@@ -1223,6 +1222,7 @@ ssize_t tpm_read(struct file *file, char __user *buf,
                mutex_unlock(&chip->buffer_mutex);
        }  
+       atomic_set(&chip->data_pending, 0);
        return ret_size;
 }

If we reset chip->data_pending after the buffer was copied to userspace,
it's guaranteed that tpm_write() won't touch such buffer before tpm_read()
handles it, given it polls chip->data_pending first.

Rajiv


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