Re: [PATCH] drm: Fix an unwanted master inheritance

[Thomas Hellstrom <thellstrom@vmware.com>]

On 12/01/2015 11:57 AM, Emil Velikov wrote:
> Hi Thomas,
>
> Something doesn't feel quite right, please read on.
>
> On 30 November 2015 at 12:44, Thomas Hellstrom <thellstrom@vmware.com> wrote:
>> A client calling drmSetMaster() using a file descriptor that was opened
>> when another client was master would inherit the latter client's master
>> object and all it's authenticated clients.
>>
>> This is unwanted behaviour, and when this happens, instead allocate a
>> brand new master object for the client calling drmSetMaster().
>>
>> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
>> ---
>>  drivers/gpu/drm/drm_drv.c  | 12 +++++++
>>  drivers/gpu/drm/drm_fops.c | 80 ++++++++++++++++++++++++++++++----------------
>>  include/drm/drmP.h         |  6 ++++
>>  3 files changed, 70 insertions(+), 28 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>> index 9362609..1f072ba 100644
>> --- a/drivers/gpu/drm/drm_drv.c
>> +++ b/drivers/gpu/drm/drm_drv.c
>> @@ -160,6 +160,18 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
>>                 goto out_unlock;
>>         }
>>
>> +       if (!file_priv->allowed_master) {
>> +               struct drm_master *saved_master = file_priv->master;
>> +
>> +               ret = drm_new_set_master(dev, file_priv);
>> +               if (ret)
>> +                       file_priv->master = saved_master;
> Imho this shouldn'e belong here but in drm_new_set_master() - i.e. it
> should unwind things on error. Similarly, although we restore the old
> drm_master (below), we still have is_master, allowed_master and
> authenticated set. Thus one can reuse the elevated credentials (is
> this the right terminology?) despite that the ioctl has failed.
>
>> +               else
>> +                       drm_master_put(&saved_master);
>> +
>> +               goto out_unlock;
>> +       }
>> +
>>         file_priv->minor->master = drm_master_get(file_priv->master);
>>         file_priv->is_master = 1;
>>         if (dev->driver->master_set) {
>> diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
>> index c59ce4d..4b5c11c 100644
>> --- a/drivers/gpu/drm/drm_fops.c
>> +++ b/drivers/gpu/drm/drm_fops.c
>> @@ -126,6 +126,56 @@ static int drm_cpu_valid(void)
>>  }
>>
>>  /**
>> + * drm_new_set_master - Allocate a new master object and become master for the
>> + * associated master realm.
>> + *
>> + * @dev: The associated device.
>> + * @fpriv: File private identifying the client.
>> + *
>> + * This function must be called with dev::struct_mutex held. Returns negative
>> + * error code on failure, zero on success.
>> + */
>> +int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
>> +{
>> +       int ret;
>> +
>> +       lockdep_assert_held_once(&dev->master_mutex);
>> +       /* create a new master */
>> +       fpriv->minor->master = drm_master_create(fpriv->minor);
>> +       if (!fpriv->minor->master)
>> +               return -ENOMEM;
>> +
>> +       fpriv->is_master = 1;
>> +       fpriv->allowed_master = 1;
>> +
>> +       /* take another reference for the copy in the local file priv */
>> +       fpriv->master = drm_master_get(fpriv->minor->master);
>> +
>> +       fpriv->authenticated = 1;
>> +
>> +       if (dev->driver->master_create) {
>> +               ret = dev->driver->master_create(dev, fpriv->master);
>> +               if (ret) {
>> +                       /* drop both references if this fails */
>> +                       drm_master_put(&fpriv->minor->master);
>> +                       drm_master_put(&fpriv->master);
>> +                       return ret;
>> +               }
>> +       }
>> +       if (dev->driver->master_set) {
>> +               ret = dev->driver->master_set(dev, fpriv, true);
>> +               if (ret) {
> Afaics both of these callbacks are available from legacy(UMS) drivers
> aren't they ? With the radeon UMS removal patches in the works, this
> leaves vmwgfx.
>
> Either way, perhaps we should set is_master, allowed_master and
> authenticated after these two ? Or alternatively restore the original
> values on error.
>
> Did I miss something or the above sounds about right ?
It does. I'll address this and resend.

Thanks for reviewing!

/Thomas



>
> Regards,
> Emil
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