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

[Thomas Hellstrom <thellstrom@vmware.com>]

On 12/02/2015 04:54 PM, Daniel Vetter wrote:
> On Tue, Dec 01, 2015 at 12:58:13PM +0100, Thomas Hellstrom wrote:
>> 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.
> Just wanted to pull this in and noticed there's still this open. New
> version incoming soon?
>
> Thanks, Daniel
OK. Sent out v2. I'd prefer if this could go through -fixes and I've
also cc'd stable since it fixes a real kernel crash on vmwgfx.

Thomas

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