Re: [PATCH] drm/i915: Fix use after free in lpe_audio_platdev_destroy()

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Apr 12, 2017 at 06:59:55AM +0200, Takashi Iwai wrote:
> On Tue, 11 Apr 2017 23:27:07 +0200,
> Chris Wilson wrote:
> > 
> > On Tue, Apr 11, 2017 at 10:20:56PM +0100, Chris Wilson wrote:
> > > On Tue, Apr 11, 2017 at 11:01:57PM +0200, Takashi Iwai wrote:
> > > > On Tue, 11 Apr 2017 22:41:12 +0200,
> > > > Chris Wilson wrote:
> > > > Oh, this fell into a crack as it was sent just before my vacation.
> > > > 
> > > > About the change:
> > > > 
> > > > > > ---
> > > > > >  drivers/gpu/drm/i915/intel_lpe_audio.c | 8 ++++++--
> > > > > >  1 file changed, 6 insertions(+), 2 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/gpu/drm/i915/intel_lpe_audio.c b/drivers/gpu/drm/i915/intel_lpe_audio.c
> > > > > > index 7a5b41b1c024..32000902a204 100644
> > > > > > --- a/drivers/gpu/drm/i915/intel_lpe_audio.c
> > > > > > +++ b/drivers/gpu/drm/i915/intel_lpe_audio.c
> > > > > > @@ -131,8 +131,12 @@ lpe_audio_platdev_create(struct drm_i915_private *dev_priv)
> > > > > >  
> > > > > >  static void lpe_audio_platdev_destroy(struct drm_i915_private *dev_priv)
> > > > > >  {
> > > > > > -	platform_device_unregister(dev_priv->lpe_audio.platdev);
> > > > > > -	kfree(dev_priv->lpe_audio.platdev->dev.dma_mask);
> > > > > > +	struct platform_device *platdev = dev_priv->lpe_audio.platdev;
> > > > > > +
> > > > > > +	kfree(platdev->dev.dma_mask);
> > > > > > +	platdev->dev.dma_mask = NULL;
> > > > > > +
> > > > > > +	platform_device_unregister(platdev);
> > > > 
> > > > I'm not sure whether it's good idea to fiddle dma_mask bits before the
> > > > unregister call.  Interestingly, this is the only driver that calls
> > > > kfree() for pdev's dma_mask.  Either we do something wrong, or
> > > > everyone forgot this?
> > > 
> > > Afaict, everyone else chose the blissful ignorance strategy and we are
> > > the only fools to try and free the dma_mask.
> > > 
> > > Would you feel more comfortable with:
> > > 
> > > 	void *mask = platdev->dev.dma_mask;
> > > 
> > > 	platform_device_unregister(platdev);
> > > 
> > > 	/* XXX see platform_device_register_full():
> > > 	 * "This memory isn't freed when the device is put."
> > > 	 * It's not clear why it hasn't been fixed in a decade...
> > > 	 */
> > > 	kfree(mask);
> > 
> > Still has the issue that unregister may not the final put, it should but
> > still...
> > 
> > I think we just leak the memory. We are not the owner and so shouldn't
> > be fiddling around trying to free it.
> 
> Agreed, IMO, we should handle better in the platform device side.

Maybe just use dma_coerce_mask_and_coherent() and avoid the stupid
kmalloc()?

-- 
Ville Syrjälä
Intel OTC
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