[Freedreno] [PATCH V2] drm: hold gem reference until object is no longer accessed

[cohens at codeaurora.org]

On 2020-07-27 16:11, daniel at ffwll.ch wrote:
> On Mon, Jul 27, 2020 at 09:55:07PM +0200, Greg KH wrote:
>> On Mon, Jul 20, 2020 at 06:30:50PM -0400, Steve Cohen wrote:
>> > A use-after-free in drm_gem_open_ioctl can happen if the
>> > GEM object handle is closed between the idr lookup and
>> > retrieving the size from said object since a local reference
>> > is not being held at that point. Hold the local reference
>> > while the object can still be accessed to fix this and
>> > plug the potential security hole.
>> >
>> > Signed-off-by: Steve Cohen <cohens at codeaurora.org>
>> > ---
>> >  drivers/gpu/drm/drm_gem.c | 10 ++++------
>> >  1 file changed, 4 insertions(+), 6 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
>> > index 7bf628e..ee2058a 100644
>> > --- a/drivers/gpu/drm/drm_gem.c
>> > +++ b/drivers/gpu/drm/drm_gem.c
>> > @@ -871,9 +871,6 @@ drm_gem_flink_ioctl(struct drm_device *dev, void *data,
>> >   * @file_priv: drm file-private structure
>> >   *
>> >   * Open an object using the global name, returning a handle and the size.
>> > - *
>> > - * This handle (of course) holds a reference to the object, so the object
>> > - * will not go away until the handle is deleted.
>> >   */
>> >  int
>> >  drm_gem_open_ioctl(struct drm_device *dev, void *data,
>> > @@ -898,14 +895,15 @@ drm_gem_open_ioctl(struct drm_device *dev, void *data,
>> >
>> >  	/* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
>> >  	ret = drm_gem_handle_create_tail(file_priv, obj, &handle);
>> > -	drm_gem_object_put_unlocked(obj);
>> >  	if (ret)
>> > -		return ret;
>> > +		goto err;
>> >
>> >  	args->handle = handle;
>> >  	args->size = obj->size;
>> >
>> > -	return 0;
>> > +err:
>> > +	drm_gem_object_put_unlocked(obj);
>> > +	return ret;
>> >  }
>> >
>> >  /**
>> 
>> As this seems to fix an important issue, any reason it wasn't cc: 
>> stable
>> on it so that it gets backported properly?
>> 
>> How about a "Fixes:" tag so that we know what commit id it fixes so we
>> know how far back to backport things?
>> 
>> And a hint to the maintainers that "this is an issue that needs to get
>> into 5.8-final, it shouldn't wait around longer please" would have 
>> also
>> been nice to see :)
>> 
>> And what chagned from v1, aren't you supposed to list that somewhere 
>> in
>> the changelog or below the --- line (never remember what DRM drivers
>> want here...)
>> 
>> Care to send a v3?
> 
> Don't worry, I'm pushing this to drm-misc-fixes now, should still make 
> it
> to 5.8. Plus cc: stable. I didn't bother with Fixes: since I think the 
> bug
> is rather old. Also, worst case you leak 32bit of some kernel memory 
> that
> got reused already (but yeah I know that's often enough to get the foot 
> in
> somewhere nasty and crack the door open).
> 
> I think it fell through cracks because Sam said he'll apply, guess that
> didn't happen.

Sam added his Reviewed-By on V1 with a comment to rename the goto label,
but in V2 I also updated the API documentation and the commit text for
a more complete change and thought he would re-add the tag.

> Also yes a changelog, somewhere, for next time around.

Apologies, it won't happen again. Should I still submit a V3?
It looks like you've got Greg's concerns covered.

-Steve

> -Daniel
> 
> 
>> 
>> thanks,
>> 
>> greg k-h