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

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

[Steve Cohen]

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@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;
 }
 
 /**
-- 
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a Linux Foundation Collaborative Project

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

[Greg KH]

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@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?

thanks,

greg k-h
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Re: [PATCH V2] drm: hold gem reference until object is no longer accessed

[daniel]

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@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.

Also yes a changelog, somewhere, for next time around.
-Daniel


> 
> thanks,
> 
> greg k-h

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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Re: [Freedreno] [PATCH V2] drm: hold gem reference until object is no longer accessed

[cohens]

On 2020-07-27 16:11, daniel@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@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
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Re: [Freedreno] [PATCH V2] drm: hold gem reference until object is no longer accessed

[daniel]

On Mon, Jul 27, 2020 at 05:54:59PM -0400, cohens@codeaurora.org wrote:
> On 2020-07-27 16:11, daniel@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@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.

Uh no, but we need another patch to re-add the kerneldoc you deleted. I
missed that when merging your patch. Also that's kinda what patch
changelogs are for, for blind reviewers like me :-)
-Daniel

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

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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