[PATCH] drm/vgem: fix use-after-free when drm_gem_handle_create() fails

[Eric Biggers]

On Tue, Feb 26, 2019 at 09:01:29PM +0000, Chris Wilson wrote:
> Quoting Eric Biggers (2019-02-26 20:47:26)
> > From: Eric Biggers <ebiggers at google.com>
> > 
> > If drm_gem_handle_create() fails in vgem_gem_create(), then the
> > drm_vgem_gem_object is freed twice: once when the reference is dropped
> > by drm_gem_object_put_unlocked(), and again by __vgem_gem_destroy().
> > 
> > This was hit by syzkaller using fault injection.
> > 
> > Fix it by skipping the second free.
> > 
> > Reported-by: syzbot+e73f2fb5ed5a5df36d33 at syzkaller.appspotmail.com
> > Fixes: 5ba6c9ff961a ("drm/vgem: Fix mmaping")
> 
> That's the wrong fixes line, it's
> Fixes: af33a9190d02 ("drm/vgem: Enable dmabuf import interfaces")
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Laura Abbott <labbott at redhat.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> 
> Sadly I reviewed it so I'm still culpable, but the fix is correct as the
> put purposely frees it on error.
> 

You're right; I misread the code at that commit.
I'll resend with the correct tags.

> > Cc: stable at vger.kernel.org
> > Signed-off-by: Eric Biggers <ebiggers at google.com>
> 
> > ---
> >  drivers/gpu/drm/vgem/vgem_drv.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
> > index 5930facd6d2d8..70646d9da1596 100644
> > --- a/drivers/gpu/drm/vgem/vgem_drv.c
> > +++ b/drivers/gpu/drm/vgem/vgem_drv.c
> > @@ -189,15 +189,13 @@ static struct drm_gem_object *vgem_gem_create(struct drm_device *dev,
> >                 return ERR_CAST(obj);
> >  
> >         ret = drm_gem_handle_create(file, &obj->base, handle);
> > +
> >         drm_gem_object_put_unlocked(&obj->base);
> > +
> 
> The pattern in the other GEM drivers is not to have these extra
> newlines.
> 
> Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> 
> >         if (ret)
> > -               goto err;
> > +               return ERR_PTR(ret);
> >  
> >         return &obj->base;
> > -
> > -err:
> > -       __vgem_gem_destroy(obj);
> > -       return ERR_PTR(ret);
> >  }
> >  
> >  static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev,
> > -- 
> > 2.21.0.rc2.261.ga7da99ff1b-goog
> >