linux-kernel - Re: [PATCH] drm/prime: Fix use after free in mmap with drm_gem_ttm

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Date:   Wed, 3 Nov 2021 11:12:25 +1100
From:   "Anand K. Mistry" <amistry@...gle.com>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     dri-devel@...ts.freedesktop.org, nartemiev@...gle.com,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/prime: Fix use after free in mmap with drm_gem_ttm_mmap

Any movement on merging this patch? Not urgent on my end (we have this
patch in our tree), but I think other amd users might run into this
UAF.

On Thu, 30 Sept 2021 at 17:21, Thomas Zimmermann <tzimmermann@...e.de> wrote:
>
> Hi
>
> Am 30.09.21 um 01:00 schrieb Anand K Mistry:
> > drm_gem_ttm_mmap() drops a reference to the gem object on success. If
> > the gem object's refcount == 1 on entry to drm_gem_prime_mmap(), that
> > drop will free the gem object, and the subsequent drm_gem_object_get()
> > will be a UAF. Fix by grabbing a reference before calling the mmap
> > helper.
> >
> > This issue was forseen when the reference dropping was adding in
> > commit 9786b65bc61ac ("drm/ttm: fix mmap refcounting"):
> >    "For that to work properly the drm_gem_object_get() call in
> >    drm_gem_ttm_mmap() must be moved so it happens before calling
> >    obj->funcs->mmap(), otherwise the gem refcount would go down
> >    to zero."
> >
> > Signed-off-by: Anand K Mistry <amistry@...gle.com>
>
> Acked-by: Thomas Zimmermann <tzimmermann@...e.de>
>
> This looks fine to me, but it affects many drivers. Let's maybe wait a
> bit if more reviews come it.
>
> Best regards
> Thomas
>
> > ---
> >
> >   drivers/gpu/drm/drm_prime.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> > index 2a54f86856af..e1854fd24bb0 100644
> > --- a/drivers/gpu/drm/drm_prime.c
> > +++ b/drivers/gpu/drm/drm_prime.c
> > @@ -719,11 +719,13 @@ int drm_gem_prime_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> >       if (obj->funcs && obj->funcs->mmap) {
> >               vma->vm_ops = obj->funcs->vm_ops;
> >
> > +             drm_gem_object_get(obj);
> >               ret = obj->funcs->mmap(obj, vma);
> > -             if (ret)
> > +             if (ret) {
> > +                     drm_gem_object_put(obj);
> >                       return ret;
> > +             }
> >               vma->vm_private_data = obj;
> > -             drm_gem_object_get(obj);
> >               return 0;
> >       }
> >
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer

-- 
Anand K. Mistry
Software Engineer
Google Australia