Re: KASAN: use-after-free Read in drm_gem_object_release

From: Daniel Vetter <daniel@ffwll.ch>
To: Dan Carpenter <dan.carpenter@oracle.com>,
	Thomas Zimmermann <tzimmermann@suse.de>
Cc: Rob Clark <robdclark@chromium.org>,
	security@kernel.org, Dave Airlie <airlied@linux.ie>,
	butt3rflyh4ck <butterflyhuangxx@gmail.com>,
	syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	"Wilson, Chris" <chris@chris-wilson.co.uk>,
	Sean Paul <seanpaul@chromium.org>,
	Sam Ravnborg <sam@ravnborg.org>,
	Emil Velikov <emil.velikov@collabora.com>
Subject: Re: KASAN: use-after-free Read in drm_gem_object_release
Date: Mon, 13 Jul 2020 18:12:11 +0200	[thread overview]
Message-ID: <CAKMK7uE1idHW4msihsBSbo8aWf602gqT-Z_BCR-gSJCRfugu=w@mail.gmail.com> (raw)
In-Reply-To: <20200710115240.GI2571@kadam>

Adding Thomas, who's the main author for vram helpers.
-Daniel

On Fri, Jul 10, 2020 at 1:53 PM Dan Carpenter <dan.carpenter@oracle.com> wrote:
>
> On Fri, Jul 10, 2020 at 04:24:03PM +0800, butt3rflyh4ck wrote:
> > I report a bug (in linux-5.8.0-rc4) found by syzkaller.
> >
> > kernel config: https://github.com/butterflyhack/syzkaller-fuzz/blob/master/v5.8.0-rc4.config
> >
> > I test the reproducer and crash too.
> >
> > In the drm_em_vram_t() function,  ttm_bo_init() function call
>          ^^^^^^^^^^^^^
> This a typo.  The function name is drm_gem_vram_init().
>
> > ttm_bo_init_reserved(),
> > the ttm_bo_init_reserved() function  call ttm_bo_put(), it will free
> > gbo->bo that is struct ttm_buffer_object.
> >
> > then, goto the err_drm_gem_object_release lable,
> > drm_gem_object_release() function will free gbo->bo.base, so cause use
> > after free.
> >
>
> There is a third free in drm_gem_vram_create().  This is a triple free
> bug.  The correct place to free this is in drm_gem_vram_create() because
> that's where it was allocated.
>
> This code is quite subtle so I'm not going to attempt to fix it because
> I can't test it.
>
> regards,
> dan carpenter
>


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