Re: [PATCH] fbdev: potential information leak in do_fb_ioctl()

From: Arnd Bergmann <arnd@arndb.de>
To: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	Joe Perches <joe@perches.com>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	Andrea Righi <righi.andrea@gmail.com>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Sam Ravnborg <sam@ravnborg.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Peter Rosin <peda@axentia.se>, Gerd Hoffmann <kraxel@redhat.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	kernel-janitors@vger.kernel.org, security@kernel.org,
	Kees Cook <keescook@chromium.org>,
	Julia Lawall <Julia.Lawall@lip6.fr>
Subject: Re: [PATCH] fbdev: potential information leak in do_fb_ioctl()
Date: Mon, 13 Jan 2020 13:49:30 +0100	[thread overview]
Message-ID: <CAK8P3a2uLm9pJtx42qDXJSdD71-dVW6+iDcRAnEB85Ajak-HLw@mail.gmail.com> (raw)
In-Reply-To: <fd4e6f01-074b-def7-7ffb-9a9197930c31@samsung.com>

On Fri, Jan 3, 2020 at 2:09 PM Bartlomiej Zolnierkiewicz
<b.zolnierkie@samsung.com> wrote:
> On 10/29/19 8:02 PM, Eric W. Biederman wrote:
> >
> > The goal is to avoid memory that has values of the previous users of
> > that memory region from leaking to userspace.  Which depending on who
> > the previous user of that memory region is could tell userspace
> > information about what the kernel is doing that it should not be allowed
> > to find out.
> >
> > I tried to trace through where "info" and thus presumably "info->fix" is
> > coming from and only made it as far as  register_framebuffer.  Given
>
> "info" (and thus "info->fix") comes from framebuffer_alloc() (which is
> called by fbdev device drivers prior to registering "info" with
> register_framebuffer()). framebuffer_alloc() does kzalloc() on "info".
>
> Therefore shouldn't memcpy() (as suggested by Jeo Perches) be enough?

Is it guaranteed that all drivers call framebuffer_alloc() rather than
open-coding it somewhere?

Here is a list of all files that call register_framebuffer() without first
calling framebuffer_alloc:

$ git grep -wl register_framebuffer | xargs grep -L framebuffer_alloc
Documentation/fb/framebuffer.rst
drivers/media/pci/ivtv/ivtvfb.c
drivers/media/platform/vivid/vivid-osd.c
drivers/video/fbdev/68328fb.c
drivers/video/fbdev/acornfb.c
drivers/video/fbdev/amba-clcd.c
drivers/video/fbdev/atafb.c
drivers/video/fbdev/au1100fb.c
drivers/video/fbdev/controlfb.c
drivers/video/fbdev/core/fbmem.c
drivers/video/fbdev/cyber2000fb.c
drivers/video/fbdev/fsl-diu-fb.c
drivers/video/fbdev/g364fb.c
drivers/video/fbdev/goldfishfb.c
drivers/video/fbdev/hpfb.c
drivers/video/fbdev/macfb.c
drivers/video/fbdev/matrox/matroxfb_base.c
drivers/video/fbdev/matrox/matroxfb_crtc2.c
drivers/video/fbdev/maxinefb.c
drivers/video/fbdev/ocfb.c
drivers/video/fbdev/pxafb.c
drivers/video/fbdev/sa1100fb.c
drivers/video/fbdev/stifb.c
drivers/video/fbdev/valkyriefb.c
drivers/video/fbdev/vermilion/vermilion.c
drivers/video/fbdev/vt8500lcdfb.c
drivers/video/fbdev/wm8505fb.c
drivers/video/fbdev/xilinxfb.c

It's possible (even likely, the ones I looked at are fine) that they
all correctly
zero out the fb_info structure first, but it seems hard to guarantee, so
Eric's suggestion would possibly still be the safer choice.

      Arnd