linux-kernel - Re: [PATCH] fbdev: potential information leak in do_fb

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Message-ID: <CAK8P3a2uLm9pJtx42qDXJSdD71-dVW6+iDcRAnEB85Ajak-HLw@mail.gmail.com>
Date:   Mon, 13 Jan 2020 13:49:30 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Joe Perches <joe@...ches.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Andrea Righi <righi.andrea@...il.com>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        Sam Ravnborg <sam@...nborg.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Peter Rosin <peda@...ntia.se>,
        Gerd Hoffmann <kraxel@...hat.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org, security@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Julia Lawall <Julia.Lawall@...6.fr>
Subject: Re: [PATCH] fbdev: potential information leak in do_fb_ioctl()

On Fri, Jan 3, 2020 at 2:09 PM Bartlomiej Zolnierkiewicz
<b.zolnierkie@...sung.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