[PATCH] fbmem: prevent potential use-after-free issues with console_lock()

[Daniel Vetter]

On Thu, 5 Jan 2023 at 11:21, Daniel Vetter <daniel at ffwll.ch> wrote:
>
> Hi Helge
>
> On Mon, 2 Jan 2023 at 16:28, Helge Deller <deller at gmx.de> wrote:
> >
> > On 12/30/22 07:35, Hang Zhang wrote:
> > > In do_fb_ioctl(), user specified "fb_info" can be freed in the callee
> > > fbcon_get_con2fb_map_ioctl() -> set_con2fb_map() ->
> > > con2fb_release_oldinfo(), this free operation is protected by
> > > console_lock() in fbcon_set_con2fb_map_ioctl(), it also results in
> > > the change of certain states such as "minfo->dead" in matroxfb_remove(),
> > > so that it can be checked to avoid use-after-free before the use sites
> > > (e.g., the check at the beginning of matroxfb_ioctl()). However,
> > > the problem is that the use site is not protected by the same locks
> > > as for the free operation, e.g., "default" case in do_fb_ioctl()
> > > can lead to "matroxfb_ioctl()" but it's not protected by console_lock(),
> > > which can invalidate the aforementioned state set and check in a
> > > concurrent setting.
> > >
> > > Prevent the potential use-after-free issues by protecting the "default"
> > > case in do_fb_ioctl() with console_lock(), similarly as for many other
> > > cases like "case FBIOBLANK" and "case FBIOPAN_DISPLAY".
> > >
> > > Signed-off-by: Hang Zhang <zh.nvgt at gmail.com>
> >
> > applied to fbdev git tree.
>
> The patch above makes no sense at all to me:
>
> - fb_info is protected by lock_fb_info and
> - the lifetime of fb_info is protected by the get/put functions
> - yes there's the interaction with con2fb, which is protected by
> console_lock, but the lifetime guarantees are ensured by the device
> removal
> - which means any stuff happening in matroxfb_remove is also not a
> concern here (unless matroxfb completely gets all the device lifetime
> stuff wrong, but it doesn't look like it's any worse than any of the
> other fbdev drivers that we haven't recently fixed up due to the
> takeover issues with firmware drivers

I have also a really hard timing finding the con2fb map use in the
matroxfb ioctl code, but that just might be that I didn't look
carefully enough. Maybe that would shed some light on this.
-Daniel


>
> On the very clear downside this now means we take console_lock for the
> vblank ioctl (which is a device driver extension for reasons, despite
> that it's a standard fbdev ioctl), which is no good at all given how
> console_lock() is a really expensive lock.
>
> Unless I'm massively missing something, can you pls push the revert
> before this lands in Linus' tree?
>
> Thanks, Daniel
>
> > Thanks,
> > Helge
> >
> > > ---
> > >   drivers/video/fbdev/core/fbmem.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> > > index 1e70d8c67653..8b1a1527d18a 100644
> > > --- a/drivers/video/fbdev/core/fbmem.c
> > > +++ b/drivers/video/fbdev/core/fbmem.c
> > > @@ -1182,6 +1182,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
> > >               console_unlock();
> > >               break;
> > >       default:
> > > +             console_lock();
> > >               lock_fb_info(info);
> > >               fb = info->fbops;
> > >               if (fb->fb_ioctl)
> > > @@ -1189,6 +1190,7 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
> > >               else
> > >                       ret = -ENOTTY;
> > >               unlock_fb_info(info);
> > > +             console_unlock();
> > >       }
> > >       return ret;
> > >   }
> >
>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch



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