[PATCH] fbmem: pull fbcon_update_vcs() out of fb_set_var()

[Daniel Vetter]

On Thu, Jul 30, 2020 at 12:47 AM Tetsuo Handa
<penguin-kernel at i-love.sakura.ne.jp> wrote:
>
> syzbot is reporting OOB read bug in vc_do_resize() [1] caused by memcpy()
> based on outdated old_{rows,row_size} values, for resize_screen() can
> recurse into vc_do_resize() which changes vc->vc_{cols,rows} that outdates
> old_{rows,row_size} values which were saved before calling resize_screen().
>
> Daniel Vetter explained that resize_screen() should not recurse into
> fbcon_update_vcs() path due to FBINFO_MISC_USEREVENT being still set
> when calling resize_screen().
>
> Instead of masking FBINFO_MISC_USEREVENT before calling fbcon_update_vcs(),
> we can remove FBINFO_MISC_USEREVENT by calling fbcon_update_vcs() only if
> fb_set_var() returned 0. This change assumes that it is harmless to call
> fbcon_update_vcs() when fb_set_var() returned 0 without reaching
> fb_notifier_call_chain().
>
> [1] https://syzkaller.appspot.com/bug?id=c70c88cfd16dcf6e1d3c7f0ab8648b3144b5b25e
>
> Reported-and-tested-by: syzbot <syzbot+c37a14770d51a085a520 at syzkaller.appspotmail.com>
> Suggested-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
> ---
>  drivers/video/fbdev/core/fbmem.c   | 8 ++------
>  drivers/video/fbdev/core/fbsysfs.c | 4 ++--
>  drivers/video/fbdev/ps3fb.c        | 4 ++--
>  include/linux/fb.h                 | 2 --
>  4 files changed, 6 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 30e73ec..da7c88f 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -957,7 +957,6 @@ static int fb_check_caps(struct fb_info *info, struct fb_var_screeninfo *var,
>  int
>  fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>  {
> -       int flags = info->flags;
>         int ret = 0;
>         u32 activate;
>         struct fb_var_screeninfo old_var;
> @@ -1052,9 +1051,6 @@ static int fb_check_caps(struct fb_info *info, struct fb_var_screeninfo *var,
>         event.data = &mode;
>         fb_notifier_call_chain(FB_EVENT_MODE_CHANGE, &event);
>
> -       if (flags & FBINFO_MISC_USEREVENT)
> -               fbcon_update_vcs(info, activate & FB_ACTIVATE_ALL);
> -
>         return 0;
>  }
>  EXPORT_SYMBOL(fb_set_var);
> @@ -1105,9 +1101,9 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>                         return -EFAULT;
>                 console_lock();
>                 lock_fb_info(info);
> -               info->flags |= FBINFO_MISC_USEREVENT;
>                 ret = fb_set_var(info, &var);
> -               info->flags &= ~FBINFO_MISC_USEREVENT;
> +               if (!ret)
> +                       fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
>                 unlock_fb_info(info);
>                 console_unlock();
>                 if (!ret && copy_to_user(argp, &var, sizeof(var)))
> diff --git a/drivers/video/fbdev/core/fbsysfs.c b/drivers/video/fbdev/core/fbsysfs.c
> index d54c88f..65dae05 100644
> --- a/drivers/video/fbdev/core/fbsysfs.c
> +++ b/drivers/video/fbdev/core/fbsysfs.c
> @@ -91,9 +91,9 @@ static int activate(struct fb_info *fb_info, struct fb_var_screeninfo *var)
>
>         var->activate |= FB_ACTIVATE_FORCE;
>         console_lock();
> -       fb_info->flags |= FBINFO_MISC_USEREVENT;
>         err = fb_set_var(fb_info, var);
> -       fb_info->flags &= ~FBINFO_MISC_USEREVENT;
> +       if (!err)
> +               fbcon_update_vcs(fb_info, var->activate & FB_ACTIVATE_ALL);
>         console_unlock();
>         if (err)
>                 return err;
> diff --git a/drivers/video/fbdev/ps3fb.c b/drivers/video/fbdev/ps3fb.c
> index 9df78fb..4b4a99f 100644
> --- a/drivers/video/fbdev/ps3fb.c
> +++ b/drivers/video/fbdev/ps3fb.c
> @@ -824,12 +824,12 @@ static int ps3fb_ioctl(struct fb_info *info, unsigned int cmd,
>                                 var = info->var;
>                                 fb_videomode_to_var(&var, vmode);
>                                 console_lock();
> -                               info->flags |= FBINFO_MISC_USEREVENT;
>                                 /* Force, in case only special bits changed */
>                                 var.activate |= FB_ACTIVATE_FORCE;
>                                 par->new_mode_id = val;
>                                 retval = fb_set_var(info, &var);
> -                               info->flags &= ~FBINFO_MISC_USEREVENT;
> +                               if (!retval)
> +                                       fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);

Patch looks good, except ... does this compile? fbcon_update_vcs is
defined in fbcon.h, and that doesn't seem to be included here ...
Maybe what we want is an fb_set_var_ioctl in fbmem.c so that the fbcon
interaction is a bit better hidden (but that's a bikeshed, feel free
to ignore). Also I have no idea what trickery you need to compile-test
ps3fb, that's why I'm asking :-)
-Daniel

>                                 console_unlock();
>                         }
>                         break;
> diff --git a/include/linux/fb.h b/include/linux/fb.h
> index 3b4b2f0..b11eb02 100644
> --- a/include/linux/fb.h
> +++ b/include/linux/fb.h
> @@ -400,8 +400,6 @@ struct fb_tile_ops {
>  #define FBINFO_HWACCEL_YPAN            0x2000 /* optional */
>  #define FBINFO_HWACCEL_YWRAP           0x4000 /* optional */
>
> -#define FBINFO_MISC_USEREVENT          0x10000 /* event request
> -                                                 from userspace */
>  #define FBINFO_MISC_TILEBLITTING       0x20000 /* use tile blitting */
>
>  /* A driver may set this flag to indicate that it does want a set_par to be
> --
> 1.8.3.1
>
>


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