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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 14 May 2021 10:37:47 -0700
Message-ID: <CAHk-=whN=zrJ8BOLaGsJDH8d3sXLEoXA2DVsCq40OAUc1npOOw@mail.gmail.com>
Subject: Re: [PATCH] video: fbdev: vga16fb: fix OOB write in vga16fb_imageblit()
To:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Cc:     dri-devel <dri-devel@lists.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Daniel Vetter <daniel@ffwll.ch>,
        syzbot <syzbot+1f29e126cf461c4de3b3@syzkaller.appspotmail.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Colin King <colin.king@canonical.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jani Nikula <jani.nikula@intel.com>,
        Jiri Slaby <jirislaby@kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        "Antonino A. Daplas" <adaplas@gmail.com>
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On Fri, May 14, 2021 at 10:29 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> So why not just say "that clearly already doesn't work, so make it
> explicitly not permitted"?

IOW, something like this would seem fairly simple and straightforward:

  diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
  index 01645e87b3d5..f24e627b7402 100644
  --- a/drivers/tty/vt/vt.c
  +++ b/drivers/tty/vt/vt.c
  @@ -1171,8 +1171,13 @@ static inline int resize_screen(struct
vc_data *vc, int width, int height,
          /* Resizes the resolution of the display adapater */
          int err = 0;

  -       if (vc->vc_mode != KD_GRAPHICS && vc->vc_sw->con_resize)
  +       if (vc->vc_sw->con_resize) {
  +               // If we have a resize function but are in KD_GRAPHICS mode,
  +               // we can't actually do a resize and need to error out.
  +               if (vc->vc_mode == KD_GRAPHICS)
  +                       return -EINVAL;
                  err = vc->vc_sw->con_resize(vc, width, height, user);
  +       }

          return err;
   }

not tested, but it looks ObviouslyCorrect(tm), and since we know the
old case didn't work right, it seems very safe to do.

           Linus