Re: [PATCH 4.19] fbmem: add margin check to fb_check_caps()

[Dongliang Mu <mudongliangabcd@gmail.com>]

On Thu, Sep 2, 2021 at 9:15 PM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>
> Hi Dongliang,
>
> On Thu, Sep 2, 2021 at 8:04 AM Dongliang Mu <mudongliangabcd@gmail.com> wrote:
> > [ Upstream commit a49145acfb975d921464b84fe00279f99827d816 ]
>
> Oops, looks like I missed when that one was submitted for review...

This patch cannot directly apply to old stable trees. Maybe that's the reason.

>
> > A fb_ioctl() FBIOPUT_VSCREENINFO call with invalid xres setting
> > or yres setting in struct fb_var_screeninfo will result in a
> > KASAN: vmalloc-out-of-bounds failure in bitfill_aligned() as
> > the margins are being cleared. The margins are cleared in
> > chunks and if the xres setting or yres setting is a value of
> > zero upto the chunk size, the failure will occur.
> >
> > Add a margin check to validate xres and yres settings.
>
> Shouldn't (the caller of) bitfill_aligned() be fixed instead?
> Can this be triggered by e.g. using the mini_4x6 font?

I am sorry. I don't know much detail about this subsystem. I just
found syzkaller can still trigger this bug in linux-4.19.

Combined with the bug information, I found this patch is not merged
into the old kernel trees. So I send this patch rebased on linux-4.19.
Also I have tested it on linux-4.14 and below.

>
> > Note that, this patch needs special handling to backport it to linux
> > kernel 4.19, 4.14, 4.9, 4.4.
> >
> > Signed-off-by: George Kennedy <george.kennedy@oracle.com>
> > Reported-by: syzbot+e5fd3e65515b48c02a30@syzkaller.appspotmail.com
> > Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
> > Cc: Dhaval Giani <dhaval.giani@oracle.com>
> > Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
> > Link: https://patchwork.freedesktop.org/patch/msgid/1594149963-13801-1-git-send-email-george.kennedy@oracle.com
> > Signed-off-by: Sasha Levin <sashal@kernel.org>
> > ---
> >  drivers/video/fbdev/core/fbmem.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> > index 84845275dbef..de04c097d67c 100644
> > --- a/drivers/video/fbdev/core/fbmem.c
> > +++ b/drivers/video/fbdev/core/fbmem.c
> > @@ -991,6 +991,10 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
> >                         goto done;
> >                 }
> >
> > +               /* bitfill_aligned() assumes that it's at least 8x8 */
> > +               if (var->xres < 8 || var->yres < 8)
> > +                       return -EINVAL;
>
> Are you sure there don't exist such small displays (e.g. OLED)?
>
> > +
> >                 ret = info->fbops->fb_check_var(var, info);
> >
> >                 if (ret)
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds