linux-kernel - Re: [PATCH] fbmem: don't allow too huge resolutions

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Date:   Tue, 31 Aug 2021 18:20:09 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        syzbot <syzbot+04168c8063cfdde1db5e@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Colin King <colin.king@...onical.com>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] fbmem: don't allow too huge resolutions

On Tue, Aug 31, 2021 at 5:24 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> On 2021/08/31 15:48, Geert Uytterhoeven wrote:
> > Furthermore, this restricts the virtual frame buffer size on 64-bit,
> > too, while graphics cards can have much more than 4 GiB of RAM.
>
> Excuse me, but do you mean that some hardware allows allocating more than
> UINT_MAX bytes of memory for kernel frame buffer drivers?
>
> > IMHO that should be fixed in vga16fb, too.
>
> According to https://elixir.bootlin.com/linux/v5.14/A/ident/fb_check_var ,
> there are 89 files. Randomly picking up drivers/video/fbdev/udlfb.c as
> an example. dlfb_is_valid_mode() from dlfb_ops_check_var() is doing
>
>   if (mode->xres * mode->yres > dlfb->sku_pixel_limit)
>     return 0;
>   return 1;
>
> where max dlfb->sku_pixel_limit seems to be 2048 * 1152 but I think we need
> same overflow check. I want to avoid patching individual modules if possible.
> That depends on whether some hardware needs to allocate more than UINT_MAX
> bytes of memory.

Yeah basic input validation makes no sense to push into each driver.
That's just asking that most of the fbdev drivers will never be fixed.

Same for not-so-basic input validation, if there's no driver that
actually needs the flexibility (like the virtual vs physical size
thing that's floating around maybe).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch