linux-kernel - Re: [PATCH] video: fbdev: vga16fb: fix OOB write in vga16fb

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Date:   Fri, 14 May 2021 13:32:42 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        syzbot <syzbot+1f29e126cf461c4de3b3@...kaller.appspotmail.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Colin King <colin.king@...onical.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jani Nikula <jani.nikula@...el.com>,
        Jiri Slaby <jirislaby@...nel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "Antonino A. Daplas" <adaplas@...il.com>
Subject: Re: [PATCH] video: fbdev: vga16fb: fix OOB write in vga16fb_imageblit()

On Fri, May 14, 2021 at 1:25 PM Maciej W. Rozycki <macro@...am.me.uk> wrote:
>
>  Overall I think it does make sense to resize the text console at any
> time, even if the visible console (VT) chosen is in the graphics mode,

It might make sense, but only if we call the function to update the
low-level data.

Not calling it, and then starting to randomly use the (wrong)
geometry, and just limiting it so that it's all within the buffer -
THAT does not make sense.

So I think your patch is fundamentally wrong. It basically says "let's
use random stale incorrect data, but just make sure that the end
result is still within the allocated buffer".

My patch is at least conceptually sane.

An alternative would be to just remove the "vcmode != KD_GRAPHICS"
check entirely, and always call con_resize() to update the low-level
data, but honestly, that seems very likelty to break something very
fundamentally, since it's not how any of fbcon has ever been tested,

Another alternative would be to just delay the resize to when vcmode
is put back to text mode again. That sounds somewhat reasonable to me,
but it's a pretty big thing.

But no, your patch to just "knowingly use entirely wrong values, then
add a limit check because we know the values are possibly garbage and
not consistent with reality" is simply not acceptable.

              Linus