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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Mon, 17 May 2021 15:10:19 +0200
Message-ID: <CAKMK7uFfuV1GH7rvTwLCovnWvd0hYXPGbAyYELoX9PqZn3u2gQ@mail.gmail.com>
Subject: Re: [PATCH] video: fbdev: vga16fb: fix OOB write in vga16fb_imageblit()
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        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>,
        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 Mon, May 17, 2021 at 3:07 PM Daniel Vetter <daniel@ffwll.ch> wrote:
>
> On Fri, May 14, 2021 at 10:33 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Fri, May 14, 2021 at 1:25 PM Maciej W. Rozycki <macro@orcam.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,
>
> Just an aside: I think with fbdev drivers this would go boom, because
> you'd have fbcon interferring with a direct /dev/fb/* user.

Boom here means a bit of screen corruption, because fbcon overdraws
your X sessions. Fixed by the next redraw of X.

> But if your fbdev driver is actually a drm modeset driver, then we
> have additional limitations: If the userspace accesses the display
> through /dev/dri/card0, then the kernel blocks all access through
> /dev/fb/* (including fbcon) to the actual display (it only goes into
> the buffer used for fbdev emulation). And everything would be fine.
>
> Also generally you'd get away with this even in problematic cases,
> since usually you resize your console when looking at it, not when X
> or something else is using your fbdev direct access.
>
> The one thing that's left out here a bit in the cold is userspace
> modeset drivers in X. Those would get hosed. But also, we stopped
> supporting those in at least i915/amd/radeon/nouveau drivers,
> automatically falling back to the fbdev stuff in most cases (with or
> without the drm drivers underneath that), and no one screamed. So
> probably not many users left.

This one could lead to incosistent hw state, which would be worse.

> So I /think/ we could wager this, if it's the least intrusive fix from
> the kernel pov. But it has some risks that we need to revert again if
> we break some of the really old use-cases here.

Cheers, Daniel

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


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