linux-kernel - Re: Indirect call in vesafb driver

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Message-ID: <20190320082616.twns6kjvgapdg4lq@sirius.home.kraxel.org>
Date:   Wed, 20 Mar 2019 09:26:16 +0100
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Alexander Pateenok <pateenoc@...il.com>,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: Indirect call in vesafb driver

On Tue, Mar 19, 2019 at 04:46:51PM +0000, Alan Cox wrote:
> On Wed, 13 Mar 2019 17:54:18 +0300
> Alexander Pateenok <pateenoc@...il.com> wrote:
> 
> > Hi,
> > 
> > There're several indirect calls in inline assembly in vesafb driver
> > (drivers/video/fbdev/vesafb.c), and these calls cannot be automatically
> > changed to retpolines. It's in vesafb_pan_display():
> > 
> >    73        __asm__ __volatile__(
> >    74                "call *(%%edi)"
> > 
> > and in vesa_setpalette():
> > 
> >   113        __asm__ __volatile__(
> >   114         "call *(%%esi)"
> > 
> > Is there need to use CALL_NOSPEC ?
> 
> Vesafb is from the time on the dinosaurs but yes any vesa bios code will
> not be speculatively hardened. I'd also doubt anyone is actually using
> vesafb in the first place but it should use nospec

Note that even when using vesafb the display panning is disabled by
default, and vesa_setpalette() is only used with 8bit depth (256 color
mode).

Also note that only 32bit builds will try to call vesa bios code ...

cheers,
  Gerd