Re: [PATCH v2 6/6] OvmfPkg/QemuVideoDxe: Bypass NULL pointer detection during VBE SHIM installing

["Wang, Jian J" <jian.j.wang@intel.com>]

You're right that there's no such need. I just saw that this driver is loaded
before EndOfDxe but missed the fact that it's actually started after that.
So BIT7 of PcdNullPointerDetectionPropertyMask is enough.

And thanks a lot for other feedbacks in another emails, especially for the 
catching of potential attributes overridden issue, which also exists in other 
part of code. 

> -----Original Message-----
> From: Laszlo Ersek [mailto:lersek@redhat.com]
> Sent: Friday, September 22, 2017 11:29 PM
> To: Wang, Jian J <jian.j.wang@intel.com>; edk2-devel@lists.01.org
> Cc: Zeng, Star <star.zeng@intel.com>; Dong, Eric <eric.dong@intel.com>; Yao,
> Jiewen <jiewen.yao@intel.com>; Kinney, Michael D
> <michael.d.kinney@intel.com>; Justen, Jordan L <jordan.l.justen@intel.com>;
> Wolman, Ayellet <ayellet.wolman@intel.com>
> Subject: Re: [PATCH v2 6/6] OvmfPkg/QemuVideoDxe: Bypass NULL pointer
> detection during VBE SHIM installing
> 
> On 09/22/17 13:50, Laszlo Ersek wrote:
> > This patch looks great to me, I would like to request a few small
> > updates:
> >
> > On 09/21/17 07:20, Jian J Wang wrote:
> >> QemuVideoDxe driver will install VBE SHIM into page 0. If NULL pointer
> >
> > (1) please replace the word "install" with "link".
> >
> > The VBE Shim is technically installed into the "real-mode" C segment,
> > only the int 0x10 vector lives in page 0.
> >
> >> detection is enabled, this driver will fail to load. NULL pointer detection
> >> bypassing code is added to prevent such problem during boot.
> >>
> >> Please note that Windows 7 will try to access VBE SHIM during boot if it's
> >> installed, and then cause boot failure. This can be fixed by setting BIT7
> >> of PcdNullPointerDetectionPropertyMask to disable NULL pointer detection
> >> after EndOfDxe. As far as we know, there's no other OSs has such issue.
> >
> > This is not a request, just a comment: I verified the default value in
> > the .dec, and I see it is 0. So there's no need to post an additional
> > patch for the OVMF DSC files, in order to set BIT7.
> 
> Actually, let me take a step back, and re-think the necessity of all
> this work for QemuVideoDxe!
> 
> The facts are:
> 
> (1) The *only* purpose of the VBE Shim is to allow Windows 7 to boot in
> pure UEFI mode (i.e. without a CSM).
> 
> (2) If I understand correctly, you guys have verified that Windows 7
> cannot boot with the page0 protection enabled, *regardless* of what we
> do in QemuVideoDxe. Can you confirm this please?
> 
> With the above in mind, let's consider the effects of the
> "PcdNullPointerDetectionPropertyMask" bits:
> 
> * BIT0 clear:
>   - The page0 protection is completely disabled.
>   - This patch does nothing, in effect.
>   - The VBE Shim works.
>   - Windows 7 boots.
> 
> * BIT0 set, BIT7 also set:
>   - The page0 protection is disabled in the DXE core at the end of DXE.
>   - This patch does nothing, in effect.
>   - The VBE Shim works, because it is a UEFI driver, and it connects its
>     devices (and installs the shim) after End-of-Dxe, at which point
>     page0 protection is no longer in effect.
>   - Windows 7 boots fine, again because it is loaded after End-of-Dxe.
> 
> * BIT0 set, BIT7 clear:
>   - The page0 protection is never disabled until the OS (loader)
>     installs its own page tables.
>   - This patch enables the VBE Shim to work, by temporarily disabling
>     page0 protection.
>   - However, Windows 7 will fail to boot nonetheless, because it cannot
>     cope with page0 protection. (This is fact (2).)
> 
> Now, if you consider fact (1) as well: given that Windows 7 cannot boot
> with page0 protection enabled *anyway*, why mess with the VBE Shim at
> all?
> 
> How about the following patch instead:
> 
> > diff --git a/OvmfPkg/QemuVideoDxe/VbeShim.c
> b/OvmfPkg/QemuVideoDxe/VbeShim.c
> > index e45a08e8873f..8ba5522cde3c 100644
> > --- a/OvmfPkg/QemuVideoDxe/VbeShim.c
> > +++ b/OvmfPkg/QemuVideoDxe/VbeShim.c
> > @@ -75,6 +75,20 @@ InstallVbeShim (
> >    UINTN                Printed;
> >    VBE_MODE_INFO        *VbeModeInfo;
> >
> > +  if ((PcdGet8 (PcdNullPointerDetectionPropertyMask) & (BIT0|BIT7)) == BIT0)
> {
> > +    DEBUG ((
> > +      DEBUG_WARN,
> > +      "%a: page 0 protected, not installing VBE shim\n",
> > +      __FUNCTION__
> > +      ));
> > +    DEBUG ((
> > +      DEBUG_WARN,
> > +      "%a: page 0 protection prevents Windows 7 from booting anyway\n",
> > +      __FUNCTION__
> > +      ));
> > +    return;
> > +  }
> > +
> >    Segment0 = 0x00000;
> >    SegmentC = 0xC0000;
> >    SegmentF = 0xF0000;
> 
> Thanks!
> Laszlo