Re: [edk2-devel] [PATCH 1/1] OvmfPkg/QemuVideoDxe: purge VbeShim

["Ard Biesheuvel" <ardb@kernel.org>]

On Tue, 13 Feb 2024 at 15:55, Pedro Falcato <pedro.falcato@gmail.com> wrote:
>
> On Tue, Feb 13, 2024 at 2:02 PM Gerd Hoffmann <kraxel@redhat.com> wrote:
> >
> > On Tue, Feb 13, 2024 at 11:44:17AM +0100, Laszlo Ersek wrote:
> > > Hi Gerd,
> > >
> > > On 2/13/24 09:59, Gerd Hoffmann wrote:
> > > > Removing CSM support broke VbeShim.
> > > >
> > > > I see ASSERTS being triggered, both in vbeshim code and elsewhere.
> > > > Didn't investigate in detail, but apparently the real mode address space
> > > > is not considered being special any more, which invalidates some
> > > > assumptions of the vbeshim code.
> > > >
> > > > The guest os which depends on vbeshim for video support is -- according
> > > > to the comments -- Windows 2008 R2.  Which went EOL in January 2020,
> > > > more than four years ago.
> > > >
> > > > Time to retire VbeShim.  RIP.
> > >
> > > I don't understand this, and/or disagree.
> > >
> > > (1) Please collect more information about the ASSERT()s (the failure mode you are seeing).
> > > Because, (a) in the code that's being removed in this patch, I don't see any ASSERT() that should fire, and (b) I cannot reproduce the issue. (In fact my log contains "InstallVbeShim: VBE shim installed".)
> >
> > Fair enough.  The ASSERT() I see firing is this:
> >
> > ASSERT /home/kraxel/projects/edk2/OvmfPkg/QemuVideoDxe/VbeShim.c(142): Int0x10->Segment == 0x0000
> >
> > It reproduces for me when booting with a bunch of hot-pluggable CPUs (qemu -smp 2,maxcpus=32).
> >
> > I bisected this, and it seems indeed totally unrelated to CSM removal:
> >
> >    725acd0b9cc047b20a1b9379e46f05d3ebcf485e is the first bad commit
> >    commit 725acd0b9cc047b20a1b9379e46f05d3ebcf485e
> >    Author: Dun Tan <dun.tan@intel.com>
> >    Date:   Fri Dec 1 10:53:32 2023 +0800
> >
> >        UefiCpuPkg: Avoid assuming only one smmbasehob
> >
> > New theory: This is a NULL pointer dereference corrupting the zero page.
>
> Tangentially related: Maybe it's time to unmap the NULL page? :)
>


That would break many 3rd party GPU and network drivers.


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