[Qemu-devel] Regression with windows 7 VMs and VGA CVE-2016-3712 fix (2.6.0 and 2.5.1.1)

[Thomas Lamprecht]

Hi all,

I recently ran into Problems when trying to install some Windows VMs
this was after an update to QEMU 2.5.1.1, the VM shows Windows loading
files for the installation, then the "Starting Windows" screen appears
here it hangs and never continues.

Changing the "-vga" option to cirrus solves this, the installation can
proceed and finish. When changing back to std (or also qxl, vmware) the
installed VM also hangs on the "Starting Windows" screen while qemu
showing a little but no excessive load.

This phenomena appears also with QEMU 2.6.0 but not with 2.6.0-rc4, a
git bisect shows fd3c136b3e1482cd0ec7285d6bc2a3e6a62c38d7 (vga: make
sure vga register setup for vbe stays intact (CVE-2016-3712)) as the
culprit for this regression, as its a fix for a DoS its not an option to
just revert it, I guess.
The (short) bisect log is:

git bisect start
# bad: [bfc766d38e1fae5767d43845c15c79ac8fa6d6af] Update version for v2.6.0 
release
git bisect bad bfc766d38e1fae5767d43845c15c79ac8fa6d6af
# good: [975eb6a547f809608ccb08c221552f666611af25] Update version for 
v2.6.0-rc4 release
git bisect good 975eb6a547f809608ccb08c221552f666611af25
# good: [2068192dcccd8a80dddfcc8df6164cf9c26e0fc4] vga: update vga register 
setup on vbe changes
git bisect good 2068192dcccd8a80dddfcc8df6164cf9c26e0fc4
# bad: [53db932604dfa7bb9241d132e0173894cf54261c] Merge remote-tracking branch 
'remotes/kraxel/tags/pull-vga-20160509-1' into staging
git bisect bad 53db932604dfa7bb9241d132e0173894cf54261c

I could reproduce that with QEMU 2.5.1 and QEMU 2.6 on a Debian derivate
(Promox VE) with 4.4 Kernel and also with QEMU 2.6 on an Arch Linux
System with a 4.5 Kernel, so it should not be host distro depended. Both
machines have Intel x86_64 processors.
The problem should be reproducible with said Versions or a build from
git including the above mentioned commit (fd3c136) by starting a VM with
an Windows 7 ISO, e.g.:

Hanging installation
./x86_64-softmmu/qemu-system-x86_64 -boot d -cdrom win7.iso -m 1024

Working installation:
./x86_64-softmmu/qemu-system-x86_64 -boot d -cdrom win7.iso -m 1024 -vga cirrus

Noteworthy may be that Windows 10 is working, I do not had time to get
other Windows versions and test them, I'll do that as soon as possible.
Various Linux system also seems to work fine, at least I did not ran
into an issue there yet.

I also tried testing with SeaBIOS and OVMF, as initially I had no idea
what broke, both lead to the same result - without the CVE-2016-3712 fix
they both work, with not.
Further, KVM enabled and disabled does not make any difference.

If I can take any further step, e.g. open a bug report at another place
or help with testing I'd glad to do so.

best regards,
Thomas