Re: [libvirt] [PATCH] qemu: Put format=raw onto cmd line for SCSI passthrough
On Fri, Oct 12, 2018 at 01:17:42PM +0100, Daniel P. Berrangé wrote:
On Fri, Oct 12, 2018 at 01:14:51PM +0200, Michal Privoznik wrote:
> https://bugzilla.redhat.com/show_bug.cgi?id=1632833
>
> When doing a SCSI passthrough we don't put format= onto the
> command line. This causes qemu to probe the format automatically
> which ends up in a warning in the domain log and possible qemu
> disabling writes to the first block (according to the warning
> message).
If the warning message is correct, this should have been reported
as a security bug to libvirt and given a CVE.
On the other hand if the warning from QEMU isn't correct, then
QEMU shouldn't have printed the warning about it being dangerous.
So something is missing here either way.
I used 'modprobe scsi_debug' to create a fake SCSI device
which appears as "scsi_host6" in my host OS, and has a
/dev/sg3 and /dev/sdd device nodes.
When I use this XML:
<hostdev mode='subsystem' type='scsi' managed='no'
sgio='filtered' rawio='yes'>
<source>
<adapter name='scsi_host6'/>
<address bus='0' target='0' unit='0'/>
</source>
<alias name='hostdev0'/>
<address type='drive' controller='0' bus='0'
target='0' unit='0'/>
</hostdev>
libvirt spawns QEMU pointing to /dev/sg3
-drive file=/dev/sg3,if=none,id=drive-hostdev0
Inside the guest, I can successfully run
qemu-img create -f qcow2 /dev/sda 4M
and on the host OS this now appears visible in the host
# qemu-img info /dev/sdd
image: /dev/sdd
file format: qcow2
virtual size: 6.0M (6291456 bytes)
disk size: 0
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false
this will *not* have an effect on this QEMU binary if
it reboots, however, because QEMU does not appear to
actually trigger format probing when used via the
"scsi-generic" device type.
Attempting to run qemu-img info against the /dev/sg3
device in the host fails as you can't read from an
sg device directly. Presumably this is why QEMU won't
do format probing either.
Thus I don't think there is any security flaw here.
The QEMU warning appears bogus.
So to shut QEMU up:
Reviewed-by: Daniel P. Berrangé <berrange(a)redhat.com>
Regards,
Daniel
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