[XSA-258] Information leak via crafted user-supplied CDROM

ISSUE DESCRIPTION

QEMU handles many different file formats for virtual disks (e.g., raw,
qcow2, vhd, &c). Some of these formats are "snapshots" that specify
"patches" to an alternate disk image, whose filename is included in
the snapshot file.

When qemu is given a disk but the type is not specified, it attempts
to guess the file format by reading it. If a disk image is intended
to be 'raw', but the image is entirely controlled by an attacker, the
attacker could write a header to the image, describing one of these
"snapshot" formats, and pointing to an arbitrary file as the "backing"
file.

When attaching disks via command-line parameters at boot time
(including both "normal" disks and CDROMs), libxl specifies the
format; however, when inserting a CDROM live via QMP, the format was
not specified.

IMPACT

An attacker supplying a crafted CDROM image can read any file (or
device node) on the dom0 filesystem with the permissions of the qemu
devicemodel process. (The virtual CDROM device is read-only, so
no data can be written.)

VULNERABLE SYSTEMS

Only x86 HVM guests with a virtual CDROM device are affected. ARM
guests, x86 PV guests, x86 PVH guests, and x86 HVM guests without a
virtual CDROM device are not affected.

Only systems with qemu running in dom0 are affected; systems running
stub domains are not affected. Only systems using qemu-xen (aka
"qemu-upstream" are affected; systems running qemu-xen-traditional
are not affected.

Only systems in which an attacker can provide a raw CDROM image, and
cause that image to be virtually inserted while the guest is running,
are affected. Systems which only have host administrator-supplied
CDROM images, or systems which allow images to be added only at boot
time, are not affected.