Re: [libvirt] [PATCHv2 01/33] qemu: process: Refresh backing chain info when reconnecting to qemu

On 05/22/2014 07:47 AM, Peter Krempa wrote: Not necessarily ideal. Reading metadata from a file that is also simultaneously opened read-write by qemu might, in super-rare circumstances, hit a race where qemu is in the middle of updating metadata due to the completion of some job that was started before the libvirtd restart. Although qcow2 has a hard cap of header fitting in one cluster (typically 64k) (at least, after CVE-2014-0144, qemu.git commit 24342f2ca), it is still plausible that qemu is updating the qcow2 header via sectors (perhaps as small as 512-byte chunks) rather than by full clusters; and there are scenarios where an update touches two non-adjacent sectors (such as adjusting the backing file name), such that if a reader sees the old content of one sector but the new content of the other, then it is completely broken in interpreting the data. A slightly safer alternative might be using a QMP command to ask qemu what the backing chain currently is when we reconnect - although that may require correlation back to filenames we passed in, since in the face of fd passing, qemu may only know the name "/dev/fdset/NNN" instead of the filename the earlier libvirtd opened and passed in by fd. And that presupposes that qemu can always be trusted (we don't want to introduce a CVE where a guest that compromises qemu into returning bogus QMP results can then cause libvirt to mismanage the host filesystem). So with those points made, a metadata read race is extremely rare, and I see no point in implementing a half-way solution that requires parsing a QMP command if we are eventually going to switch over to full XML tracking anyways. Therefore, since _this_ patch is a clear improvement (even if not theoretically perfect), we shouldn't hold it up waiting for some other solution. ACK as-is. and thanks for the comment to remind us what we should improve. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org