Re: plug pre-created tap devices to libvirt guests

On Tue, Jun 30, 2020 at 04:02:05PM +0100, Daniel P. Berrangé wrote: > On Tue, Jun 30, 2020 at 12:59:03PM +0200, Miguel Duarte de Mora Barroso wrote: > > On Mon, Apr 6, 2020 at 4:03 PM Laine Stump <lstump redhat com> wrote: > > > > > > On 4/6/20 9:54 AM, Daniel P. Berrangé wrote: > > > > On Mon, Apr 06, 2020 at 03:47:01PM +0200, Miguel Duarte de Mora Barroso wrote: > > > >> Hi all, > > > >> > > > >> I'm aware that it is possible to plug pre-created macvtap devices to > > > >> libvirt guests - tracked in RFE [0]. > > > >> > > > >> My interpretation of the wording in [1] and [2] is that it is also > > > >> possible to plug pre-created tap devices into libvirt guests - that > > > >> would be a requirement to allow kubevirt to run with less capabilities > > > >> in the pods that encapsulate the VMs. > > > >> > > > >> I took a look at the libvirt code ([3] & [4]), and, from my limited > > > >> understanding, I got the impression that plugging existing interfaces > > > >> via `managed='no' ` is only possible for macvtap interfaces. > > > > > > > > > No, it works for standard tap devices as well. > > > > > > > > > The reason the BZs and commit logs talk mostly about macvtap rather than > > > tap is because 1) that's what kubevirt people had asked for and 2) it > > > already *mostly* worked for tap devices, so most of the work was related > > > to macvtap (my memory is already fuzzy, but I think there were a couple > > > privileged operations we still tried to do for standard tap devices even > > > if they were precreated (standard disclaimer: I often misremember, so > > > this memory could be wrong! But definitely precreated tap devices do work). > > > > > > > It's been a while since I've started this thread, but lately I've > > understood better how tap devices work, and that new insight makes me > > wonder about a couple of things. > > > > Our ultimate goal In kubevirt is to consume a pre-created tap device > > by a kubernetes pod that doesn't have the NET_ADMIN capability. > > > > After looking at the current libvirt code, I don't think that is > > currently supported, since we'll *always* enter the > > `virNetDevTapCreate` function in [1] (I'm interested in the *tap* > > scenario). > > > > The tap device is effectively created in that function - [2] - by > > opening the clone device (/dev/net/tun), and calling `ioctl(fd, > > TUNSETIFF,...)` in it. AFAIK, both of those operations *require* the > > NET_ADMIN capability. If I'm correct, this means that the current > > libvirt implementation makes our goals impossible to achieve. > > AFAIK, that is not correct - CAP_NET_ADMIN isn't required to open > or create a tap device - only to add the tap device to a bridge. > > So if you create the tap device & attach it to a bridge ahead of > time, libvirt should then be able to open it and give it to QEMU https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/d... ((uid_valid(tun->owner) && !uid_eq(cred->euid, tun->owner)) || (gid_valid(tun->group) && !in_egroup_p(tun->group))) && !ns_capable(net->user_ns, CAP_NET_ADMIN); This is called by the TUNSETIFF code. AFAICT, that means if you fchown(tapfd, uid, gid), to the uid+gid of libvirtd, it should not require CAP_NET_ADMIN. Regards, Daniel