Re: [PATCH v2] network: add rule to nftables backend that zeroes checksum of DHCP responses

Hi, On Tue, Oct 29, 2024 at 09:30:27AM -0400, Laine Stump wrote: > On 10/29/24 8:46 AM, Daniel P. Berrangé wrote: >> On Tue, Oct 29, 2024 at 12:22:42PM +0000, Daniel P. Berrangé wrote: >>> On Tue, Oct 29, 2024 at 06:03:26AM -0500, Andrea Bolognani wrote: >>>> On Mon, Oct 28, 2024 at 06:07:14PM +0000, Daniel P. Berrangé wrote: >>>>> On Mon, Oct 28, 2024 at 10:32:55AM -0700, Andrea Bolognani wrote: >>>>>> I did some testing of my own and I can confirm that FreeBSD and >>>>>> OpenBSD are fine with this change, as are various Linux flavors >>>>>> (Alpine, CirrOS, Debian, Fedora, openSUSE, Ubuntu). >>>>>> >>>>>> However, a few other operating systems aren't: namely GNU/Hurd, Haiku >>>>>> and NetBSD break with this change. Interestingly, these were all fine >>>>>> with the nftables backend before it. >>>>> >>>>> Well that's odd. I've checked NetBSD source code and found no less >>>>> than 3 DHCP client impls, and all of them cope with checksum == 0. >>>>> >>>>> https://github.com/NetBSD/src/blob/trunk/usr.bin/rump_dhcpclient/net.c#L497 >>>>> >>>>> https://github.com/NetBSD/src/blob/trunk/external/bsd/dhcpcd/dist/src/dhc... >>>>> >>>>> https://github.com/NetBSD/src/blob/trunk/external/mpl/dhcp/dist/common/pa... >>>>> >>>>> the middle impl also directly copes with partial checksums >>>> >>>> The boot log contains >>>> >>>> Starting dhcpcd. >>>> wm0: checksum failure from 192.168.124.1 >>>> >>>> so I guess the second implementation is the relevant one. >>> >>> I've just tested netBSD 10.0 and get exactly the same failure >>> as you. >>> >>> I've tried "tcpdump -vv -i vnetXXX port 68" on the host and >>> on the guest and that is reporting that the checksum is bad. >>> It is *not* getting set to zero. >>> >>> Meanwhile, if I run the same tcpdump with OpenBSD guest, I >>> see tcpdump reporting a zero checksum as expected. >>> >>> WTF ? >>> >>> Somehow our nftables rule is not having an effect, or worse, >>> it is have a non-deterministic effect where it works for >>> packets on some guests, but not others. >>> >>> I checked the rule counters and packets are hitting the rule, >>> but not getting their checksum zerod. >> >> Further research shows tcpdump on packets leaving 'virbr0' have >> the checksum correctly zerod. Our nftables rule is working. >> >> A concurrent tcpdump on packets leaving 'vnetNNN' shows the >> checksum is mangled. >> >> With our old iptables rules, we set a valid checksum when leaving >> virbr0, and I presume this causes all subsequent code to not touch >> the checksum field. >> >> With our new nftables rules, we set a zero checksum when leaving >> virbr0, and "zero checksum" conceptually means "not present (yet)". >> >> I think there must be logic somewhere in the kernel/QEMU which >> sees "not present" and decides it needs to do <something> with >> the checksum field. > > Yikes! > >> >> A key difference that is probably relevant is that netbsd is >> using an e1000 NIC in QEMU, while openbsd is using a virtio-net >> NIC. At least when created by virt-manager. > > >> AFAIR, QEMU's magic checksum offload only happens for virtio-net, >> so presumably our rules are incompatible with non-virtio-net NICs >> in someway. > > Double and triple yikes! > > So something in the packet path for non-virtio-net NICs is noticing that > the packet checksum is 0, and then "fixing" it with the *wrong* checksum? > > But in the past when it already had the correct checksum, that same bit > of code said "Huh. The checksum is already correct" and left it alone. Maybe this is due to the partial checksum thing in kernel which updates the checksum if not using virtio "shortcut". > So when the extra rules are removed, then those same guests begin > working? (You can easily remove the checksum rules with: > > nft delete chain ip libvirt_network postroute_mangle > > BTW, I just now tried an e1000e NIC on Fedora guest and it continues to > work with the 0-checksum rules removed. In this case tcpdump on virbr0 > shows "bad cksum", but when I look at tcpdump on the guest, it shows > "udp cksum ok" though, so something else somewhere is setting the > checksum to the correct value. FWIW, I just tested an alternative workaround using tc. This works for me with a FreeBSD guest and NIC switched to either e1000 or virtio: # tc qd add dev vnetbr0 root handle 1: htb # tc filter add dev vnetbr0 prio 1 protocol ip parent 1: \ u32 match ip sport 67 ffff match ip dport 68 ffff \ action csum ip and udp