Public bug reported: BugLink: https://bugs.launchpad.net/bugs/
[Impact] In environments which have CHECKSUM iptables rules set, the following kernel call trace will be created when a GSO skb is processed by the CHECKSUM target: WARNING: CPU: 34 PID: 806048 at /build/linux-zdslHp/linux-4.4.0/net/core/dev.c:2456 skb_warn_bad_offload+0xcf/0x110() qr-f78bfdf7-fe: caps=(0x000000000fdb58e9, 0x000000000fdb58e9) len=1955 data_len=479 gso_size=1448 gso_type=1 ip_summed=3 CPU: 34 PID: 806048 Comm: haproxy Tainted: G W OE 4.4.0-138-generic #164-Ubuntu Call Trace: dump_stack+0x63/0x90 warn_slowpath_common+0x82/0xc0 warn_slowpath_fmt+0x5c/0x80 ? ___ratelimit+0xa2/0xe0 skb_warn_bad_offload+0xcf/0x110 skb_checksum_help+0x185/0x1a0 checksum_tg+0x22/0x29 [xt_CHECKSUM] ipt_do_table+0x301/0x730 [ip_tables] ? ipt_do_table+0x349/0x730 [ip_tables] iptable_mangle_hook+0x39/0x107 [iptable_mangle] nf_iterate+0x68/0x80 nf_hook_slow+0x73/0xd0 ip_output+0xcf/0xe0 ? __ip_flush_pending_frames.isra.43+0x90/0x90 ip_local_out+0x3b/0x50 ip_queue_xmit+0x154/0x390 __tcp_transmit_skb+0x52b/0x9b0 tcp_write_xmit+0x1dd/0xf50 __tcp_push_pending_frames+0x31/0xd0 tcp_push+0xec/0x110 tcp_sendmsg+0x749/0xba0 inet_sendmsg+0x6b/0xa0 sock_sendmsg+0x3e/0x50 SYSC_sendto+0x101/0x190 ? __sys_sendmsg+0x51/0x90 SyS_sendto+0xe/0x10 entry_SYSCALL_64_fastpath+0x22/0xc1 The CHECKSUM target does not support GSO skbs, and when a GSO skb is passed to skb_checksum_help(), it errors out and skb_warn_bad_offload() is called. The above call trace was found in a customer environment which has an Openstack deployment, with the following sorts of iptables rules set: -A neutron-l3-agent-POSTROUTING -o qr-+ -p tcp -m tcp --sport 9697 -j CHECKSUM --checksum-fill -A neutron-dhcp-age-POSTROUTING -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill This was causing haproxy running on the node to crash and restart every time a GSO skb was processed by the CHECKSUM target. I recommend reading the netdev mailing list thread for more details: https://www.spinics.net/lists/netdev/msg517366.html [Fix] This was fixed in 4.19 upstream with the below commit: commit 10568f6c5761db24249c610c94d6e44d5505a0ba Author: Florian Westphal <[email protected]> Date: Wed Aug 22 11:33:27 2018 +0200 Subject: netfilter: xt_checksum: ignore gso skbs This commit adds a check to see if the current skb is a gso skb, and if it is, skips skb_checksum_help(). It then continues on to check if the packet uses udp, and if it does, exits early. Otherwise it prints a single warning that CHECKSUM should be avoided, and if really needed, only for use with outbound udp. Note, 10568f6c5761db24249c610c94d6e44d5505a0ba was included in upstream stable version 4.18.13, and was backported to bionic in 4.15.0-58.64 by LP #1836426. This patch required minor backporting for 4.4, by slightly adjusting the context in the final patch hunk. [Testcase] You can reproduce this by adding the following iptables rule: -A POSTROUTING -p tcp -m tcp --sport 80 -j CHECKSUM --checksum-fill and running traffic over port 80. I built a test kernel, which is available here: https://launchpad.net/~mruffell/+archive/ubuntu/sf216537-test For unpatched kernels, this causes the process which was handeling the socket to crash, as seen by haproxy crashing on a node in production which hits this issue. On patched kernels you see the warning printed to dmesg and no crashes occur. [Regression Potential] The changes are limited only to users which have CHECKSUM rules enabled in their iptables configs. Openstack commonly configures such rules on deployment, even though they are not necessary, as almost all packets have their checksum calculated by NICs these days, and CHECKSUM is only around to service old dhcp clients which would discard UDP packets with empty checksums. This commit was selected for upstream -stable 4.18.13, and has made its way into bionic 4.15.0-58.64 by LP #1836426. There have been no reported problems and those kernels would have had sufficient testing with Openstack and its configured iptables rules. If any users are affected by regression, then they can simply delete any CHECKSUM entries in their iptables configs. ** Affects: linux (Ubuntu) Importance: Undecided Status: New ** Affects: linux (Ubuntu Xenial) Importance: Undecided Status: New ** Also affects: linux (Ubuntu Xenial) Importance: Undecided Status: New -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1840619 Title: skb_warn_bad_offload kernel splat due to CHECKSUM target not compatible with GSO skbs To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1840619/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
