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During ILA address translations, the L4 checksums can be handled in
different ways. One of them, adj-transport, consist in parsing the
transport layer and updating any found checksum. This logic relies on
inet_proto_csum_replace_by_diff and produces an incorrect skb->csum when
in state CHECKSUM_COMPLETE.
This bug can be reproduced with a simple ILA to SIR mapping, assuming
packets are received with CHECKSUM_COMPLETE:
$ ip a show dev eth0
14: eth0@if15: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
link/ether 62:ae:35:9e:0f:8d brd ff:ff:ff:ff:ff:ff link-netnsid 0
inet6 3333:0:0:1::c078/64 scope global
valid_lft forever preferred_lft forever
inet6 fd00:10:244:1::c078/128 scope global nodad
valid_lft forever preferred_lft forever
inet6 fe80::60ae:35ff:fe9e:f8d/64 scope link proto kernel_ll
valid_lft forever preferred_lft forever
$ ip ila add loc_match fd00:10:244:1 loc 3333:0:0:1 \
csum-mode adj-transport ident-type luid dev eth0
Then I hit [fd00:10:244:1::c078]:8000 with a server listening only on
[3333:0:0:1::c078]:8000. With the bug, the SYN packet is dropped with
SKB_DROP_REASON_TCP_CSUM after inet_proto_csum_replace_by_diff changed
skb->csum. The translation and drop are visible on pwru [1] traces:
IFACE TUPLE FUNC
eth0:9 [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp) ipv6_rcv
eth0:9 [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp) ip6_rcv_core
eth0:9 [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp) nf_hook_slow
eth0:9 [fd00:10:244:3::3d8]:51420->[fd00:10:244:1::c078]:8000(tcp) inet_proto_csum_replace_by_diff
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) tcp_v6_early_demux
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) ip6_route_input
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) ip6_input
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) ip6_input_finish
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) ip6_protocol_deliver_rcu
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) raw6_local_deliver
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) ipv6_raw_deliver
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) tcp_v6_rcv
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) __skb_checksum_complete
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) kfree_skb_reason(SKB_DROP_REASON_TCP_CSUM)
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) skb_release_head_state
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) skb_release_data
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) skb_free_head
eth0:9 [fd00:10:244:3::3d8]:51420->[3333:0:0:1::c078]:8000(tcp) kfree_skbmem
This is happening because inet_proto_csum_replace_by_diff is updating
skb->csum when it shouldn't. The L4 checksum is updated such that it
"cancels" the IPv6 address change in terms of checksum computation, so
the impact on skb->csum is null.
Note this would be different for an IPv4 packet since three fields
would be updated: the IPv4 address, the IP checksum, and the L4
checksum. Two would cancel each other and skb->csum would still need
to be updated to take the L4 checksum change into account.
This patch fixes it by passing an ipv6 flag to
inet_proto_csum_replace_by_diff, to skip the skb->csum update if we're
in the IPv6 case. Note the behavior of the only other user of
inet_proto_csum_replace_by_diff, the BPF subsystem, is left as is in
this patch and fixed in the subsequent patch.
With the fix, using the reproduction from above, I can confirm
skb->csum is not touched by inet_proto_csum_replace_by_diff and the TCP
SYN proceeds to the application after the ILA translation.
Link: https://github.com/cilium/pwru [1]
Fixes:
|
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.