The REMCSUM option carries an absolute checksum start and checksum field
offset. gue_remcsum() passes them to skb_remcsum_process(), whose
partial path stores offset - start in the u16 skb->csum_offset variable.
If offset is less than start, this underflows.
A forwarded packet can retain CHECKSUM_PARTIAL and reach a NETIF_F_HW_CSUM
driver which trusts the metadata, leading skb_copy_and_csum_dev() to write
two bytes about 64 KiB beyond the destination buffer.
Reject reversed tuples in validate_gue_flags(), after the existing length
validation, so all GUE parsers enforce the ordering in one place.
Fixes: fe881ef11c ("gue: Use checksum partial with remote checksum offload")
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Link: https://patch.msgid.link/20260915124806.2852293-2-Jeremy.Jean@oss.cyber.gouv.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
2566866fc3