net: gue: reject invalid REMCSUM offsets

Overview

In the Linux kernel, the following vulnerability has been resolved:

net: gue: reject invalid REMCSUM offsets

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.

Affected

• Linux >= fe881ef11cf0220f118816181930494d484c4883 < 4b9bc21b2396c396d88dd909caaf73d5be6ff593
• Linux >= fe881ef11cf0220f118816181930494d484c4883 < 43efb1f997e976f386aa89716f55870cd2cff9ad
• Linux >= fe881ef11cf0220f118816181930494d484c4883 < 1eeec0c51fd5598da44080ed96bbd9880021c28b
• Linux >= fe881ef11cf0220f118816181930494d484c4883 < 1d664be4ca49d1696e85239236f4988dfc4653c3
• Linux >= fe881ef11cf0220f118816181930494d484c4883 < 97f907203321eee2cc9132d5c9eb47698fb41d35
• Linux >= fe881ef11cf0220f118816181930494d484c4883 < 40aeafbcbc7bc4018e8f74bbbb4d29ed8846f57f
• Linux >= fe881ef11cf0220f118816181930494d484c4883 < c20f755338ca234e523dfc943719f194dfc1f903
• Linux >= fe881ef11cf0220f118816181930494d484c4883 < 2566866fc30965d915d0b52b5c3323b362619f0e
• Linux 4.0

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.