flow_dissector: avoid u16 truncation of skb->len when computing thoff

Overview

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

flow_dissector: avoid u16 truncation of skb->len when computing thoff

__skb_flow_dissect() computes key_control->thoff using:

key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);

min_t(u16, ...) casts both arguments to u16 before comparing them. Whenever skb->len (or hlen) modulo 65536 is smaller than nhoff, the truncated length wins and thoff is set to a bogus small value, even when the dissection succeeded. For example, an IPv4/TCP frame with skb->len == 65540 and nhoff == 34 gets thoff == 4, so callers such as skb_probe_transport_header() point the transport header inside the Ethernet header.

Such skbs are not exotic:

• At the time of commit d0c081b49137 ("flow_dissector: properly cap thoff field"), AF_PACKET with PACKET_VNET_HDR could already build GSO skbs larger than 64KB (MTU checks are skipped for GSO, and alloc_skb_with_frags() accepted up to MAX_SKB_FRAGS (17) order-0 pages on top of the linear part).
• BIG TCP now makes skbs larger than 64KB common.
• The following patch makes tun_get_user() dissect IFF_TAP frames before eth_type_trans() pulls the Ethernet header, so a GSO frame carrying a 65522..65535 byte L3 packet will be dissected with skb->len in [65536, 65549].

Compare as u32 instead. If the resulting offset cannot be represented in the u16 key_control->thoff, cap it to U16_MAX and report the dissection as failed rather than silently returning a wrong transport offset. thoff is still set on failure, as some callers (such as eth_get_headlen()) use it regardless of the return value.

Affected

• Linux >= d0c081b49137cd3200f2023c0875723be66e7ce5 < d9e08836d06c84abb18470c94b12b131eaccdc7d
• Linux >= d0c081b49137cd3200f2023c0875723be66e7ce5 < 50d0cd575cab763f833d6ac6eb6550b23a0b3dae
• Linux >= d0c081b49137cd3200f2023c0875723be66e7ce5 < 9993e6a0ac1b5b3101ffba702cacb9f42b9909d8
• Linux >= d0c081b49137cd3200f2023c0875723be66e7ce5 < 52efce4755a4b8b82039caa773645ffc80b25362
• Linux >= d0c081b49137cd3200f2023c0875723be66e7ce5 < 99bda1ecbd56905f8bd934b9cdccd3bd251192b1
• Linux d35cd5e279881ec36ff8cd82a2d9caebc0cce3fc
• Linux eecfa2eeefe3caef4c8d6b2284f6066c76ab26a1
• Linux 80f327285cab6ab8e10c3ea03278add5dd99fb02
• Linux >= 4.4.114 < 4.5
• Linux >= 4.9.79 < 4.10
• Linux >= 4.14.16 < 4.15
• Linux 4.15

Remediation

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