Merge branch 'net-fix-ip6gre-header-length-before-capping-tunnel-headroom'

Zhiling Zou says:

====================
net: fix IP6GRE header length before capping tunnel headroom

We found and validated an issue in IP tunnel headroom accounting. The bug is
reachable by a non-root user via user and net namespace.
We've tested it, and it should not affect any other functionality.

We will provide detailed information about the bug
in this email, along with a PoC to trigger it.

---- details below ----

Bug details:

IP tunnel devices derive advertised headroom from lower output devices. A
namespace-local stack of tunnel devices can make that advertised reservation
larger than the 16-bit skb header offsets can represent. Once IP output
reserves that space and records network or transport header offsets, later skb
head expansion can wrap those offsets and leave header helpers pointing into
headroom instead of the packet area.

For IP6GRE, there is an earlier accounting bug that has to be fixed first:
ip6gre_tnl_link_config_route() folds the lower device's hard_header_len into
the tunnel device's hard_header_len whenever header_ops is set. That is wrong
for both header_ops users. ip6gretap and ip6erspan have a fixed Ethernet
hardware header length, while an NBMA ip6gre tunnel's header_ops creates only
the tunnel header: GRE, optional FOU or GUE, and the outer IPv6 header. The
lower device header is needed headroom, not part of the tunnel device's
hardware header.

This series first fixes that IP6GRE hardware-header accounting, then caps the
advertised IP tunnel needed_headroom at the same 512-byte limit already used
by the runtime tunnel transmit path. Tunnel transmit can still expand the skb
when a packet needs more headroom, so nonsensical stacked configurations may
pay an extra reallocation but cannot publish an unbounded reservation to upper
layers.
====================

Link: https://patch.msgid.link/cover.1786542637.git.zhilinz@nebusec.ai
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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