Subject: CVE-2024-35884: udp: do not accept non-tunnel GSO skbs landing in a tunnel

From: Greg Kroah-Hartman <[email-masked]>

Description
===========

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

udp: do not accept non-tunnel GSO skbs landing in a tunnel

When rx-udp-gro-forwarding is enabled UDP packets might be GROed when
being forwarded. If such packets might land in a tunnel this can cause
various issues and udp_gro_receive makes sure this isn't the case by
looking for a matching socket. This is performed in
udp4/6_gro_lookup_skb but only in the current netns. This is an issue
with tunneled packets when the endpoint is in another netns. In such
cases the packets will be GROed at the UDP level, which leads to various
issues later on. The same thing can happen with rx-gro-list.

We saw this with geneve packets being GROed at the UDP level. In such
case gso_size is set; later the packet goes through the geneve rx path,
the geneve header is pulled, the offset are adjusted and frag_list skbs
are not adjusted with regard to geneve. When those skbs hit
skb_fragment, it will misbehave. Different outcomes are possible
depending on what the GROed skbs look like; from corrupted packets to
kernel crashes.

One example is a BUG_ON[1] triggered in skb_segment while processing the
frag_list. Because gso_size is wrong (geneve header was pulled)
skb_segment thinks there is "geneve header size" of data in frag_list,
although it's in fact the next packet. The BUG_ON itself has nothing to
do with the issue. This is only one of the potential issues.

Looking up for a matching socket in udp_gro_receive is fragile: the
lookup could be extended to all netns (not speaking about performances)
but nothing prevents those packets from being modified in between and we
could still not find a matching socket. It's OK to keep the current
logic there as it should cover most cases but we also need to make sure
we handle tunnel packets being GROed too early.

This is done by extending the checks in udp_unexpected_gso: GSO packets
lacking the SKB_GSO_UDP_TUNNEL/_CSUM bits and landing in a tunnel must
be segmented.

[1] kernel BUG at net/core/skbuff.c:4408!
    RIP: 0010:skb_segment+0xd2a/0xf70
    __udp_gso_segment+0xaa/0x560

The Linux kernel CVE team has assigned CVE-2024-35884 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.6 with commit 9fd1ff5d2ac7 and fixed in 5.10.215 with commit 3391b157780b
	Issue introduced in 5.6 with commit 9fd1ff5d2ac7 and fixed in 5.15.154 with commit d49ae15a5767
	Issue introduced in 5.6 with commit 9fd1ff5d2ac7 and fixed in 6.1.85 with commit d12245080cb2
	Issue introduced in 5.6 with commit 9fd1ff5d2ac7 and fixed in 6.6.26 with commit 3001e7aa43d6
	Issue introduced in 5.6 with commit 9fd1ff5d2ac7 and fixed in 6.8.5 with commit 35fe0e0b5c00
	Issue introduced in 5.6 with commit 9fd1ff5d2ac7 and fixed in 6.9 with commit 3d010c8031e3

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35884
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	include/linux/udp.h
	net/ipv4/udp.c
	net/ipv4/udp_offload.c
	net/ipv6/udp.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/3391b157780bbedf8ef9f202cbf10ee90bf6b0f8
	https://git.kernel.org/stable/c/d49ae15a5767d4e9ef8bbb79e42df1bfebc94670
	https://git.kernel.org/stable/c/d12245080cb259d82b34699f6cd4ec11bdb688bd
	https://git.kernel.org/stable/c/3001e7aa43d6691db2a878b0745b854bf12ddd19
	https://git.kernel.org/stable/c/35fe0e0b5c00bef7dde74842a2564c43856fbce4
	https://git.kernel.org/stable/c/3d010c8031e39f5fa1e8b13ada77e0321091011f