CVE-2024-35884

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