CVE-2021-47036 udp: skip L4 aggregation for UDP tunnel packets
A Linux kernel flaw in UDP tunnel handling can cause inner protocol corruption and packet holding, leading to denial of service when specific GRO features are enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2021-47036?
CVE-2021-47036 is a critical denial of service vulnerability in Linux kernel, published on 28 Feb 2024. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
udp: skip L4 aggregation for UDP tunnel packets
If NETIF_F_GRO_FRAGLIST or NETIF_F_GRO_UDP_FWD are enabled, and there are UDP tunnels available in the system, udp_gro_receive() could end-up doing L4 aggregation (either SKB_GSO_UDP_L4 or SKB_GSO_FRAGLIST) at the outer UDP tunnel level for packets effectively carrying and UDP tunnel header.
That could cause inner protocol corruption. If e.g. the relevant packets carry a vxlan header, different vxlan ids will be ignored/ aggregated to the same GSO packet. Inner headers will be ignored, too, so that e.g. TCP over vxlan push packets will be held in the GRO engine till the next flush, etc.
Just skip the SKB_GSO_UDP_L4 and SKB_GSO_FRAGLIST code path if the current packet could land in a UDP tunnel, and let udp_gro_receive() do GRO via udp_sk(sk)->gro_receive.
The check implemented in this patch is broader than what is strictly needed, as the existing UDP tunnel could be e.g. configured on top of a different device: we could end-up skipping GRO at-all for some packets.
Anyhow, that is a very thin corner case and covering it will add quite a bit of complexity.
v1 -> v2: - hopefully clarify the commit message
Which versions are affected by CVE-2021-47036?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2021-47036?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2021-47036 being exploited?
There is no known exploitation of CVE-2021-47036 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as DEBIAN-CVE-2021-47036, GHSA-h958-4cw7-rqgr, ROOT-OS-DEBIAN-11-CVE-2021-47036, UBUNTU-CVE-2021-47036.