CVE-2025-37959

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

bpf: Scrub packet on bpf_redirect_peer

When bpf_redirect_peer is used to redirect packets to a device in another network namespace, the skb isn't scrubbed. That can lead skb information from one namespace to be "misused" in another namespace.

As one example, this is causing Cilium to drop traffic when using bpf_redirect_peer to redirect packets that just went through IPsec decryption to a container namespace. The following pwru trace shows (1) the packet path from the host's XFRM layer to the container's XFRM layer where it's dropped and (2) the number of active skb extensions at each function.

NETNS       MARK  IFACE  TUPLE                                FUNC
4026533547  d00   eth0   10.244.3.124:35473->10.244.2.158:53  xfrm_rcv_cb
                         .active_extensions = (__u8)2,
4026533547  d00   eth0   10.244.3.124:35473->10.244.2.158:53  xfrm4_rcv_cb
                         .active_extensions = (__u8)2,
4026533547  d00   eth0   10.244.3.124:35473->10.244.2.158:53  gro_cells_receive
                         .active_extensions = (__u8)2,
[...]
4026533547  0     eth0   10.244.3.124:35473->10.244.2.158:53  skb_do_redirect
                         .active_extensions = (__u8)2,
4026534999  0     eth0   10.244.3.124:35473->10.244.2.158:53  ip_rcv
                         .active_extensions = (__u8)2,
4026534999  0     eth0   10.244.3.124:35473->10.244.2.158:53  ip_rcv_core
                         .active_extensions = (__u8)2,
[...]
4026534999  0     eth0   10.244.3.124:35473->10.244.2.158:53  udp_queue_rcv_one_skb
                         .active_extensions = (__u8)2,
4026534999  0     eth0   10.244.3.124:35473->10.244.2.158:53  __xfrm_policy_check
                         .active_extensions = (__u8)2,
4026534999  0     eth0   10.244.3.124:35473->10.244.2.158:53  __xfrm_decode_session
                         .active_extensions = (__u8)2,
4026534999  0     eth0   10.244.3.124:35473->10.244.2.158:53  security_xfrm_decode_session
                         .active_extensions = (__u8)2,
4026534999  0     eth0   10.244.3.124:35473->10.244.2.158:53  kfree_skb_reason(SKB_DROP_REASON_XFRM_POLICY)
                         .active_extensions = (__u8)2,

In this case, there are no XFRM policies in the container's network namespace so the drop is unexpected. When we decrypt the IPsec packet, the XFRM state used for decryption is set in the skb extensions. This information is preserved across the netns switch. When we reach the XFRM policy check in the container's netns, __xfrm_policy_check drops the packet with LINUX_MIB_XFRMINNOPOLS because a (container-side) XFRM policy can't be found that matches the (host-side) XFRM state used for decryption.

This patch fixes this by scrubbing the packet when using bpf_redirect_peer, as is done on typical netns switches via veth devices except skb->mark and skb->tstamp are not zeroed.

The BPF disabled by default. Affects systems using BPF with cross-netns redirects and IPsec. May lead to traffic drops or metadata leakage due to un-scrubbed skb. The security impact is limited, because no privileges escalation possibility.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.