CVE-2025-21864 Information

Description

In the Linux kernel the following vulnerability has been resolved:

tcp: drop secpath at the same time as we currently drop dst

Xiumei reported hitting the WARN in xfrm6_tunnel_net_exit while running tests that boil down to:

• create a pair of netns
• run a basic TCP test over ipcomp6
• delete the pair of netns

The xfrm_state found on spi_byaddr was not deleted at the time we delete the netns because we still have a reference on it. This lingering reference comes from a secpath (which holds a ref on the xfrm_state) which is still attached to an skb. This skb is not leaked it ends up on sk_receive_queue and then gets defer-free’d by skb_attempt_defer_free.

The problem happens when we defer freeing an skb (push it on one CPU’s defer_list) and don’t flush that list before the netns is deleted. In that case we still have a reference on the xfrm_state that we don’t expect at this point.

We already drop the skb’s dst in the TCP receive path when it’s no longer needed so let’s also drop the secpath. At this point tcp_filter has already called into the LSM hooks that may require the secpath so it should not be needed anymore. However in some of those places the MPTCP extension has just been attached to the skb so we cannot simply drop all extensions.

Reference

https://git.kernel.org/stable/c/69cafd9413084cd5012cf5d7c7ec6f3d493726d9 https://git.kernel.org/stable/c/87858bbf21da239ace300d61dd209907995c0491 https://git.kernel.org/stable/c/9b6412e6979f6f9e0632075f8f008937b5cd4efd https://git.kernel.org/stable/c/cd34a07f744451e2ecf9005bb7d24d0b2fb83656 https://git.kernel.org/stable/c/f1d5e6a5e468308af7759cf5276779d3155c5e98