[PATCH net v2] tcp: drop secpath at the same time as we currently drop dst

[Paul Moore]

On Mon, Feb 17, 2025 at 5:23 AM Sabrina Dubroca <sd at queasysnail.net> wrote:
>
> 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.

I don't recall seeing any follow up in the v1 patchset regarding
IP_CMSG_PASSSEC/security_socket_getpeersec_dgram(), can you confirm
that the secpath is preserved for that code path?

https://lore.kernel.org/linux-security-module/CAHC9VhQZ+k1J0UidJ-bgdBGBuVX9M18tQ+a+fuqXQM_L-PFvzA@mail.gmail.com

> However, in some of those
> places, the MPTCP extension has just been attached to the skb, so we
> cannot simply drop all extensions.
>
> Fixes: 68822bdf76f1 ("net: generalize skb freeing deferral to per-cpu lists")
> Reported-by: Xiumei Mu <xmu at redhat.com>
> Signed-off-by: Sabrina Dubroca <sd at queasysnail.net>
> ---
> v1: drop all extensions just before calling skb_attempt_defer_free
>     https://lore.kernel.org/netdev/879a4592e4e4bd0c30dbe29ca189e224ec1739a5.1739201151.git.sd@queasysnail.net/
> v2: - drop only secpath, as soon as possible - per Eric's feedback
>     - add debug warns if trying to add to sk_receive_queue an skb with
>       a dst or a secpath
>
> @Eric feel free to add some tags (Suggested-by? sign-off?) for the
> code I adapted from
> https://lore.kernel.org/netdev/CANn89i+JdDukwEhZ%3D41FxY-w63eER6JVixkwL+s2eSOjo6aWEQ@mail.gmail.com/
>
>  include/net/tcp.h       | 14 ++++++++++++++
>  net/ipv4/tcp_fastopen.c |  4 ++--
>  net/ipv4/tcp_input.c    |  8 ++++----
>  net/ipv4/tcp_ipv4.c     |  2 +-
>  4 files changed, 21 insertions(+), 7 deletions(-)

-- 
paul-moore.com