AppArmor: TCP Fast Open bypasses connect mediation (last unaddressed LSM)

[Ryan Lee]

On Thu, Jun 18, 2026 at 6:12 PM Bryam Vargas <hexlabsecurity at proton.me> wrote:
>
> Hello John, and LSM folks,
>
> I have been working on the Landlock TCP Fast Open connect bypass [1]. Stephen
> Smalley's SELinux fix for the same issue [3] -- "Similar to Landlock, SELinux was
> not updated when TCP Fast Open support was introduced ..." -- made me go back and
> check the rest of the connect-mediating LSMs, since I had only been looking at
> Landlock. With Landlock [2], SELinux [3], and now TOMOYO [4] all getting fixes,
> AppArmor is the last one with the same gap and no fix yet.
>
> Root cause (shared with the others)
> -----------------------------------
> security_socket_connect() has a single call site, net/socket.c (the connect(2)
> syscall). TCP Fast Open performs an implicit connect inside sendmsg:
>
>   tcp_sendmsg -> tcp_sendmsg_fastopen -> __inet_stream_connect(..., is_sendmsg=1)
>               -> sk->sk_prot->connect()                 net/ipv4/{tcp.c,af_inet.c}
>
> This never calls security_socket_connect(); the only LSM hook on the path is
> security_socket_sendmsg(). mptcp_sendmsg_fastopen reaches the same code and is a
> second producer.
>
> AppArmor
> --------
> apparmor_socket_connect() requests AA_MAY_CONNECT; apparmor_socket_sendmsg() (via
> aa_sock_msg_perm) requests AA_MAY_SEND. These are distinct bits, and apparmor_parser
> compiles them independently: "network send inet stream," yields accept mask 0x02
> while "network connect inet stream," yields 0x40. So an egress-restriction profile
> that grants send but not connect is bypassed by MSG_FASTOPEN.
>
> Reproduced on 6.12.88 with apparmor active. Under a profile granting the inet/inet6
> stream lifecycle except connect:
>
>   aa-exec -p egress_restricted -- ./probe
>   [TCP ] connect(2)=EACCES(blocked)  sendto(MSG_FASTOPEN)=OK(reached)  => connection established
>   [TCP6] connect(2)=EACCES(blocked)  sendto(MSG_FASTOPEN)=OK(reached)  => connection established
>
> (The coarse "network inet stream," idiom grants connect anyway, so this only bites the
> fine-grained "allow send, deny connect" policy that the asymmetry is meant to serve.)
>
> Fix
> ---
> Same shape as the TOMOYO [4] and SELinux [3] fixes: in apparmor_socket_sendmsg (or
> aa_sock_msg_perm), when MSG_FASTOPEN is set and msg_name carries a destination on a
> not-yet-connected stream socket, additionally require aa_sk_perm(OP_CONNECT,
> AA_MAY_CONNECT, sk). I am happy to send that patch and the reproducer.

We would appreciate having the patch and the reproducer to look over.
Ideally, the reproducer could be integrated as a regression test into
the upstream repo at
https://gitlab.com/apparmor/apparmor/-/tree/master/tests/regression/apparmor?ref_type=heads,
but we can also assist with that step.
>
> (A single core check in __inet_stream_connect(), gated on is_sendmsg, would have
> covered all five LSMs and both the TCP and MPTCP producers in one place -- the kernel
> already mediates the analogous implicit-connect-on-send for AF_UNIX via
> security_unix_may_send and for SCTP via security_sctp_bind_connect. But since the
> other four LSMs are taking per-hook fixes, AppArmor matching them is the consistent
> move; mentioning the core option only in case it is preferred.)
>
> [1] Landlock: LANDLOCK_ACCESS_NET_CONNECT_TCP bypass via TCP Fast Open (report)
>     https://lore.kernel.org/r/20260616201615.275032-1-hexlabsecurity@proton.me
> [2] landlock: fix TCP Fast Open connection bypass (Matthieu Buffet)
>     https://lore.kernel.org/r/20260617180526.15627-2-matthieu@buffet.re
> [3] selinux: check connect-related permissions on TCP Fast Open (Stephen Smalley)
>     https://lore.kernel.org/r/20260618175513.112443-2-stephen.smalley.work@gmail.com
> [4] tomoyo: Enforce connect policy in TCP Fast Open (Matthieu Buffet)
>     https://lore.kernel.org/r/20260619002207.61104-1-matthieu@buffet.re
>
> Thanks,
> Bryam Vargas
>
>