This allows to ksock_send() to various destinations by allowing to call
ksock_connect() in the send path instead of initializing it once for all
in the SYSCALL program. It was limiting in cases ksock was used to reply
to specific destinations.

We need to break this potential cycle between the two LSM hooks and BPF
programs calling kfuncs triggering themselves:

socket_sendmsg program
  bpf_ksock_connect()
    connect_socket() -> security_socket_connect()
      socket_connect program
	bpf_ksock_send()
          kernel_sendmsg() -> sock_sendmsg() -> __sock_sendmsg()
            security_socket_sendmsg()
              socket_sendmsg program

Breaking this on one edge is sufficient, so in this commit we prevent
bpf_ksock_connect() to be called from LSM programs attached to the
security_socket_sendmsg() hook.

Signed-off-by: Mahe Tardy <mahe.tardy@gmail.com>
---
 net/core/bpf_ksock.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/net/core/bpf_ksock.c b/net/core/bpf_ksock.c
index 6c508c7a0adc..87329c4a2f8e 100644
--- a/net/core/bpf_ksock.c
+++ b/net/core/bpf_ksock.c
@@ -144,7 +144,7 @@ bpf_ksock_create(const struct bpf_ksock_create_opts *opts, u32 opts__sz,
  * Connects the socket to the specified remote address and port.
  *
  * This function may sleep while connecting the socket, so it can only be used
- * in sleepable BPF programs (SYSCALL).
+ * in sleepable BPF programs.
  *
  * Return: 0 on success, negative errno on error.
  */
@@ -243,7 +243,6 @@ __bpf_kfunc_end_defs();

 BTF_KFUNCS_START(ksock_init_kfunc_btf_ids)
 BTF_ID_FLAGS(func, bpf_ksock_create, KF_ACQUIRE | KF_RET_NULL | KF_SLEEPABLE)
-BTF_ID_FLAGS(func, bpf_ksock_connect, KF_SLEEPABLE)
 BTF_KFUNCS_END(ksock_init_kfunc_btf_ids)

 static const struct btf_kfunc_id_set ksock_init_kfunc_set = {
@@ -254,6 +253,7 @@ static const struct btf_kfunc_id_set ksock_init_kfunc_set = {
 BTF_KFUNCS_START(ksock_kfunc_btf_ids)
 BTF_ID_FLAGS(func, bpf_ksock_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_ksock_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_ksock_connect, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_ksock_send, KF_SLEEPABLE)
 BTF_KFUNCS_END(ksock_kfunc_btf_ids)

@@ -265,6 +265,14 @@ BTF_ID(func, bpf_lsm_socket_sendmsg)
 BTF_ID_UNUSED
 #endif
 BTF_ID_LIST_SINGLE(bpf_ksock_send_id, func, bpf_ksock_send)
+
+BTF_ID_LIST(bpf_lsm_socket_connect_id)
+#ifdef CONFIG_SECURITY_NETWORK
+BTF_ID(func, bpf_lsm_socket_connect)
+#else
+BTF_ID_UNUSED
+#endif
+BTF_ID_LIST_SINGLE(bpf_ksock_connect_id, func, bpf_ksock_connect)
 #endif

 static int bpf_ksock_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id)
@@ -278,7 +286,17 @@ static int bpf_ksock_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id)
 #ifdef CONFIG_BPF_LSM
 	if (prog->type == BPF_PROG_TYPE_LSM) {
 		if (prog->aux->attach_btf_id == bpf_lsm_socket_sendmsg_id[0] &&
-		    kfunc_id == bpf_ksock_send_id[0])
+		    (kfunc_id == bpf_ksock_send_id[0] ||
+		    /*
+		     * Break the socket_sendmsg -> bpf_ksock_connect() ->
+		     * socket_connect -> bpf_ksock_send() -> socket_sendmsg'
+		     * cycle. Blocking this edge is enough.
+		     */
+		     kfunc_id == bpf_ksock_connect_id[0]))
+			return -EACCES;
+
+		if (prog->aux->attach_btf_id == bpf_lsm_socket_connect_id[0] &&
+		    kfunc_id == bpf_ksock_connect_id[0])
 			return -EACCES;

 		return 0;
--
2.43.0