bpf: Hooks for sys_sendmsg

In addition to already existing BPF hooks for sys_bind and sys_connect,
the patch provides new hooks for sys_sendmsg.

It leverages existing BPF program type `BPF_PROG_TYPE_CGROUP_SOCK_ADDR`
that provides access to socket itlself (properties like family, type,
protocol) and user-passed `struct sockaddr *` so that BPF program can
override destination IP and port for system calls such as sendto(2) or
sendmsg(2) and/or assign source IP to the socket.

The hooks are implemented as two new attach types:
`BPF_CGROUP_UDP4_SENDMSG` and `BPF_CGROUP_UDP6_SENDMSG` for UDPv4 and
UDPv6 correspondingly.

UDPv4 and UDPv6 separate attach types for same reason as sys_bind and
sys_connect hooks, i.e. to prevent reading from / writing to e.g.
user_ip6 fields when user passes sockaddr_in since it'd be out-of-bound.

The difference with already existing hooks is sys_sendmsg are
implemented only for unconnected UDP.

For TCP it doesn't make sense to change user-provided `struct sockaddr *`
at sendto(2)/sendmsg(2) time since socket either was already connected
and has source/destination set or wasn't connected and call to
sendto(2)/sendmsg(2) would lead to ENOTCONN anyway.

Connected UDP is already handled by sys_connect hooks that can override
source/destination at connect time and use fast-path later, i.e. these
hooks don't affect UDP fast-path.

Rewriting source IP is implemented differently than that in sys_connect
hooks. When sys_sendmsg is used with unconnected UDP it doesn't work to
just bind socket to desired local IP address since source IP can be set
on per-packet basis by using ancillary data (cmsg(3)). So no matter if
socket is bound or not, source IP has to be rewritten on every call to
sys_sendmsg.

To do so two new fields are added to UAPI `struct bpf_sock_addr`;
* `msg_src_ip4` to set source IPv4 for UDPv4;
* `msg_src_ip6` to set source IPv6 for UDPv6.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 18 insertions, 2 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index d71f1f3e1155..3c27d00b5730 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -901,6 +901,7 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 {
 	struct inet_sock *inet = inet_sk(sk);
 	struct udp_sock *up = udp_sk(sk);
+	DECLARE_SOCKADDR(struct sockaddr_in *, usin, msg->msg_name);
 	struct flowi4 fl4_stack;
 	struct flowi4 *fl4;
 	int ulen = len;
@@ -955,8 +956,7 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	/*
 	 *	Get and verify the address.
 	 */
-	if (msg->msg_name) {
-		DECLARE_SOCKADDR(struct sockaddr_in *, usin, msg->msg_name);
+	if (usin) {
 		if (msg->msg_namelen < sizeof(*usin))
 			return -EINVAL;
 		if (usin->sin_family != AF_INET) {
@@ -1010,6 +1010,22 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		rcu_read_unlock();
 	}
 
+	if (cgroup_bpf_enabled && !connected) {
+		err = BPF_CGROUP_RUN_PROG_UDP4_SENDMSG_LOCK(sk,
+					    (struct sockaddr *)usin, &ipc.addr);
+		if (err)
+			goto out_free;
+		if (usin) {
+			if (usin->sin_port == 0) {
+				/* BPF program set invalid port. Reject it. */
+				err = -EINVAL;
+				goto out_free;
+			}
+			daddr = usin->sin_addr.s_addr;
+			dport = usin->sin_port;
+		}
+	}
+
 	saddr = ipc.addr;
 	ipc.addr = faddr = daddr;