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, 8 insertions, 0 deletions

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 9b8c6e310e9a..cc68787f2d97 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -160,6 +160,8 @@ enum bpf_attach_type {
 	BPF_CGROUP_INET6_CONNECT,
 	BPF_CGROUP_INET4_POST_BIND,
 	BPF_CGROUP_INET6_POST_BIND,
+	BPF_CGROUP_UDP4_SENDMSG,
+	BPF_CGROUP_UDP6_SENDMSG,
 	__MAX_BPF_ATTACH_TYPE
 };
 
@@ -2363,6 +2365,12 @@ struct bpf_sock_addr {
 	__u32 family;		/* Allows 4-byte read, but no write */
 	__u32 type;		/* Allows 4-byte read, but no write */
 	__u32 protocol;		/* Allows 4-byte read, but no write */
+	__u32 msg_src_ip4;	/* Allows 1,2,4-byte read an 4-byte write.
+				 * Stored in network byte order.
+				 */
+	__u32 msg_src_ip6[4];	/* Allows 1,2,4-byte read an 4-byte write.
+				 * Stored in network byte order.
+				 */
 };
 
 /* User bpf_sock_ops struct to access socket values and specify request ops