net, sunrpc: Remap EPERM in case of connection failure in xs_tcp_setup_socket

When using a BPF program on kernel_connect(), the call can return -EPERM. This
causes xs_tcp_setup_socket() to loop forever, filling up the syslog and causing
the kernel to potentially freeze up.

Neil suggested:

  This will propagate -EPERM up into other layers which might not be ready
  to handle it. It might be safer to map EPERM to an error we would be more
  likely to expect from the network system - such as ECONNREFUSED or ENETDOWN.

ECONNREFUSED as error seems reasonable. For programs setting a different error
can be out of reach (see handling in 4fbac77d) in particular on kernels
which do not have f10d0596 ("bpf: Make BPF_PROG_RUN_ARRAY return -err
instead of allow boolean"), thus given that it is better to simply remap for
consistent behavior. UDP does handle EPERM in xs_udp_send_request().

Fixes: d74bad4e ("bpf: Hooks for sys_connect")
Fixes: 4fbac77d ("bpf: Hooks for sys_bind")
Co-developed-by: Lex Siegel <usiegl00@gmail.com>
Signed-off-by: Lex Siegel <usiegl00@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Neil Brown <neilb@suse.de>
Cc: Trond Myklebust <trondmy@kernel.org>
Cc: Anna Schumaker <anna@kernel.org>
Link: https://github.com/cilium/cilium/issues/33395
Link: https://lore.kernel.org/bpf/171374175513.12877.8993642908082014881@noble.neil.brown.name
Link: https://patch.msgid.link/9069ec1d59e4b2129fc23433349fd5580ad43921.1720075070.git.daniel@iogearbox.netSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/sunrpc/xprtsock.c

@@ -2441,6 +2441,13 @@ static void xs_tcp_setup_socket(struct work_struct *work)
 		transport->srcport = 0;
 		status = -EAGAIN;
 		break;
+	case -EPERM:
+		/* Happens, for instance, if a BPF program is preventing
+		 * the connect. Remap the error so upper layers can better
+		 * deal with it.
+		 */
+		status = -ECONNREFUSED;
+		fallthrough;
 	case -EINVAL:
 		/* Happens, for instance, if the user specified a link
 		 * local IPv6 address without a scope-id.