net: add skeleton of bpfilter kernel module

bpfilter.ko consists of bpfilter_kern.c (normal kernel module code)
and user mode helper code that is embedded into bpfilter.ko

The steps to build bpfilter.ko are the following:
- main.c is compiled by HOSTCC into the bpfilter_umh elf executable file
- with quite a bit of objcopy and Makefile magic the bpfilter_umh elf file
  is converted into bpfilter_umh.o object file
  with _binary_net_bpfilter_bpfilter_umh_start and _end symbols
  Example:
  $ nm ./bld_x64/net/bpfilter/bpfilter_umh.o
  0000000000004cf8 T _binary_net_bpfilter_bpfilter_umh_end
  0000000000004cf8 A _binary_net_bpfilter_bpfilter_umh_size
  0000000000000000 T _binary_net_bpfilter_bpfilter_umh_start
- bpfilter_umh.o and bpfilter_kern.o are linked together into bpfilter.ko

bpfilter_kern.c is a normal kernel module code that calls
the fork_usermode_blob() helper to execute part of its own data
as a user mode process.

Notice that _binary_net_bpfilter_bpfilter_umh_start - end
is placed into .init.rodata section, so it's freed as soon as __init
function of bpfilter.ko is finished.
As part of __init the bpfilter.ko does first request/reply action
via two unix pipe provided by fork_usermode_blob() helper to
make sure that umh is healthy. If not it will kill it via pid.

Later bpfilter_process_sockopt() will be called from bpfilter hooks
in get/setsockopt() to pass iptable commands into umh via bpfilter.ko

If admin does 'rmmod bpfilter' the __exit code bpfilter.ko will
kill umh as well.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 111 insertions, 0 deletions

diff --git a/net/bpfilter/bpfilter_kern.c b/net/bpfilter/bpfilter_kern.c
new file mode 100644
index 000000000000..7596314b61c7
--- /dev/null
+++ b/net/bpfilter/bpfilter_kern.c
@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/umh.h>
+#include <linux/bpfilter.h>
+#include <linux/sched.h>
+#include <linux/sched/signal.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include "msgfmt.h"
+
+#define UMH_start _binary_net_bpfilter_bpfilter_umh_start
+#define UMH_end _binary_net_bpfilter_bpfilter_umh_end
+
+extern char UMH_start;
+extern char UMH_end;
+
+static struct umh_info info;
+/* since ip_getsockopt() can run in parallel, serialize access to umh */
+static DEFINE_MUTEX(bpfilter_lock);
+
+static void shutdown_umh(struct umh_info *info)
+{
+	struct task_struct *tsk;
+
+	tsk = pid_task(find_vpid(info->pid), PIDTYPE_PID);
+	if (tsk)
+		force_sig(SIGKILL, tsk);
+	fput(info->pipe_to_umh);
+	fput(info->pipe_from_umh);
+}
+
+static void __stop_umh(void)
+{
+	if (bpfilter_process_sockopt) {
+		bpfilter_process_sockopt = NULL;
+		shutdown_umh(&info);
+	}
+}
+
+static void stop_umh(void)
+{
+	mutex_lock(&bpfilter_lock);
+	__stop_umh();
+	mutex_unlock(&bpfilter_lock);
+}
+
+static int __bpfilter_process_sockopt(struct sock *sk, int optname,
+				      char __user *optval,
+				      unsigned int optlen, bool is_set)
+{
+	struct mbox_request req;
+	struct mbox_reply reply;
+	loff_t pos;
+	ssize_t n;
+	int ret;
+
+	req.is_set = is_set;
+	req.pid = current->pid;
+	req.cmd = optname;
+	req.addr = (long)optval;
+	req.len = optlen;
+	mutex_lock(&bpfilter_lock);
+	n = __kernel_write(info.pipe_to_umh, &req, sizeof(req), &pos);
+	if (n != sizeof(req)) {
+		pr_err("write fail %zd\n", n);
+		__stop_umh();
+		ret = -EFAULT;
+		goto out;
+	}
+	pos = 0;
+	n = kernel_read(info.pipe_from_umh, &reply, sizeof(reply), &pos);
+	if (n != sizeof(reply)) {
+		pr_err("read fail %zd\n", n);
+		__stop_umh();
+		ret = -EFAULT;
+		goto out;
+	}
+	ret = reply.status;
+out:
+	mutex_unlock(&bpfilter_lock);
+	return ret;
+}
+
+static int __init load_umh(void)
+{
+	int err;
+
+	/* fork usermode process */
+	err = fork_usermode_blob(&UMH_start, &UMH_end - &UMH_start, &info);
+	if (err)
+		return err;
+	pr_info("Loaded bpfilter_umh pid %d\n", info.pid);
+
+	/* health check that usermode process started correctly */
+	if (__bpfilter_process_sockopt(NULL, 0, 0, 0, 0) != 0) {
+		stop_umh();
+		return -EFAULT;
+	}
+	bpfilter_process_sockopt = &__bpfilter_process_sockopt;
+	return 0;
+}
+
+static void __exit fini_umh(void)
+{
+	stop_umh();
+}
+module_init(load_umh);
+module_exit(fini_umh);
+MODULE_LICENSE("GPL");