net: bpfilter: disallow to remove bpfilter module while being used

The bpfilter.ko module can be removed while functions of the bpfilter.ko
are executing. so panic can occurred. in order to protect that, locks can
be used. a bpfilter_lock protects routines in the
__bpfilter_process_sockopt() but it's not enough because __exit routine
can be executed concurrently.

Now, the bpfilter_umh can not run in parallel.
So, the module do not removed while it's being used and it do not
double-create UMH process.
The members of the umh_info and the bpfilter_umh_ops are protected by
the bpfilter_umh_ops.lock.

test commands:
   while :
   do
	iptables -I FORWARD -m string --string ap --algo kmp &
	modprobe -rv bpfilter &
   done

splat looks like:
[  298.623435] BUG: unable to handle kernel paging request at fffffbfff807440b
[  298.628512] #PF error: [normal kernel read fault]
[  298.633018] PGD 124327067 P4D 124327067 PUD 11c1a3067 PMD 119eb2067 PTE 0
[  298.638859] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  298.638859] CPU: 0 PID: 2997 Comm: iptables Not tainted 4.20.0+ torvalds#154
[  298.638859] RIP: 0010:__mutex_lock+0x6b9/0x16a0
[  298.638859] Code: c0 00 00 e8 89 82 ff ff 80 bd 8f fc ff ff 00 0f 85 d9 05 00 00 48 8b 85 80 fc ff ff 48 bf 00 00 00 00 00 fc ff df 48 c1 e8 03 <80> 3c 38 00 0f 85 1d 0e 00 00 48 8b 85 c8 fc ff ff 49 39 47 58 c6
[  298.638859] RSP: 0018:ffff88810e7777a0 EFLAGS: 00010202
[  298.638859] RAX: 1ffffffff807440b RBX: ffff888111bd4d80 RCX: 0000000000000000
[  298.638859] RDX: 1ffff110235ff806 RSI: ffff888111bd5538 RDI: dffffc0000000000
[  298.638859] RBP: ffff88810e777b30 R08: 0000000080000002 R09: 0000000000000000
[  298.638859] R10: 0000000000000000 R11: 0000000000000000 R12: fffffbfff168a42c
[  298.638859] R13: ffff888111bd4d80 R14: ffff8881040e9a05 R15: ffffffffc03a2000
[  298.638859] FS:  00007f39e3758700(0000) GS:ffff88811ae00000(0000) knlGS:0000000000000000
[  298.638859] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  298.638859] CR2: fffffbfff807440b CR3: 000000011243e000 CR4: 00000000001006f0
[  298.638859] Call Trace:
[  298.638859]  ? mutex_lock_io_nested+0x1560/0x1560
[  298.638859]  ? kasan_kmalloc+0xa0/0xd0
[  298.638859]  ? kmem_cache_alloc+0x1c2/0x260
[  298.638859]  ? __alloc_file+0x92/0x3c0
[  298.638859]  ? alloc_empty_file+0x43/0x120
[  298.638859]  ? alloc_file_pseudo+0x220/0x330
[  298.638859]  ? sock_alloc_file+0x39/0x160
[  298.638859]  ? __sys_socket+0x113/0x1d0
[  298.638859]  ? __x64_sys_socket+0x6f/0xb0
[  298.638859]  ? do_syscall_64+0x138/0x560
[  298.638859]  ? entry_SYSCALL_64_after_hwframe+0x49/0xbe
[  298.638859]  ? __alloc_file+0x92/0x3c0
[  298.638859]  ? init_object+0x6b/0x80
[  298.638859]  ? cyc2ns_read_end+0x10/0x10
[  298.638859]  ? cyc2ns_read_end+0x10/0x10
[  298.638859]  ? hlock_class+0x140/0x140
[  298.638859]  ? sched_clock_local+0xd4/0x140
[  298.638859]  ? sched_clock_local+0xd4/0x140
[  298.638859]  ? check_flags.part.37+0x440/0x440
[  298.638859]  ? __lock_acquire+0x4f90/0x4f90
[  298.638859]  ? set_rq_offline.part.89+0x140/0x140
[ ... ]
Fixes: d2ba09c ("net: add skeleton of bpfilter kernel module")
Signed-off-by: Taehee Yoo <[email protected]>

Author: TaeheeYoo

include/linux/bpfilter.h

@@ -12,6 +12,8 @@ int bpfilter_ip_get_sockopt(struct sock *sk, int optname, char __user *optval,
 			    int __user *optlen);
 struct bpfilter_umh_ops {
 	struct umh_info info;
+	/* since ip_getsockopt() can run in parallel, serialize access to umh */
+	struct mutex lock;
 	int (*sockopt)(struct sock *sk, int optname,
 		       char __user *optval,
 		       unsigned int optlen, bool is_set);

net/bpfilter/bpfilter_kern.c

@@ -13,9 +13,6 @@
 extern char bpfilter_umh_start;
 extern char bpfilter_umh_end;
 
-/* since ip_getsockopt() can run in parallel, serialize access to umh */
-static DEFINE_MUTEX(bpfilter_lock);
-
 static void shutdown_umh(void)
 {
 	struct task_struct *tsk;
@@ -36,13 +33,6 @@ static void __stop_umh(void)
 		shutdown_umh();
 }
 
-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)
@@ -58,7 +48,6 @@ static int __bpfilter_process_sockopt(struct sock *sk, int optname,
 	req.cmd = optname;
 	req.addr = (long __force __user)optval;
 	req.len = optlen;
-	mutex_lock(&bpfilter_lock);
 	if (!bpfilter_ops.info.pid)
 		goto out;
 	n = __kernel_write(bpfilter_ops.info.pipe_to_umh, &req, sizeof(req),
@@ -80,7 +69,6 @@ static int __bpfilter_process_sockopt(struct sock *sk, int optname,
 	}
 	ret = reply.status;
 out:
-	mutex_unlock(&bpfilter_lock);
 	return ret;
 }
 
@@ -99,7 +87,7 @@ static int start_umh(void)
 
 	/* health check that usermode process started correctly */
 	if (__bpfilter_process_sockopt(NULL, 0, NULL, 0, 0) != 0) {
-		stop_umh();
+		shutdown_umh();
 		return -EFAULT;
 	}
 
@@ -110,22 +98,26 @@ static int __init load_umh(void)
 {
 	int err;
 
+	mutex_lock(&bpfilter_ops.lock);
 	err = start_umh();
 	if (!err && IS_ENABLED(CONFIG_INET)) {
 		bpfilter_ops.sockopt = &__bpfilter_process_sockopt;
 		bpfilter_ops.start = &start_umh;
 	}
+	mutex_unlock(&bpfilter_ops.lock);
 
 	return err;
 }
 
 static void __exit fini_umh(void)
 {
+	mutex_lock(&bpfilter_ops.lock);
 	if (IS_ENABLED(CONFIG_INET)) {
+		shutdown_umh();
 		bpfilter_ops.start = NULL;
 		bpfilter_ops.sockopt = NULL;
 	}
-	stop_umh();
+	mutex_unlock(&bpfilter_ops.lock);
 }
 module_init(load_umh);
 module_exit(fini_umh);

net/ipv4/bpfilter/sockopt.c

@@ -14,10 +14,12 @@ EXPORT_SYMBOL_GPL(bpfilter_ops);
 
 static void bpfilter_umh_cleanup(struct umh_info *info)
 {
+	mutex_lock(&bpfilter_ops.lock);
 	bpfilter_ops.stop = true;
 	fput(info->pipe_to_umh);
 	fput(info->pipe_from_umh);
 	info->pid = 0;
+	mutex_unlock(&bpfilter_ops.lock);
 }
 
 static int bpfilter_mbox_request(struct sock *sk, int optname,
@@ -26,20 +28,28 @@ static int bpfilter_mbox_request(struct sock *sk, int optname,
 {
 	int err;
 
+	mutex_lock(&bpfilter_ops.lock);
 	if (!bpfilter_ops.sockopt) {
+		mutex_unlock(&bpfilter_ops.lock);
 		err = request_module("bpfilter");
+		mutex_lock(&bpfilter_ops.lock);
 
 		if (err)
-			return err;
-		if (!bpfilter_ops.sockopt)
-			return -ECHILD;
+			goto out;
+		if (!bpfilter_ops.sockopt) {
+			err = -ECHILD;
+			goto out;
+		}
 	}
 	if (bpfilter_ops.stop) {
 		err = bpfilter_ops.start();
 		if (err)
-			return err;
+			goto out;
 	}
-	return bpfilter_ops.sockopt(sk, optname, optval, optlen, is_set);
+	err =  bpfilter_ops.sockopt(sk, optname, optval, optlen, is_set);
+out:
+	mutex_unlock(&bpfilter_ops.lock);
+	return err;
 }
 
 int bpfilter_ip_set_sockopt(struct sock *sk, int optname, char __user *optval,
@@ -61,6 +71,7 @@ int bpfilter_ip_get_sockopt(struct sock *sk, int optname, char __user *optval,
 
 static int __init bpfilter_sockopt_init(void)
 {
+	mutex_init(&bpfilter_ops.lock);
 	bpfilter_ops.stop = true;
 	bpfilter_ops.info.cmdline = "bpfilter_umh";
 	bpfilter_ops.info.cleanup = &bpfilter_umh_cleanup;