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

bpfilter.ko module can be removed while functions of 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.

test commands:
while :
do

	iptables -I FORWARD -m string --string ap --algo kmp &
	modprobe -rv bpfilter &

done

splat looks like:

[ 1074.741758] BUG: unable to handle kernel paging request at fffffbfff805540b
[ 1074.741864] PGD 124327067 P4D 124327067 PUD 11c1a3067 PMD 119eaf067 PTE 0
[ 1074.755849] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[ 1074.755849] CPU: 0 PID: 11561 Comm: iptables Not tainted 4.20.0-rc6+ #62
[ 1074.763907] RIP: 0010:__mutex_lock+0x6b9/0x16a0
[ 1074.763907] Code: d2 00 00 e8 d9 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
[ 1074.763907] RSP: 0018:ffff88810286f7a0 EFLAGS: 00010202
[ 1074.763907] RAX: 1ffffffff805540b RBX: ffff888112760040 RCX: 0000000000000000
[ 1074.763907] RDX: 1ffff110235ff806 RSI: ffff8881127607f8 RDI: dffffc0000000000
[ 1074.763907] RBP: ffff88810286fb30 R08: 0000000080000002 R09: 0000000000000000
[ 1074.763907] R10: 0000000000000000 R11: 0000000000000000 R12: fffffbfff5288220
[ 1074.763907] R13: ffff888112760040 R14: ffff88810d421a05 R15: ffffffffc02aa000
[ 1074.763907] FS:  00007fd35394e700(0000) GS:ffff88811ae00000(0000) knlGS:0000000000000000
[ 1074.763907] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 1074.763907] CR2: fffffbfff805540b CR3: 000000010d182000 CR4: 00000000001006f0
[ 1074.763907] Call Trace:
[ 1074.763907]  ? mutex_lock_io_nested+0x1560/0x1560
[ 1074.763907]  ? kasan_kmalloc+0xa0/0xd0
[ 1074.763907]  ? kmem_cache_alloc+0x1c2/0x260
[ 1074.763907]  ? __alloc_file+0x92/0x3c0
[ 1074.763907]  ? alloc_empty_file+0x43/0x120
[ 1074.763907]  ? alloc_file_pseudo+0x220/0x330
[ 1074.763907]  ? sock_alloc_file+0x39/0x160
[ 1074.763907]  ? __sys_socket+0x113/0x1d0
[ 1074.763907]  ? __x64_sys_socket+0x6f/0xb0
[ 1074.763907]  ? do_syscall_64+0x138/0x560
[ 1074.763907]  ? entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 1074.763907]  ? trace_hardirqs_on_caller+0x9a/0x220
[ 1074.763907]  ? trace_hardirqs_off_caller+0x220/0x220
[ 1074.763907]  ? __alloc_file+0x92/0x3c0
[ 1074.763907]  ? __alloc_file+0x92/0x3c0
[ 1074.763907]  ? __alloc_file+0x92/0x3c0
[ 1074.763907]  ? cyc2ns_read_end+0x10/0x10
[ 1074.763907]  ? cyc2ns_read_end+0x10/0x10
[ 1074.763907]  ? hlock_class+0x140/0x140
[ 1074.763907]  ? sched_clock_local+0xd4/0x140
[ 1074.763907]  ? sched_clock_local+0xd4/0x140
[ 1074.763907]  ? check_flags.part.35+0x440/0x440
[ 1074.763907]  ? __lock_acquire+0x4f90/0x4f90
[ 1074.763907]  ? perf_trace_sched_switch+0xee0/0xee0
[ 1074.763907]  ? __fget_light+0xb3/0x360
[ 1074.763907]  ? fget_raw+0x10/0x10
[ 1074.763907]  ? bpfilter_ip_get_sockopt+0x30/0x60
[ 1074.763907]  ? ip_getsockopt+0xc7/0x1a0

[ ... ]

---
 include/linux/bpfilter.h     | 16 ++++++++++----
 net/bpfilter/bpfilter_kern.c | 27 ++++++++---------------
 net/ipv4/bpfilter/sockopt.c  | 42 ++++++++++++++++++++++++------------
 3 files changed, 49 insertions(+), 36 deletions(-)

index 3039361cd65a..87afd0d2b9bf 100644
@@ -9,9 +9,17 @@ int bpfilter_ip_set_sockopt(struct sock *sk, int optname, char __user *optval,
 			    unsigned int optlen);
 int bpfilter_ip_get_sockopt(struct sock *sk, int optname, char __user *optval,
 			    int __user *optlen);
-extern int (*bpfilter_process_sockopt)(struct sock *sk, int optname,
-				       char __user *optval,
-				       unsigned int optlen, bool is_set);
-extern int (*bpfilter_start_umh)(void);
+
+struct bpfilter_umh_ops {
+	int (*process_sockopt)(struct sock *sk, int optname,
+			       char __user *optval,
+			       unsigned int optlen, bool is_set);
+	int (*start_umh)(void);
+	/*
+	 * since ip_getsockopt() can run in parallel, serialize access to umh.
+	 */
+	struct mutex mutex;
+};
+extern struct bpfilter_umh_ops bpfilter_ops;
 
 #endif

index 7397f5e8da2f..a868c9b2e9c7 100644
@@ -14,8 +14,6 @@ extern char bpfilter_umh_start;
 extern char bpfilter_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)
 {

@@ -33,21 +31,14 @@ static void shutdown_umh(struct umh_info *info)
 	info->pid = 0;
 }
 
-static void __stop_umh(void)
+static void stop_umh(void)
 {
 	if (IS_ENABLED(CONFIG_INET)) {
-		bpfilter_process_sockopt = NULL;
+		bpfilter_ops.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)

@@ -63,13 +54,12 @@ 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 (!info.pid)
 		goto out;
 	n = __kernel_write(info.pipe_to_umh, &req, sizeof(req), &pos);
 	if (n != sizeof(req)) {
 		pr_err("write fail %zd\n", n);
-		__stop_umh();
+		stop_umh();
 		ret = -EFAULT;
 		goto out;
 	}

@@ -77,13 +67,12 @@ static int __bpfilter_process_sockopt(struct sock *sk, int optname,
 	n = kernel_read(info.pipe_from_umh, &reply, sizeof(reply), &pos);
 	if (n != sizeof(reply)) {
 		pr_err("read fail %zd\n", n);
-		__stop_umh();
+		stop_umh();
 		ret = -EFAULT;
 		goto out;
 	}
 	ret = reply.status;
 out:
-	mutex_unlock(&bpfilter_lock);
 	return ret;
 }
 

@@ -106,7 +95,7 @@ int start_umh(void)
 		return -EFAULT;
 	}
 	if (IS_ENABLED(CONFIG_INET))
-		bpfilter_process_sockopt = &__bpfilter_process_sockopt;
+		bpfilter_ops.process_sockopt = &__bpfilter_process_sockopt;
 
 	return 0;
 }

@@ -114,16 +103,18 @@ int start_umh(void)
 static int __init load_umh(void)
 {
 	if (IS_ENABLED(CONFIG_INET))
-		bpfilter_start_umh = &start_umh;
+		bpfilter_ops.start_umh = &start_umh;
 
 	return start_umh();
 }
 
 static void __exit fini_umh(void)
 {
+	mutex_lock(&bpfilter_ops.mutex);
 	if (IS_ENABLED(CONFIG_INET))
-		bpfilter_start_umh = NULL;
+		bpfilter_ops.start_umh = NULL;
 	stop_umh();
+	mutex_unlock(&bpfilter_ops.mutex);
 }
 module_init(load_umh);
 module_exit(fini_umh);

2.17.1