[PATCH] Bluetooth: 6lowpan: hold L2CAP conn across debugfs control

[PATCH] Bluetooth: 6lowpan: hold L2CAP conn across debugfs control

[Cen Zhang]

get_l2cap_conn() looks up an LE hci_conn under hdev protection, but
then drops that protection before reading hcon->l2cap_data and before
lowpan_control_write() later dereferences conn->hcon.  A disconnect or
device close can tear down the same L2CAP connection in that window.

The buggy scenario involves two paths, with each column showing the order
within that path:

6LoWPAN control write:              HCI disconnect/device close:
  1. get_l2cap_conn() finds hcon      1. hci_disconn_cfm() dispatches
     and hcon->l2cap_data.               the L2CAP disconnect callback.
  2. get_l2cap_conn() drops hdev      2. l2cap_conn_del() clears
     protection and returns conn.        hcon->l2cap_data and drops the
                                         L2CAP connection reference.
  3. lowpan_control_write() reads     3. hci_conn_del() removes and drops
     conn->hcon.                         the HCI connection.

Take a reference to the L2CAP connection with
l2cap_conn_hold_unless_zero() while hdev is still locked, and drop that
reference after the debugfs command's last use of conn.  This mirrors the
existing L2CAP ACL receive-side handoff and keeps the connection
dereferenceable after leaving hdev protection.  Export the existing helper
so the bluetooth_6lowpan module can use the same lifetime primitive.

Validation reproduced this kernel report:
BUG: KASAN: slab-use-after-free in lowpan_control_write+0x374/0x520
The buggy address belongs to the object at ffff888111b9d000 which belongs
to the cache kmalloc-1k of size 1024
The buggy address is located 0 bytes inside of freed 1024-byte region
[ffff888111b9d000, ffff888111b9d400)
Read of size 8
Call trace:
  dump_stack_lvl+0x66/0xa0
  print_report+0xce/0x5f0
  lowpan_control_write+0x374/0x520 (net/bluetooth/6lowpan.c:1131)
  srso_alias_return_thunk+0x5/0xfbef5
  __virt_addr_valid+0x19f/0x330
  kasan_report+0xe0/0x110
  __debugfs_file_get+0xf7/0x400
  full_proxy_write+0x9e/0xd0
  vfs_write+0x1b0/0x810
  ksys_write+0xd2/0x170
  dnotify_flush+0x32/0x220
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
Allocated by task stack:
  kasan_save_stack+0x33/0x60
  kasan_save_track+0x17/0x60
  __kasan_kmalloc+0xaa/0xb0
  l2cap_conn_add+0x45/0x520
  l2cap_chan_connect+0xac6/0xd90
  l2cap_sock_connect+0x216/0x350
  __sys_connect+0x101/0x130
  __x64_sys_connect+0x40/0x50
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
Freed by task stack:
  kasan_save_stack+0x33/0x60
  kasan_save_track+0x17/0x60
  kasan_save_free_info+0x3b/0x60
  __kasan_slab_free+0x5f/0x80
  kfree+0x313/0x590
  hci_conn_hash_flush+0xc0/0x140
  hci_dev_close_sync+0x41a/0xb00
  hci_dev_close+0x12f/0x160
  hci_sock_ioctl+0x157/0x570
  sock_do_ioctl+0xf7/0x210
  sock_ioctl+0x32f/0x490
  __x64_sys_ioctl+0xc7/0x110
  do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  kasan_record_aux_stack+0xa7/0xc0
  insert_work+0x32/0x100
  __queue_work+0x262/0xa60
  queue_work_on+0xad/0xb0
  l2cap_connect_cfm+0x4ef/0x670
  hci_le_remote_feat_complete_evt+0x247/0x430
  hci_event_packet+0x360/0x6f0
  hci_rx_work+0x2ae/0x7a0
  process_one_work+0x4fd/0xbc0
  worker_thread+0x2d8/0x570
  kthread+0x1ad/0x1f0
  ret_from_fork+0x3c9/0x540
  ret_from_fork_asm+0x1a/0x30

Fixes: 6b8d4a6a0314 ("Bluetooth: 6LoWPAN: Use connected oriented channel instead of fixed one")
Assisted-by: Codex:gpt-5.5
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
 net/bluetooth/6lowpan.c    | 21 +++++++++++++++------
 net/bluetooth/l2cap_core.c |  1 +
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/net/bluetooth/6lowpan.c b/net/bluetooth/6lowpan.c
index cb1e329d66fd..2fa1cc3de796 100644
--- a/net/bluetooth/6lowpan.c
+++ b/net/bluetooth/6lowpan.c
@@ -1029,16 +1029,19 @@ static int get_l2cap_conn(char *buf, bdaddr_t *addr, u8 *addr_type,
 
 	hci_dev_lock(hdev);
 	hcon = hci_conn_hash_lookup_le(hdev, addr, le_addr_type);
-	hci_dev_unlock(hdev);
-	hci_dev_put(hdev);
-
-	if (!hcon)
+	if (!hcon) {
+		hci_dev_unlock(hdev);
+		hci_dev_put(hdev);
 		return -ENOENT;
+	}
 
-	*conn = (struct l2cap_conn *)hcon->l2cap_data;
+	*conn = l2cap_conn_hold_unless_zero(hcon->l2cap_data);
 
 	BT_DBG("conn %p dst %pMR type %u", *conn, &hcon->dst, hcon->dst_type);
 
+	hci_dev_unlock(hdev);
+	hci_dev_put(hdev);
+
 	return 0;
 }
 
@@ -1185,18 +1188,22 @@ static ssize_t lowpan_control_write(struct file *fp,
 		if (conn) {
 			struct lowpan_peer *peer;
 
-			if (!is_bt_6lowpan(conn->hcon))
+			if (!is_bt_6lowpan(conn->hcon)) {
+				l2cap_conn_put(conn);
 				return -EINVAL;
+			}
 
 			peer = lookup_peer(conn);
 			if (peer) {
 				BT_DBG("6LoWPAN connection already exists");
+				l2cap_conn_put(conn);
 				return -EALREADY;
 			}
 
 			BT_DBG("conn %p dst %pMR type %d user %u", conn,
 			       &conn->hcon->dst, conn->hcon->dst_type,
 			       addr_type);
+			l2cap_conn_put(conn);
 		}
 
 		ret = bt_6lowpan_connect(&addr, addr_type);
@@ -1212,6 +1219,8 @@ static ssize_t lowpan_control_write(struct file *fp,
 			return ret;
 
 		ret = bt_6lowpan_disconnect(conn, addr_type);
+		if (conn)
+			l2cap_conn_put(conn);
 		if (ret < 0)
 			return ret;
 
diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 62133eef9d2f..ed668a782d3d 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -7702,6 +7702,7 @@ struct l2cap_conn *l2cap_conn_hold_unless_zero(struct l2cap_conn *c)
 
 	return c;
 }
+EXPORT_SYMBOL(l2cap_conn_hold_unless_zero);
 
 int l2cap_recv_acldata(struct hci_dev *hdev, u16 handle,
 		       struct sk_buff *skb, u16 flags)
-- 
2.43.0

RE: Bluetooth: 6lowpan: hold L2CAP conn across debugfs control

[bluez.test.bot]

[-- Attachment #1: Type: text/plain, Size: 1557 bytes --]

This is automated email and please do not reply to this email!

Dear submitter,

Thank you for submitting the patches to the linux bluetooth mailing list.
This is a CI test results with your patch series:
PW Link:https://patchwork.kernel.org/project/bluetooth/list/?series=1115490

---Test result---

Test Summary:
CheckPatch                    PASS      1.39 seconds
VerifyFixes                   PASS      0.23 seconds
VerifySignedoff               PASS      0.24 seconds
GitLint                       PASS      0.56 seconds
SubjectPrefix                 PASS      0.26 seconds
BuildKernel                   PASS      27.06 seconds
CheckAllWarning               PASS      30.15 seconds
CheckSparse                   PASS      28.73 seconds
BuildKernel32                 PASS      26.22 seconds
CheckKernelLLVM               SKIP      0.00 seconds
TestRunnerSetup               PASS      578.67 seconds
TestRunner_l2cap-tester       FAIL      59.26 seconds
TestRunner_6lowpan-tester     PASS      22.98 seconds
IncrementalBuild              PASS      26.09 seconds

Details
##############################
Test: CheckKernelLLVM - SKIP
Desc: Build kernel with LLVM + context analysis
Output:
Clang not found
##############################
Test: TestRunner_l2cap-tester - FAIL
Desc: Run l2cap-tester with test-runner
Output:
Total: 96, Passed: 95 (99.0%), Failed: 1, Not Run: 0

Failed Test Cases
L2CAP BR/EDR Server - Set PHY 2M                     Failed       0.253 seconds


https://github.com/bluez/bluetooth-next/pull/341

---
Regards,
Linux Bluetooth