[PATCH V2 5/7] Bluetooth: Fix HCI UART HCI_UART_PROTO_SET locking

[Dean Jenkins <Dean_Jenkins@mentor.com>]

From: Vignesh Raman <Vignesh_Raman@mentor.com>

Here is a NULL pointer dereference that causes a Bluetooth crash:

(hci_uart_tty_receive+0x0/0x84 [hci_uart]) from (flush_to_ldisc+0x104/0x190)
(flush_to_ldisc+0x0/0x190) from (tty_flip_buffer_push+0x50/0x5c)
(tty_flip_buffer_push+0x0/0x5c) from (imx_rxint+0x228/0x23c)
(imx_rxint+0x0/0x23c) from (imx_int+0xa8/0x174)
(imx_int+0x0/0x174) from (handle_irq_event_percpu+0x90/0x280)
(handle_irq_event_percpu+0x0/0x280) from (handle_irq_event+0x44/0x64)
(handle_irq_event+0x0/0x64) from (handle_fasteoi_irq+0xc0/0x108)
(handle_fasteoi_irq+0x0/0x108) from (generic_handle_irq+0x28/0x38)
(generic_handle_irq+0x0/0x38) from (handle_IRQ+0x6c/0x94)
(handle_IRQ+0x0/0x94) from (gic_handle_irq+0x44/0x68)
(gic_handle_irq+0x0/0x68) from (__irq_svc+0x40/0x70)
(bcsp_open+0x0/0xcc [hci_uart]) from (hci_uart_tty_ioctl+0xa8/0x238 [hci_uart])
(hci_uart_tty_ioctl+0x0/0x238 [hci_uart]) from (tty_ioctl+0xa88/0xae8)
(tty_ioctl+0x0/0xae8) from (vfs_ioctl+0x30/0x44)
(vfs_ioctl+0x0/0x44) from (do_vfs_ioctl+0x53c/0x590)
(do_vfs_ioctl+0x0/0x590) from (sys_ioctl+0x40/0x68)
(sys_ioctl+0x0/0x68) from (ret_fast_syscall+0x0/0x30)

It can be seen that bcsp_open() is pre-empted by an interrupt relating
to reception of UART characters. bcsp_open() is called from
hci_uart_set_proto() which is called from hci_uart_tty_ioctl() due to
HCIUARTSETPROTO handling.

The crash occurs in hci_uart_tty_receive() and analysis shows:
	if (!test_bit(HCI_UART_PROTO_SET, &hu->flags))
		return;

	spin_lock(&hu->rx_lock);
	hu->proto->recv(hu, (void *) data, count);

The crash occurs in the dereference hu->proto->recv. Presumably no recv
function has been set and a NULL pointer dereference occurs.

Note that the flag HCI_UART_PROTO_SET being clear should prevent the failure
but in fact fails. This indicates that HCI_UART_PROTO_SET is in the set state.

The HCI_UART_PROTO_SET locking fails because HCI_UART_PROTO_SET is put into
the set state in hci_uart_tty_ioctl() BEFORE calling hci_uart_set_proto()
to set the recv function pointer. Therefore, there is a race condition.

It would seem that HCI_UART_PROTO_SET was being used as a dual flag:

a) HCI_UART_PROTO_SET being used to indicate the protocol Data Link
layer had been successfully opened.

b) HCI_UART_PROTO_SET being used as a lockless solution for
hci_uart_tty_ioctl(). (mutex now added by a previous commit to prevent
concurrency so HCI_UART_PROTO_SET no longer needs to fill that role.)

The solution is to set the flag HCI_UART_PROTO_SET after successfully
calling hci_uart_set_proto() in the HCIUARTSETPROTO handling in
hci_uart_tty_ioctl(). This will prevent hci_uart_tty_receive() from
performing the recv function pointer dereference until hci_uart_set_proto()
has set the recv function pointer.

Signed-off-by: Vignesh Raman <Vignesh_Raman@mentor.com>
Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Signed-off-by: Mark Craske <Mark_Craske@mentor.com>
---
 drivers/bluetooth/hci_ldisc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
index 01590f6..1fc9817 100644
--- a/drivers/bluetooth/hci_ldisc.c
+++ b/drivers/bluetooth/hci_ldisc.c
@@ -698,10 +698,10 @@ static int hci_uart_tty_ioctl(struct tty_struct *tty, struct file *file,
 	mutex_lock(&ioctl_mutex);
 	switch (cmd) {
 	case HCIUARTSETPROTO:
-		if (!test_and_set_bit(HCI_UART_PROTO_SET, &hu->flags)) {
+		if (!test_bit(HCI_UART_PROTO_SET, &hu->flags)) {
 			err = hci_uart_set_proto(hu, arg);
-			if (err)
-				clear_bit(HCI_UART_PROTO_SET, &hu->flags);
+			if (!err)
+				set_bit(HCI_UART_PROTO_SET, &hu->flags);
 		} else
 			err = -EBUSY;
 		break;
-- 
2.7.4