On Wed, May 24, 2017 at 02:36:55AM +0100, Ben Hutchings wrote:
> On Thu, 2017-05-18 at 12:49 +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch. If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Johan Hovold <johan@xxxxxxxxxx>
> >
> > commit 95065a61e9bf25fb85295127fba893200c2bbbd8 upstream.
> >
> > Make sure to check the tty-device pointer before looking up the sibling
> > platform device to avoid dereferencing a NULL-pointer when the tty is
> > one end of a Unix98 pty.
> >
> > Fixes: 0395ffc1ee05 ("Bluetooth: hci_bcm: Add PM for BCM devices")
> > Cc: Frederic Danis <frederic.danis@xxxxxxxxxxxxxxx>
> > Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
> > Signed-off-by: Marcel Holtmann <marcel@xxxxxxxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> >
> > ---
> > drivers/bluetooth/hci_bcm.c | 5 ++++-
> > 1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > --- a/drivers/bluetooth/hci_bcm.c
> > +++ b/drivers/bluetooth/hci_bcm.c
> > @@ -287,6 +287,9 @@ static int bcm_open(struct hci_uart *hu)
> >
> > hu->priv = bcm;
> >
> > + if (!hu->tty->dev)
> > + goto out;
> > +
> > mutex_lock(&bcm_device_lock);
> > list_for_each(p, &bcm_device_list) {
> > struct bcm_device *dev = list_entry(p, struct bcm_device, list);
> > @@ -307,7 +310,7 @@ static int bcm_open(struct hci_uart *hu)
> > }
> >
> > mutex_unlock(&bcm_device_lock);
> > -
> > +out:
> > return 0;
> > }
>
> I'm a bit sceptical that this is fixing a real bug,
Please take a look at bcm_open() where the tty device is being
dereferenced whenever there is bcm platform device registered:
list_for_each(p, &bcm_device_list) {
struct bcm_device *dev = list_entry(p, struct bcm_device, list);
...
if (hu->tty->dev->parent == dev->pdev->dev.parent) {
This can be used by an unprivileged user to trigger a NULL-dereference:
Unable to handle kernel NULL pointer dereference at virtual address 00000000
...
[<bf0d5d74>] (bcm_open [hci_uart]) from [<bf0d1284>] (hci_uart_tty_ioctl+0x1b0/0x3c8 [hci_uart])
[<bf0d1284>] (hci_uart_tty_ioctl [hci_uart]) from [<c0465bcc>] (tty_ioctl+0x5bc/0xbc8)
[<c0465bcc>] (tty_ioctl) from [<c0258ad4>] (do_vfs_ioctl+0xac/0x9e8)
[<c0258ad4>] (do_vfs_ioctl) from [<c0259454>] (SyS_ioctl+0x44/0x6c)
[<c0259454>] (SyS_ioctl) from [<c01092e0>] (ret_fast_syscall+0x0/0x1c)
> but if it is -
> surely bcm_open() should fail if the tty device is not the right type?
> bcm_setup() would certainly fail after this.
The driver is written to be able to handle a non-existing platform
device (e.g. see use of bcm_device_exists() in the commit introducing
this issue), although since commit 6cc4396c8829 ("Bluetooth: hci_bcm:
Add wake-up capability") setup would indeed fail in this case (well at
least when firmware *is* available...).
Perhaps that is a separate issue that should be fixed (by failing early
in open as you suggest or by again allowing the driver to work without a
platform device), but I believe this patch is correct nonetheless. And
it does specifically fix the local DoS-attack.
Thanks,
Johan