Re: [PATCH v3] Bluetooth: Check for encryption key size on connect

[Archie Pusaka <apusaka@google.com>]

Hi Marcel,

On Sat, 26 Sep 2020 at 00:37, Marcel Holtmann <marcel@holtmann.org> wrote:
>
> Hi Archie,
>
> > When receiving connection, we only check whether the link has been
> > encrypted, but not the encryption key size of the link.
> >
> > This patch adds check for encryption key size, and reject L2CAP
> > connection which size is below the specified threshold (default 7)
> > with security block.
> >
> > Here is some btmon trace.
> > @ MGMT Event: New Link Key (0x0009) plen 26    {0x0001} [hci0] 5.847722
> >        Store hint: No (0x00)
> >        BR/EDR Address: 38:00:25:F7:F1:B0 (OUI 38-00-25)
> >        Key type: Unauthenticated Combination key from P-192 (0x04)
> >        Link key: 7bf2f68c81305d63a6b0ee2c5a7a34bc
> >        PIN length: 0
> >> HCI Event: Encryption Change (0x08) plen 4        #29 [hci0] 5.871537
> >        Status: Success (0x00)
> >        Handle: 256
> >        Encryption: Enabled with E0 (0x01)
> > < HCI Command: Read Encryp... (0x05|0x0008) plen 2  #30 [hci0] 5.871609
> >        Handle: 256
> >> HCI Event: Command Complete (0x0e) plen 7         #31 [hci0] 5.872524
> >      Read Encryption Key Size (0x05|0x0008) ncmd 1
> >        Status: Success (0x00)
> >        Handle: 256
> >        Key size: 3
> >
> > ////// WITHOUT PATCH //////
> >> ACL Data RX: Handle 256 flags 0x02 dlen 12        #42 [hci0] 5.895023
> >      L2CAP: Connection Request (0x02) ident 3 len 4
> >        PSM: 4097 (0x1001)
> >        Source CID: 64
> > < ACL Data TX: Handle 256 flags 0x00 dlen 16        #43 [hci0] 5.895213
> >      L2CAP: Connection Response (0x03) ident 3 len 8
> >        Destination CID: 64
> >        Source CID: 64
> >        Result: Connection successful (0x0000)
> >        Status: No further information available (0x0000)
> >
> > ////// WITH PATCH //////
> >> ACL Data RX: Handle 256 flags 0x02 dlen 12        #42 [hci0] 4.887024
> >      L2CAP: Connection Request (0x02) ident 3 len 4
> >        PSM: 4097 (0x1001)
> >        Source CID: 64
> > < ACL Data TX: Handle 256 flags 0x00 dlen 16        #43 [hci0] 4.887127
> >      L2CAP: Connection Response (0x03) ident 3 len 8
> >        Destination CID: 0
> >        Source CID: 64
> >        Result: Connection refused - security block (0x0003)
> >        Status: No further information available (0x0000)
> >
> > Signed-off-by: Archie Pusaka <apusaka@chromium.org>
> >
> > ---
> >
> > Changes in v3:
> > * Move the check to hci_conn_check_link_mode()
> >
> > Changes in v2:
> > * Add btmon trace to the commit message
> >
> > net/bluetooth/hci_conn.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/net/bluetooth/hci_conn.c b/net/bluetooth/hci_conn.c
> > index 9832f8445d43..89085fac797c 100644
> > --- a/net/bluetooth/hci_conn.c
> > +++ b/net/bluetooth/hci_conn.c
> > @@ -1348,6 +1348,10 @@ int hci_conn_check_link_mode(struct hci_conn *conn)
> >           !test_bit(HCI_CONN_ENCRYPT, &conn->flags))
> >               return 0;
> >
> > +     if (test_bit(HCI_CONN_ENCRYPT, &conn->flags) &&
> > +         conn->enc_key_size < conn->hdev->min_enc_key_size)
> > +             return 0;
> > +
> >       return 1;
> > }
>
> I am a bit concerned since we had that check and I on purpose moved it. See commit 693cd8ce3f88 for the change where I removed and commit d5bb334a8e17 where I initially added it.
>
> Naively adding the check in that location caused a major regression with Bluetooth 2.0 devices. This makes me a bit reluctant to re-add it here since I restructured the whole change to check the key size a different location.

I have tried this patch (both v2 and v3) to connect with a Bluetooth
2.0 device, it doesn't have any connection problem.
I suppose because in the original patch (d5bb334a8e17), there is no
check for the HCI_CONN_ENCRYPT flag.

>
> Now I have to ask, are you running an upstream kernel with both commits above that address KNOB vulnerability?

Actually no, I haven't heard of KNOB vulnerability before.
This patch is written for qualification purposes, specifically to pass
GAP/SEC/SEM/BI-05-C to BI-08-C.
However, it sounds like it could also prevent some KNOB vulnerability
as a bonus.
>
> Regards
>
> Marcel
>

Thanks,
Archie