Hi Marcel,
On Tue, 29 Sep 2020 at 00:43, Marcel Holtmann <mar...@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 <apus...@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.
> >>
> >> while that might be the case, I am still super careful. Especially also in
> >> conjunction with the email / patch from Alex trying to add just another
> >> encryption key size check. If we really need them or even both, we have to
> >> audit the whole code since I must have clearly missed something when
> >> adding the KNOB fix.
> >>
> >>>> 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.
> >>
> >> That part worries me since there should be no gaps that allows an
> >> encryption key size downgrade if our side supports Read Encryption Key
> >> Size.
> >>
> >> We really have to ensure that any L2CAP communication is stalled until we
> >> have all information from HCI connection setup that we need. So maybe the
> >> change Alex did would work as well, or as I mentioned put any L2CAP
> >> connection request as pending so that the validation happens in one place.
> >
> > I think Alex and I are solving the same problem, either one of the
> > patches should be enough.
> >
> > Here is my test method using BlueZ as both the IUT and the lower test.
> > (1) Copy the bluez/test/test-profile python script to IUT and lower test.
> > (2) Assign a fake service server to IUT
> > python test-profile -u 00001fff-0000-1000-2000-123456789abc -s -P 4097
> > (3) Assign a fake service client to lower test
> > python test-profile -u 00001fff-0000-1000-2000-123456789abc -c
> > (4) Make the lower test accept weak encryption key
> > echo 1 > /sys/kernel/debug/bluetooth/hci0/min_encrypt_key_size
> > (5) Enable ssp and disable sc on lower test
> > btmgmt ssp on
> > btmgmt sc off
> > (6) Set lower test encryption key size to 1
> > (7) initiate connection from lower test
> > dbus-send --system --print-reply --dest=org.bluez
> > /org/bluez/hci0/dev_<IUT> org.bluez.Device1.ConnectProfile
> > string:00001fff-0000-1000-2000-123456789abc
> >
> > After MITM authentication, IUT will incorrectly accept the connection,
> > even though the encryption key used is less than the one specified in
> > IUT's min_encrypt_key_size.
>
> I almost assumed that you two are chasing the same issue here. Problem is I
> really don’t yet know where to correctly put that encryption key size check.
>
> There is one case in l2cap_connect() that will not respond with L2CAP_CR_PEND.
>
> /* Force pending result for AMP controllers.
> * The connection will succeed after the
> * physical link is up.
> */
> if (amp_id == AMP_ID_BREDR) {
> l2cap_state_change(chan, BT_CONFIG);
> result = L2CAP_CR_SUCCESS;
> } else {
> l2cap_state_change(chan, BT_CONNECT2);
> result = L2CAP_CR_PEND;
> }
> status = L2CAP_CS_NO_INFO;
>
> Most services will actually use FLAG_DEFER_SETUP and then you also don’t run
> into this issue since at this stage the response is L2CAP_CR_PEND as well.
>
> One question we should answer is if we just always return L2CAP_CR_PEND or if
> we actually add the check for the encryption key size here as well. This has
> always been a shortcut to avoid an unneeded round-trip if all information are
> present. Question really is if all information are present or if this is just
> pure luck. I don’t see a guarantee that the encryption key size has been read
> in any of your patches.
>
> Everywhere else in the code we have this sequence of checks:
>
> l2cap_chan_check_security()
>
> l2cap_check_enc_key_size()
>
> This is generally how l2cap_do_start() or l2cap_conn_start() do their job. So
> we might have to restructure l2cap_connect() a little bit for following the
> same principle.
>
> Anyhow, before do this, can we try if this patch fixes this as well and check
> the btmon trace for it:
>
> diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
> index 1ab27b90ddcb..88e4c1292b98 100644
> --- a/net/bluetooth/l2cap_core.c
> +++ b/net/bluetooth/l2cap_core.c
> @@ -4156,17 +4156,8 @@ static struct l2cap_chan *l2cap_connect(struct
> l2cap_conn *conn,
> status = L2CAP_CS_AUTHOR_PEND;
> chan->ops->defer(chan);
> } else {
> - /* Force pending result for AMP controllers.
> - * The connection will succeed after the
> - * physical link is up.
> - */
> - if (amp_id == AMP_ID_BREDR) {
> - l2cap_state_change(chan, BT_CONFIG);
> - result = L2CAP_CR_SUCCESS;
> - } else {
> - l2cap_state_change(chan, BT_CONNECT2);
> - result = L2CAP_CR_PEND;
> - }
> + l2cap_state_change(chan, BT_CONNECT2);
> + result = L2CAP_CR_PEND;
> status = L2CAP_CS_NO_INFO;
> }
> } else {
>
> If this fixes your issue and puts the encryption key size check back in play,
> then I just have to think about on how to fix this.
That patch alone doesn't fix the issue I have. By applying it, the
only difference I am aware of is we would first reply "connection
pending" to the initial SDP request of the peripheral, instead of just
"connection successful". Subsequent L2CAP connections go to the
FLAG_DEFER_SETUP branch just a tad above the change in the patch, so
they are not affected at all.
>
> Regards
>
> Marcel
>
Thanks,
Archie
