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.
Regards
Marcel
