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Subject: Re: [PATCH v2] bluetooth: Enforce classic key size verification.
From:   Marcel Holtmann <marcel@holtmann.org>
In-Reply-To: <1AFDC1E2-8875-4EFC-8A75-DAB89DA9FFB5@holtmann.org>
Date:   Wed, 25 Mar 2020 19:19:02 +0100
Cc:     Alain Michaud <alainm@chromium.org>,
        Marcel Holtmann <marcel.holtmann@intel.com>,
        BlueZ <linux-bluetooth@vger.kernel.org>
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To:     Alain Michaud <alainmichaud@google.com>
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Hi Alain,

>> I suspect we'd want bluetoothd to have a configuration that can enforce a more secure posture.
>> 
>> Unfortunately when the command isn't supported, the platform is left between a rock and hard place... There isn't much we can do but to block the use of Bluetooth if the platform requires a more secure posture.
> 
> so if the BR/EDR part is not up to the policy that the host requires, we could still configure the LE part. BlueZ is set up in this way that you can run a dual-mode controller as just a LE controller.
> 
> I would also opt for the kernel just tells us what options it have. Then at least we can provide some feedback to the end-user on why Bluetooth is not available or why only selected features are available.

what about something like this:

+Read Security Features Command
+==============================
+
+       Command Code:           0x0048
+       Controller Index:       <controller id>
+       Command Parameters:
+       Return Parameters:      Security_Data_Length (2 Octets)
+                               Security_Data (0-65535 Octets)
+
+       This command is used to retrieve the supported security features
+       by the controller or the kernel.
+
+       The Security_Data_Length and Security_Data parameters provide
+       a list of security settings, features and information. It uses
+       the same format as EIR_Data, but with the namespace defined here.
+
+               Data Type       Name
+               --------------------
+               0x01            Flags
+               0x02            Max Encryption Key Size (BR/EDR)
+               0x03            Max Encryption Key Size (LE)
+               0x04            Encryption Key Size enforcement (BR/EDR)
+               0x05            Encryption Key Size enforcement (LE)
+               0x06            ECDH Public Key validation (BR/EDR)
+               0x07            ECDH Public Key validation (LE)
+
+
+       Max Encryption Key Size (BR/EDR and LE)
+
+               When the field is present, then it provides 1 Octet value
+               indicating the max encryption key size. If the field is not
+               present, then it is unknown what the max encryption key
+               size of the controller or host is in use.
+
+       Encryption Key Size Enforcement (BR/EDR and LE)
+
+               When the field is present, then it provides 1 Octet value
+               indicating the min encryption key size that is enforced by
+               the controller or host. If the field is not present, then
+               it is unknown what the controller or host are enforcing.
+
+       ECDH Public Key validation (BR/EDR and LE)
+
+               When the field is present, then it provides 1 Octet value
+               indicating if public key validation is in use (0x01) or not
+               available (0x00). If the field is not present, then it is
+               unknown if the controller or host are validating public keys.
+
+       This command generates a Command Complete event on success or
+       a Command Status event on failure.
+
+       Possible errors:        Invalid Parameters
+                               Invalid Index

Maybe this is overkill, but it would give us some flexible way of having the kernel tell us what is supported. Then bluetoothd can decide to power a controller or parts of a controller.

Regards

Marcel