Re: [PATCH v4 12/13] bluetooth/smp: ensure RNG is properly seeded before ECDH use

[Jason A. Donenfeld]

On Thu, Jun 8, 2017 at 7:05 PM, Marcel Holtmann  wrote:
> on a powered down controller, you can not do any crypto. SMP is only during a 
> connection and the RPAs are only generated when needed. So yes, doing this 
> once in hci_power_on is plenty. However we might want to limit this to LE 
> capable controllers since for BR/EDR only controllers this is not needed. For 
> A2MP I need to check that we need the random numbers seeded there. However 
> this hidden behind the high speed feature.

Okay so it sounds like certain controllers will use this and certain
won't, and so it might be slightly complicated to follow the mouse
through the tube ideally. In that case, I'd recommend continuing with
this current patchset, which just adds the wait (which is a no-op if
it's already seeded) close to the actual call sites.

Can you review that patch and give your Signed-off-by if it looks good?

Jason

Re: [PATCH v4 12/13] bluetooth/smp: ensure RNG is properly seeded before ECDH use

[Marcel Holtmann]

Hi Jason,

>> yes, there are plenty of commands needed before a controller becomes usable.
> 
> That doesn't clearly address with precision what Ted was wondering.
> Specifically, the inquiry is: can you confirm with certainty whether
> or not all calls to get_random_bytes() in the bluetooth directory are
> *necessarily* going to come after a call to hci_power_on()?

on a powered down controller, you can not do any crypto. SMP is only during a 
connection and the RPAs are only generated when needed. So yes, doing this once 
in hci_power_on is plenty. However we might want to limit this to LE capable 
controllers since for BR/EDR only controllers this is not needed. For A2MP I 
need to check that we need the random numbers seeded there. However this hidden 
behind the high speed feature.

Regards

Marcel

Re: [PATCH v4 12/13] bluetooth/smp: ensure RNG is properly seeded before ECDH use

[Jason A. Donenfeld]

Hello Marcel,

On Thu, Jun 8, 2017 at 7:04 AM, Marcel Holtmann  wrote:
> yes, there are plenty of commands needed before a controller becomes usable.

That doesn't clearly address with precision what Ted was wondering.
Specifically, the inquiry is: can you confirm with certainty whether
or not all calls to get_random_bytes() in the bluetooth directory are
*necessarily* going to come after a call to hci_power_on()?

Jason

Re: [PATCH v4 12/13] bluetooth/smp: ensure RNG is properly seeded before ECDH use

[Marcel Holtmann]

Hi Ted,

>> This protocol uses lots of complex cryptography that relies on securely
>> generated random numbers. Thus, it's important that the RNG is actually
>> seeded before use. Fortuantely, it appears we're always operating in
>> process context (there are many GFP_KERNEL allocations and other
>> sleeping operations), and so we can simply demand that the RNG is seeded
>> before we use it.
>> 
>> We take two strategies in this commit. The first is for the library code
>> that's called from other modules like hci or mgmt: here we just change
>> the call to get_random_bytes_wait, and return the result of the wait to
>> the caller, along with the other error codes of those functions like
>> usual. Then there's the SMP protocol handler itself, which makes many
>> many many calls to get_random_bytes during different phases. For this,
>> rather than have to change all the calls to get_random_bytes_wait and
>> propagate the error result, it's actually enough to just put a single
>> call to wait_for_random_bytes() at the beginning of the handler, to
>> ensure that all the subsequent invocations are safe, without having to
>> actually change them. Likewise, for the random address changing
>> function, we'd rather know early on in the function whether the RNG
>> initialization has been interrupted, rather than later, so we call
>> wait_for_random_bytes() at the top, so that later on the call to
>> get_random_bytes() is acceptable.
> 
> Do we need to do all of this?  Bluetooth folks, is it fair to assume
> that hci_power_on() has to be called before any bluetooth functions
> can be done?  If so, adding a wait_for_random_bytes() in
> hci_power_on() might be all that is necessary.

yes, there are plenty of commands needed before a controller becomes usable. 
When plugging in new Bluetooth hardware, we have to power it up and read the 
initial settings and configuration out of.

Also all the cryptographic features only apply to LE enabled controllers. The 
classic BR/EDR controllers have this all in hardware. So if you are not LE 
enabled, then there is not even a point in waiting for any seeding. However 
that said, also all LE controllers have an extra random number function we 
could call if we need extra seeding. We never bothered to hook this up since we 
thought that the kernel has enough sources.

Regards

Marcel

Re: [PATCH v4 12/13] bluetooth/smp: ensure RNG is properly seeded before ECDH use

[Theodore Ts'o]

On Tue, Jun 06, 2017 at 07:48:03PM +0200, Jason A. Donenfeld wrote:
> This protocol uses lots of complex cryptography that relies on securely
> generated random numbers. Thus, it's important that the RNG is actually
> seeded before use. Fortuantely, it appears we're always operating in
> process context (there are many GFP_KERNEL allocations and other
> sleeping operations), and so we can simply demand that the RNG is seeded
> before we use it.
> 
> We take two strategies in this commit. The first is for the library code
> that's called from other modules like hci or mgmt: here we just change
> the call to get_random_bytes_wait, and return the result of the wait to
> the caller, along with the other error codes of those functions like
> usual. Then there's the SMP protocol handler itself, which makes many
> many many calls to get_random_bytes during different phases. For this,
> rather than have to change all the calls to get_random_bytes_wait and
> propagate the error result, it's actually enough to just put a single
> call to wait_for_random_bytes() at the beginning of the handler, to
> ensure that all the subsequent invocations are safe, without having to
> actually change them. Likewise, for the random address changing
> function, we'd rather know early on in the function whether the RNG
> initialization has been interrupted, rather than later, so we call
> wait_for_random_bytes() at the top, so that later on the call to
> get_random_bytes() is acceptable.

Do we need to do all of this?  Bluetooth folks, is it fair to assume
that hci_power_on() has to be called before any bluetooth functions
can be done?  If so, adding a wait_for_random_bytes() in
hci_power_on() might be all that is necessary.

- Ted