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From:   Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Date:   Wed, 15 Jul 2020 14:08:51 -0700
Message-ID: <CABBYNZJgc5iWxaDHKTeawN6YZ6=tyjy+Fcs2E1bnxU1Jird41g@mail.gmail.com>
Subject: Re: [PATCH BlueZ 0/3] Per-device option to enable/disable internal profiles
To:     Sonny Sasaka <sonnysasaka@chromium.org>
Cc:     "linux-bluetooth@vger.kernel.org" <linux-bluetooth@vger.kernel.org>
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Sender: linux-bluetooth-owner@vger.kernel.org
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Hi Sonny,

On Wed, Jul 15, 2020 at 1:13 PM Sonny Sasaka <sonnysasaka@chromium.org> wrote:
>
> On Wed, Jul 15, 2020 at 9:25 AM Luiz Augusto von Dentz
> <luiz.dentz@gmail.com> wrote:
> >
> > Hi Sonny,
> >
> > On Tue, Jul 14, 2020 at 1:55 PM Sonny Sasaka <sonnysasaka@chromium.org> wrote:
> > >
> > > Hi Luiz,
> > >
> > > On Mon, Jul 13, 2020 at 10:28 PM Luiz Augusto von Dentz
> > > <luiz.dentz@gmail.com> wrote:
> > > >
> > > > Hi Sonny,
> > > >
> > > > On Mon, Jul 13, 2020 at 4:48 PM Sonny Sasaka <sonnysasaka@chromium.org> wrote:
> > > > >
> > > > > Hi Luiz,
> > > > >
> > > > > On Mon, Jul 13, 2020 at 3:59 PM Luiz Augusto von Dentz
> > > > > <luiz.dentz@gmail.com> wrote:
> > > > > >
> > > > > > Hi Sonny,
> > > > > >
> > > > > > On Mon, Jul 13, 2020 at 3:04 PM Sonny Sasaka <sonnysasaka@chromium.org> wrote:
> > > > > > >
> > > > > > > Hi Luiz,
> > > > > > >
> > > > > > > I considered having such an approach that gives exception to some
> > > > > > > profile to not claim exclusive access. However, I think that this
> > > > > > > approach has a drawback that it can only be guaranteed to work
> > > > > > > correctly for profiles that contain only read-only attributes. Any
> > > > > > > profile that contains writable attributes, naturally, cannot be
> > > > > > > guaranteed to always work correctly (as is the case with the Battery
> > > > > > > profile). Therefore, the usefulness of that feature will be very
> > > > > > > limited.
> > > > > > >
> > > > > > > I also considered the benefits of the AllowInternalProfiles approach:
> > > > > > > * Applications can also have control over any profile, not just
> > > > > > > Battery profile. For example, if in the future BlueZ has more internal
> > > > > > > profiles, like (Blood Pressure Profile or any other profile that may
> > > > > > > contain writable attributes), we can guarantee that applications can
> > > > > > > still opt to have access to that profile, without relying on a profile
> > > > > > > being "safe" to be shared by both BlueZ's internal and external
> > > > > > > handlers.
> > > > > > > * This has an added security benefit: applications which operate on a
> > > > > > > specific GATT profile will not unintentionally activate internal
> > > > > > > profiles such as HOG (which is able to hijack input control of the
> > > > > > > host). This is the correct and expected behavior of Android apps that
> > > > > > > connect over GATT and get access to a GATT profile.
> > > > > >
> > > > > > Not sure I follow these arguments, it seems AllowInternalProfiles may
> > > > > > actually enable hijacking the profiles so I wonder if you got this
> > > > > > backwards as we can't let things like HoG be controlled by
> > > > > > applications directly it would be too easy to implement something like
> > > > > > a keylogger, or perhaps you are suggesting that there is another layer
> > > > > > for implementing the profiles? Note that it is intended that plugins
> > > > > > shall be used for profiles that need to be integrated system wide,
> > > > > > D-Bus interface shall be restricted to only application specific
> > > > > > profiles.
> > > > >
> > > > > I think you misunderstood my point about HOG hijacking. Consider the
> > > > > following case:
> > > > > 1. A legit application (not System UI) on a host computer scans and
> > > > > connects to a nearby peer. It makes a guess about the peer device
> > > > > based on its advertising data. It intends to operate on a specific
> > > > > GATT profile (not necessarily Battery).
> > > > > 2. The peer device turns out to be malicious. It runs HOG GATT server
> > > > > and triggers the host's HOG profile to be active.
> > > > > 3. The malicious peer device's HOG GATT server can now maliciously
> > > > > make mouse movements or enter keystrokes to the host.
> > > >
> > > > I'm not sure how you would like to prevent that, we could in theory
> > > > attempt to authorize every single profile before connecting, just like
> > > > it is done for legacy, but Im sure system would not be asking the user
> > > > what profiles to connect so they just end up trusting the device,
> > > > alternatively we could make ConnectProfile to work also for LE so you
> > > > can provide a UUID and nothing else would be exposed, but note that
> > > > this guess on the AD may actually be wrong and the device may end up
> > > > malfunctioning.
> > > >
> > > > > In this case the user is considered being attacked, because he/she
> > > > > doesn't consciously interact with the System UI to connect to a nearby
> > > > > mouse/keyboard.
> > > > > My example doesn't have to be HOG. It just happens to be a profile
> > > > > which is attackable at the moment. My point is that, for applications
> > > > > it's always safest to turn off all internal profiles to avoid such
> > > > > incident. There is no use case where applications want to trigger
> > > > > internal profiles.
> > > > >
> > > > > Note 1: By "applications", I mean things like Android apps or
> > > > > JavaScript apps which are not considered System's Bluetooth UI.
> > > >
> > > > Well that doesn't make my point moot, let's say we do enable
> > > > connecting by UUID and the application try to connect HoG, it could be
> > > > a malicious application trying to eavesdrop what the user is typing,
> > > > so this problem of malicious goes both ways Im afraid, besides the
> > > > performance penalty that one would have if we need to transport HID
> > > > over D-Bus.
> > > If an application handles HOG, there will be nothing to eavesdrop
> > > because that application shouldn't have an access to UHID in the first
> > > place. If that malicious application had UHID access, that is already
> > > a problem to begin with regardless of whether there is Bluetooth or
> > > not. The security of this is handled above the Bluetooth layer. The
> > > operating system that uses this feature is responsible for higher
> > > layer security. For operating systems that don't need it I am okay
> > > with adding an option to disable this feature altogether. But I can
> > > see that there are systems that need it and I am not convinced that a
> > > general purpose Bluetooth stack should not support it.
> >
> > All a malicious application has to do is to subscribe to notification
> > of HoG characteristic, then any input generate by the device would be
> > notified back to the application and that doesn't matter if uHID is
> > accessible or not the application may not even forward the events to
> > the system, now perhaps you are imagining that applications don't have
> > direct access to the attribute objects but that would be system
> > specific which is rather tricky to define.
> When the HOG-related GATT object is available on D-Bus, that means the
> internal HOG profile is not enabled in the first place, so there is
> nothing to sniff anyway. Furthermore, the higher layer operating
> system is also responsible to prevent this if it chooses to disable
> BlueZ's internal HOG profile handler. If an operating system thinks
> that it is safer to always enable internal HOG they can do just that,
> as I mentioned I'm willing to add an option to always enable all
> internal profiles.
>
> >
> > > >
> > > > More applications could be involved and then this all becomes a mess
> > > > if they have to fight over AllowInternalProfiles, so instead of using
> > > > a theoretical example we better find real apps and devices where
> > > > conflicts happens and work out case by case, adding ConnectProfile
> > > > should actually fix most of them if there is a single profile involved
> > > > by we could also thing about an alternative to connect multiples.
> > > > There is also the possibility of exposing the btd_service as objects,
> > > > I've actually had this implemented for the car industry, that way
> > > > AutoConnect property could actually be controlled on a per service
> > > > basis instead of having just one switch for everything.
> > > To be clear, applications do not have direct access to
> > > AllowInternalProfiles. The higher layer operating system controls it.
> > > This is just the same case as the org.bluez.Adapter1.Powered property
> > > and many other examples where applications are not expected to have
> > > direct control of. Therefore there should be no problem of many things
> > > fighting over it if used correctly, just like many other properties.
> > > Again, I am okay with adding an option to disable this for operating
> > > systems that do not want it.
> >
> > I see, though you didn't comment on the idea of controlling this on a
> > per-service basis, not just have everything disabled with
> > AllowInternalProfiles, note though that there are some profiles we
> > can't really disable like GAP and GATT as that involves things that
> > bt_gatt_client itself does need to access in order to work properly.
> Right, I missed that GAP should be an exception that it cannot be
> disabled, I should've added that exception in my code.
>
> However, it seems that you still don't want a single switch to disable
> all internal profiles (even with GAP exception). I'm willing to modify
> this feature to be a blocklist of profiles per device (say
> BlockedProfiles property on Device1 interface), and this feature can
> be disabled altogether (all profiles always enabled) for operating
> systems that do not want it.
>
> That idea is also similar to your service_api branch, so I will also
> try to port your service_api branch on the master branch. I will test
> this and I am okay with using this if it serves our purposes.
> I do have a question, though: With this API design, the service
> objects are not exposed until a remote profile is detected, and
> sometimes a profile is not detected until connection takes place (if
> the profile UUID is not in the advertisement). So, how does the BlueZ
> client block certain profiles/services before connection takes place?
> We can't wait until connection takes place because we already know
> that we don't want certain profiles, and if we block a profile after
> connection takes place would that work properly? I guess we still need
> a way to block certain profiles in the Device1 API, and the blocked
> profiles also need to be stored in store_device_info.

That is a good point, but that would be a problem only for the very
first time you connect since after that the blocked state should be
recovered, we could however work out an API where one can enter the
profiles to connect and to block but usually depends on the services
being resolved so I wonder it wouldn't just be enough to block the
service discovery and in the event the internal profile has already
been connected it would just disconnect, the objects would only be
exported after you had it blocked so that means there should be any
race, but perhaps you want a dedicate Connect method where one can
enumerate the profiles to auto-connect and to block, note though that
it may not be possible to indicate a valid list in case the services
have not been resolved.

> >
> > You can find the service_api commits in here:
> >
> > https://github.com/Vudentz/BlueZ/commits/service_api
> >
> > It does allow to control both the auto_connect logic as well a block:
> >
> > https://github.com/Vudentz/BlueZ/commit/9bd6dce59fe9978b3bf415fe74f89d72254b8075
> > https://github.com/Vudentz/BlueZ/commit/42a7e479d5beb641a3d94f724a2df60db0f8221c
> >
> > > Note: I have been using the term "operating system" to refer to high
> > > level components rather than the kernel.
> > >
> > >
> > > >
> > > > > >
> > > > > > Note that we do allow external profiles to be registered with use of:
> > > > > >
> > > > > > https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/doc/profile-api.txt
> > > > > >
> > > > > > And for GATT:
> > > > > >
> > > > > > https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/doc/gatt-api.txt#n366
> > > > > >
> > > > > > We could perhaps make the assumption that once an application
> > > > > > registers itself as supporting a given profile we check if against a
> > > > > > blacklist of profiles that may have security implications, which
> > > > > > perhaps could be defined via main.conf or some other file, if that is
> > > > > > not the case the internal profile can be disabled and the D-Bus object
> > > > > > would be accessible over D-Bus. Also note that we do offer clients the
> > > > > > ability to have exclusive access with AcquireWrite and AcquireNotify.
> > > > > >
> > > > > > > Therefore I think that this approach, although more complex, has
> > > > > > > longer lasting benefits. Let me know if you have any objection to
> > > > > > > having such a feature.
> > > > > > >
> > > > > > >
> > > > > > > On Mon, Jul 13, 2020 at 1:35 PM Luiz Augusto von Dentz
> > > > > > > <luiz.dentz@gmail.com> wrote:
> > > > > > > >
> > > > > > > > Hi Sonny,
> > > > > > > >
> > > > > > > > On Mon, Jul 13, 2020 at 1:18 PM Sonny Sasaka <sonnysasaka@chromium.org> wrote:
> > > > > > > > >
> > > > > > > > > This patch series adds a mechanism for clients to choose whether to
> > > > > > > > > enable BlueZ internal profiles (e.g. A2DP, Battery) for specific
> > > > > > > > > devices.
> > > > > > > > >
> > > > > > > > > The motivation behind this feature is that some applications (e.g. Web
> > > > > > > > > Bluetooth or Android apps) need to have control over all remove GATT
> > > > > > > > > services, like Battery service. With "battery" plugin being enabled on
> > > > > > > > > BlueZ, it becomes not possible for those apps to work properly because
> > > > > > > > > BlueZ "hides" the Battery-related attributes from its GATT Client API.
> > > > > > > > > Disabling the "battery" plugin won't solve the problem either, since we
> > > > > > > > > do also need to enable the plugin so that we can use org.bluez.Battery1
> > > > > > > > > API.
> > > > > > > > >
> > > > > > > > > The solution that we propose is that clients can choose whether to
> > > > > > > > > enable internal profiles for each device. Clients know when to enable
> > > > > > > > > internal profiles (such as when a user chooses to pair/connect via a UI)
> > > > > > > > > and when to disable internal profiles (such as when the connection is
> > > > > > > > > initiated by a generic application).
> > > > > > > >
> > > > > > > > I wonder if it is not better to just have a flag indicating if the
> > > > > > > > profile shall claim exclusive access (such as GAP and GATT services),
> > > > > > > > so profiles that don't set that will have the services exposed so for
> > > > > > > > battery we can probably just have it exposed by default since it
> > > > > > > > doesn't appear to would be any conflicts on having it exposed.
> > > > > > > >
> > > > > > > > > Sonny Sasaka (3):
> > > > > > > > >   doc: Add "AllowInternalProfiles" property to org.bluez.Device1
> > > > > > > > >   device: Add "AllowInternalProfiles" property to org.bluez.Device1
> > > > > > > > >   client: Add set-allow-internal-profiles command
> > > > > > > > >
> > > > > > > > >  client/main.c      | 38 ++++++++++++++++++
> > > > > > > > >  doc/device-api.txt | 13 +++++++
> > > > > > > > >  src/device.c       | 96 ++++++++++++++++++++++++++++++++++++++++++++++
> > > > > > > > >  src/hcid.h         |  2 +
> > > > > > > > >  src/main.c         | 10 +++++
> > > > > > > > >  src/main.conf      |  4 ++
> > > > > > > > >  6 files changed, 163 insertions(+)
> > > > > > > > >
> > > > > > > > > --
> > > > > > > > > 2.26.2
> > > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > --
> > > > > > > > Luiz Augusto von Dentz
> > > > > >
> > > > > >
> > > > > >
> > > > > > --
> > > > > > Luiz Augusto von Dentz
> > > >
> > > >
> > > >
> > > > --
> > > > Luiz Augusto von Dentz
> >
> >
> >
> > --
> > Luiz Augusto von Dentz


-- 
Luiz Augusto von Dentz