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From:   Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Date:   Wed, 12 Feb 2020 13:21:33 -0800
Message-ID: <CABBYNZL9xhc5Sjoj4xb3vPSq=1XzyzG-mEhH59_5U4-s2_mBtw@mail.gmail.com>
Subject: Re: [Bluez PATCH v2] core: Add new policy for Just-Works repairing
To:     Emil Lenngren <emil.lenngren@gmail.com>
Cc:     Howard Chung <howardchung@google.com>,
        Bluez mailing list <linux-bluetooth@vger.kernel.org>,
        Luiz Augusto Von Dentz <luiz.von.dentz@intel.com>,
        chromeos-bluetooth-upstreaming@chromium.org
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Hi Emil,

On Wed, Feb 12, 2020 at 6:00 AM Emil Lenngren <emil.lenngren@gmail.com> wrote:
>
> Hi,
>
> Den ons 12 feb. 2020 kl 14:30 skrev Howard Chung <howardchung@google.com>:
> >
> > From: "howardchung@google.com" <howardchung@google.com>
> >
> > When kernel find out that the incoming Just-Works pairing is
> > initiated by a paired device, it is user space's responsibility to
> > decide the next action.
> >
> > This patch includes the following:
> > - add JustWorksRepairing policy as an option in main.conf
> > - handle the confirmation request from kernel
> >
> > ---
> > The Just-Works repairing policy could be one of the following:
> > - never: default; reject the repairing immediately.
> > - confirm: prompt a confirmation dialog to user.
> > - always: always accept the repairing.
> >
> > Changes in v2:
> > - let RequestAuthorization handle the situation
> > - remove the changes in client/
> >
> >  src/agent.c   | 16 ++++++++++++++++
> >  src/hcid.h    |  8 ++++++++
> >  src/main.c    | 27 +++++++++++++++++++++++++++
> >  src/main.conf |  5 +++++
> >  4 files changed, 56 insertions(+)
> >
> > diff --git a/src/agent.c b/src/agent.c
> > index e0ffcd22f..e013ec85f 100644
> > --- a/src/agent.c
> > +++ b/src/agent.c
> > @@ -773,12 +773,28 @@ int agent_request_authorization(struct agent *agent, struct btd_device *device,
> >                                                 GDestroyNotify destroy)
> >  {
> >         struct agent_request *req;
> > +       DBusError dbus_err;
> >         int err;
> >
> >         err = agent_has_request(agent, device, AGENT_REQUEST_AUTHORIZATION);
> >         if (err)
> >                 return err;
> >
> > +       /* Just-Works repairing policy */
> > +       if (device_is_paired(device, BDADDR_BREDR) ||
> > +                               device_is_paired(device, BDADDR_LE_PUBLIC)) {
> > +               if (main_opts.jw_repairing == JW_REPAIRING_NEVER) {
> > +                       dbus_error_init(&dbus_err);
> > +                       dbus_set_error_const(&dbus_err,
> > +                                       ERROR_INTERFACE ".Rejected", NULL);
> > +                       cb(agent, &dbus_err, user_data);
> > +                       return 0;
> > +               } else if (main_opts.jw_repairing == JW_REPAIRING_ALWAYS) {
> > +                       cb(agent, NULL, user_data);
> > +                       return 0;
> > +               }
> > +       }
> > +
> >         DBG("Calling Agent.RequestAuthorization: name=%s, path=%s",
> >                                                 agent->owner, agent->path);
> >
> > diff --git a/src/hcid.h b/src/hcid.h
> > index adea85ce2..bcd2b9fa1 100644
> > --- a/src/hcid.h
> > +++ b/src/hcid.h
> > @@ -35,6 +35,12 @@ typedef enum {
> >         BT_GATT_CACHE_NO,
> >  } bt_gatt_cache_t;
> >
> > +enum {
> > +       JW_REPAIRING_NEVER,
> > +       JW_REPAIRING_CONFIRM,
> > +       JW_REPAIRING_ALWAYS,
> > +} jw_repairing_t;
> > +
> >  struct main_opts {
> >         char            *name;
> >         uint32_t        class;
> > @@ -58,6 +64,8 @@ struct main_opts {
> >         uint16_t        gatt_mtu;
> >
> >         uint8_t         key_size;
> > +
> > +       jw_repairing_t  jw_repairing;
> >  };
> >
> >  extern struct main_opts main_opts;
> > diff --git a/src/main.c b/src/main.c
> > index 1a6ab36a3..d67f469f1 100644
> > --- a/src/main.c
> > +++ b/src/main.c
> > @@ -93,6 +93,7 @@ static const char *supported_options[] = {
> >         "MultiProfile",
> >         "FastConnectable",
> >         "Privacy",
> > +       "JustWorksRepairing",
> >         NULL
> >  };
> >
> > @@ -193,6 +194,20 @@ static bt_gatt_cache_t parse_gatt_cache(const char *cache)
> >         }
> >  }
> >
> > +static jw_repairing_t parse_jw_repairing(const char *jw_repairing)
> > +{
> > +       if (!strcmp(jw_repairing, "never")) {
> > +               return JW_REPAIRING_NEVER;
> > +       } else if (!strcmp(jw_repairing, "confirm")) {
> > +               return JW_REPAIRING_CONFIRM;
> > +       } else if (!strcmp(jw_repairing, "always")) {
> > +               return JW_REPAIRING_ALWAYS;
> > +       } else {
> > +               return JW_REPAIRING_NEVER;
> > +       }
> > +}
> > +
> > +
> >  static void check_options(GKeyFile *config, const char *group,
> >                                                 const char **options)
> >  {
> > @@ -331,6 +346,18 @@ static void parse_config(GKeyFile *config)
> >                 g_free(str);
> >         }
> >
> > +       str = g_key_file_get_string(config, "General",
> > +                                               "JustWorksRepairing", &err);
> > +       if (err) {
> > +               DBG("%s", err->message);
> > +               g_clear_error(&err);
> > +               main_opts.jw_repairing = JW_REPAIRING_NEVER;
> > +       } else {
> > +               DBG("just_works_repairing=%s", str);
> > +               main_opts.jw_repairing = parse_jw_repairing(str);
> > +               g_free(str);
> > +       }
> > +
> >         str = g_key_file_get_string(config, "General", "Name", &err);
> >         if (err) {
> >                 DBG("%s", err->message);
> > diff --git a/src/main.conf b/src/main.conf
> > index 40687a755..bb5ff5b15 100644
> > --- a/src/main.conf
> > +++ b/src/main.conf
> > @@ -72,6 +72,11 @@
> >  # Defaults to "off"
> >  # Privacy = off
> >
> > +# Specify the policy to the JUST-WORKS repairing initiated by peer
> > +# Possible values: "never", "confirm", "always"
> > +# Defaults to "never"
> > +#JustWorksRepairing = never
> > +
> >  [GATT]
> >  # GATT attribute cache.
> >  # Possible values:
> > --
> > 2.25.0.225.g125e21ebc7-goog
> >
>
> I haven't looked much into this, but have a question. What happens by
> default currently when we have an already bonded device previously
> paired using some MITM-protected mechanism, and suddenly wants to
> re-bond using a less secure mechanism (Just Works)? In my opinion
> denying this should obviously be the default in case no agent is
> available, compared to if pairing a new device then the default should
> be accept.

When no agent is registered we don't set the pairable flag, so it
shouldn't even be able do trigger a new pairing without an agent.

-- 
Luiz Augusto von Dentz