linux-kernel - Re: [PATCH v6 1/4] tee: optee: system call property

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Message-ID: <CAFA6WYNf0xk0QVKyJvRKx1cisLtbjkG_FTDQ47Q_FPAAu3WFHg@mail.gmail.com>
Date:   Thu, 11 May 2023 13:09:28 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Etienne Carriere <etienne.carriere@...aro.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        op-tee@...ts.trustedfirmware.org,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Cristian Marussi <cristian.marussi@....com>
Subject: Re: [PATCH v6 1/4] tee: optee: system call property

On Thu, 11 May 2023 at 12:50, Etienne Carriere
<etienne.carriere@...aro.org> wrote:
>
> On Thu, 11 May 2023 at 08:03, Sumit Garg <sumit.garg@...aro.org> wrote:
> > (snip)
> > > > >
> > > > >  int optee_invoke_func(struct tee_context *ctx, struct tee_ioctl_invoke_arg *arg,
> > > > > @@ -408,12 +412,15 @@ int optee_invoke_func(struct tee_context *ctx, struct tee_ioctl_invoke_arg *arg,
> > > > >         struct optee_msg_arg *msg_arg;
> > > > >         struct optee_session *sess;
> > > > >         struct tee_shm *shm;
> > > > > +       bool system_thread;
> > > > >         u_int offs;
> > > > >         int rc;
> > > > >
> > > > >         /* Check that the session is valid */
> > > > >         mutex_lock(&ctxdata->mutex);
> > > > >         sess = find_session(ctxdata, arg->session);
> > > > > +       if (sess)
> > > >
> > > > This check is redundant if we move the assignment below...
> > >
> > > Here we change the sesssion attribute while the mutex is locked, in
> > > case some equivalent call with that session is issued.
> > > Below we return to caller once mutex is unlocked.
> > > I think it is the safer behavior. What do you think?
> >
> > Aren't we only reading session attribute in order to capture value in
> > a local variable: system_thread? I don't think that it would require a
> > mutex.
>
> optee_system_session() sets session::use_sys_thread with mutex locked
> hence I think we should get the attribute with the mutex locked.
> See "[PATCH v6 3/4] tee: optee: support tracking system threads".
>

Okay I see your point. Although I don't see a practical race between
optee_invoke_func() vs optee_system_session(), you never know what
complex kernel TEE client use-case comes up. So I can live with it
being protected by a mutex.

-Sumit

> Etienne
>
> >
> > -Sumit
> >
> > >
> > > Best regards,
> > > Etienne
> > >
> > > >
> > > > > +               system_thread = sess->use_sys_thread;
> > > > >         mutex_unlock(&ctxdata->mutex);
> > > > >         if (!sess)
> > > > >                 return -EINVAL;
> > > >
> > > > ...here as:
> > > >            system_thread = sess->use_sys_thread;
> > > > (snip)