[PATCH v2] cpuidle: change enter_s2idle() prototype

[Neal Liu]

On Thu, 2020-07-09 at 14:18 +0200, Rafael J. Wysocki wrote:
> On Mon, Jul 6, 2020 at 5:13 AM Neal Liu <neal.liu at mediatek.com> wrote:
> >
> > Control Flow Integrity(CFI) is a security mechanism that disallows
> > changes to the original control flow graph of a compiled binary,
> > making it significantly harder to perform such attacks.
> >
> > init_state_node() assign same function callback to different
> > function pointer declarations.
> >
> > static int init_state_node(struct cpuidle_state *idle_state,
> >                            const struct of_device_id *matches,
> >                            struct device_node *state_node) { ...
> >         idle_state->enter = match_id->data; ...
> >         idle_state->enter_s2idle = match_id->data; }
> >
> > Function declarations:
> >
> > struct cpuidle_state { ...
> >         int (*enter) (struct cpuidle_device *dev,
> >                       struct cpuidle_driver *drv,
> >                       int index);
> >
> >         void (*enter_s2idle) (struct cpuidle_device *dev,
> >                               struct cpuidle_driver *drv,
> >                               int index); };
> >
> > In this case, either enter() or enter_s2idle() would cause CFI check
> > failed since they use same callee.
> 
> Can you please explain this in a bit more detail?
> 
> As it stands, I don't understand the problem statement enough to apply
> the patch.
> 

Okay, Let's me try to explain more details.
Control Flow Integrity(CFI) is a security mechanism that disallows
changes to the original control flow graph of a compiled binary, making
it significantly harder to perform such attacks.

There are multiple control flow instructions that could be manipulated
by the attacker and subvert control flow. The target instructions that
use data to determine the actual destination.
- indirect jump
- indirect call
- return

In this case, function prototype between caller and callee are mismatch.
Caller: (type A)funcA
Callee: (type A)funcB
Callee: (type C)funcC

funcA calls funcB -> no problem
funcA calls funcC -> CFI check failed

That's why we try to align function prototype.
Please feel free to feedback if you have any questions.

> > Align function prototype of enter() since it needs return value for
> > some use cases. The return value of enter_s2idle() is no
> > need currently.
> 
> So last time I requested you to document why ->enter_s2idle needs to
> return an int in the code, which has not been done.  Please do that.
> 
> > Signed-off-by: Neal Liu <neal.liu at mediatek.com>
> > ---
> >  drivers/acpi/processor_idle.c   |    6 ++++--
> >  drivers/cpuidle/cpuidle-tegra.c |    8 +++++---
> >  drivers/idle/intel_idle.c       |    6 ++++--
> >  include/linux/cpuidle.h         |    6 +++---
> >  4 files changed, 16 insertions(+), 10 deletions(-)
> >
> > diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> > index 75534c5..6ffb6c9 100644
> > --- a/drivers/acpi/processor_idle.c
> > +++ b/drivers/acpi/processor_idle.c
> > @@ -655,8 +655,8 @@ static int acpi_idle_enter(struct cpuidle_device *dev,
> >         return index;
> >  }
> >
> > -static void acpi_idle_enter_s2idle(struct cpuidle_device *dev,
> > -                                  struct cpuidle_driver *drv, int index)
> > +static int acpi_idle_enter_s2idle(struct cpuidle_device *dev,
> > +                                 struct cpuidle_driver *drv, int index)
> >  {
> >         struct acpi_processor_cx *cx = per_cpu(acpi_cstate[index], dev->cpu);
> >
> > @@ -674,6 +674,8 @@ static void acpi_idle_enter_s2idle(struct cpuidle_device *dev,
> >                 }
> >         }
> >         acpi_idle_do_entry(cx);
> > +
> > +       return 0;
> >  }
> >
> >  static int acpi_processor_setup_cpuidle_cx(struct acpi_processor *pr,
> > diff --git a/drivers/cpuidle/cpuidle-tegra.c b/drivers/cpuidle/cpuidle-tegra.c
> > index 1500458..a12fb14 100644
> > --- a/drivers/cpuidle/cpuidle-tegra.c
> > +++ b/drivers/cpuidle/cpuidle-tegra.c
> > @@ -253,11 +253,13 @@ static int tegra_cpuidle_enter(struct cpuidle_device *dev,
> >         return err ? -1 : index;
> >  }
> >
> > -static void tegra114_enter_s2idle(struct cpuidle_device *dev,
> > -                                 struct cpuidle_driver *drv,
> > -                                 int index)
> > +static int tegra114_enter_s2idle(struct cpuidle_device *dev,
> > +                                struct cpuidle_driver *drv,
> > +                                int index)
> >  {
> >         tegra_cpuidle_enter(dev, drv, index);
> > +
> > +       return 0;
> >  }
> >
> >  /*
> > diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> > index f449584..b178da3 100644
> > --- a/drivers/idle/intel_idle.c
> > +++ b/drivers/idle/intel_idle.c
> > @@ -175,13 +175,15 @@ static __cpuidle int intel_idle(struct cpuidle_device *dev,
> >   * Invoked as a suspend-to-idle callback routine with frozen user space, frozen
> >   * scheduler tick and suspended scheduler clock on the target CPU.
> >   */
> > -static __cpuidle void intel_idle_s2idle(struct cpuidle_device *dev,
> > -                                       struct cpuidle_driver *drv, int index)
> > +static __cpuidle int intel_idle_s2idle(struct cpuidle_device *dev,
> > +                                      struct cpuidle_driver *drv, int index)
> >  {
> >         unsigned long eax = flg2MWAIT(drv->states[index].flags);
> >         unsigned long ecx = 1; /* break on interrupt flag */
> >
> >         mwait_idle_with_hints(eax, ecx);
> > +
> > +       return 0;
> >  }
> >
> >  /*
> > diff --git a/include/linux/cpuidle.h b/include/linux/cpuidle.h
> > index ec2ef63..bee10c0 100644
> > --- a/include/linux/cpuidle.h
> > +++ b/include/linux/cpuidle.h
> > @@ -66,9 +66,9 @@ struct cpuidle_state {
> >          * suspended, so it must not re-enable interrupts at any point (even
> >          * temporarily) or attempt to change states of clock event devices.
> >          */
> > -       void (*enter_s2idle) (struct cpuidle_device *dev,
> > -                             struct cpuidle_driver *drv,
> > -                             int index);
> > +       int (*enter_s2idle)(struct cpuidle_device *dev,
> > +                           struct cpuidle_driver *drv,
> > +                           int index);
> >  };
> >
> >  /* Idle State Flags */
> > --
> > 1.7.9.5