[PATCH v2] cpuidle: change enter_s2idle() prototype

[Rafael J. Wysocki]

On Fri, Jul 24, 2020 at 12:24 PM Neal Liu <neal.liu at mediatek.com> wrote:
>
> On Fri, 2020-07-24 at 11:57 +0200, Rafael J. Wysocki wrote:
> > On Thu, Jul 23, 2020 at 9:07 PM Sami Tolvanen <samitolvanen at google.com> wrote:
> > >
> > > On Mon, Jul 20, 2020 at 04:21:34PM +0800, Neal Liu wrote:
> > > > Gentle ping on this patch.
> > > >
> > > >
> > > > On Fri, 2020-07-10 at 11:08 +0800, Neal Liu wrote:
> > > > > 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.
> > >
> > > I think you should include a better explanation in the commit message.
> > > Perhaps something like this?
> > >
> > >   init_state_node assigns the same callback function to both enter and
> > >   enter_s2idle despite mismatching function types, which trips indirect
> > >   call checking with Control-Flow Integrity (CFI).
> > >
> > > > > > > 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.
> > >
> > > Rafael, are you happy with the commit message documenting the reason,
> > > or would you prefer to also add a comment before enter_s2idle?
> >
> > As I said before, it would be good to have a comment in the code as
> > well or people will be wondering why it is necessary to return
> > anything from that callback, because its return value is never used.
> >
> > Thanks!
>
> Is it okay to add these comments before enter_s2idle?
>
> /*
>  * Align function type since init_state_node assigns the same callback

init_state_node()

>  * function to both enter and enter_s2idle despite mismatching function

->enter_s2idle

>  * types, which trips indirect call checking with Control-Flow Integrity
>  * (CFI).
>  */
> int (*enter_s2idle)(struct cpuidle_device *dev,
>                     struct cpuidle_driver *drv,
>                     int index);

But IMO it would be sufficient to add something like this to the
existing comment regarding ->enter_s2idle:

"This callback may point to the same function as ->enter if all of the
above requirements are met by it."

That would explain why the signature is the same sufficiently in my view.

Thanks!