[RFC PATCH 3/7] static_call: use helper to access non-exported key

[Ard Biesheuvel]

On Tue, 9 Nov 2021 at 20:42, Peter Zijlstra <peterz at infradead.org> wrote:
>
> On Tue, Nov 09, 2021 at 07:54:23PM +0100, Ard Biesheuvel wrote:
> > On Tue, 9 Nov 2021 at 19:53, Peter Zijlstra <peterz at infradead.org> wrote:
> > >
> > > On Tue, Nov 09, 2021 at 05:45:45PM +0100, Ard Biesheuvel wrote:
> > > > @@ -196,13 +190,21 @@ extern long __static_call_return0(void);
> > > >       EXPORT_SYMBOL_GPL(STATIC_CALL_KEY(name));                       \
> > > >       EXPORT_SYMBOL_GPL(STATIC_CALL_TRAMP(name))
> > > >
> > > > +#define EXPORT_STATIC_CALL_GETKEY_HELPER(name)                               \
> > > > +     struct static_call_key *STATIC_CALL_GETKEY(name)(void) {        \
> > > > +             BUG_ON(!core_kernel_text(                               \
> > > > +                     (unsigned long)__builtin_return_address(0)));   \
> > > > +             return &STATIC_CALL_KEY(name);                          \
> > > > +     }                                                               \
> > > > +     EXPORT_SYMBOL_GPL(STATIC_CALL_GETKEY(name))
> > >
> > > So if I were a nevarious module, I would look up the above symbol from
> > > kallsyms (it is exported and easily obtainable) and then simply
> > > read the text to discover the key address and we're in business.
> > >
> >
> > Yeah I realised that. So would you prefer to have a
> > .static_call_tramp_key section in each module and look up the keys in
> > the module loader?
>
> Think so... do you actually have a need for this?

Well, not exactly. But the XOR patch I sent earlier today does cover
this use case, and would be broken if we implemented inline static
calls on arm64.

So the easy fix is to allow EXPORT_STATIC_CALL_RO() only from the core
kernel, and require EXPORT_STATIC_CALL_RW() otherwise. That way, this
can never end up being used in a way that breaks at runtime. Would you
prefer doing that instead?