Re: [PATCH 05/15] x86: Implement function_nocfi

[Andy Lutomirski]

On Sat, Apr 17, 2021 at 4:53 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> On Sat, Apr 17 2021 at 16:19, Andy Lutomirski wrote:
> > On Fri, Apr 16, 2021 at 4:40 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> Okay, you're saying you want __builtin_gimme_body_p() to be a constant
> >> expression for the compiler, not inline asm?
> >
> > Yes.
> >
> > I admit that, in the trivial case where the asm code is *not* a
> > C-ABI-compliant function, giving a type that doesn't fool the compiler
> > into thinking that it might be is probably the best fix.  Maybe we
> > should standardize something, e.g.:
> >
> > struct raw_symbol;  /* not defined anywhere */
> > #define DECLARE_RAW_SYMBOL(x) struct raw_symbol x[]
> >
> > and then we write this:
> >
> > DECLARE_RAW_SYMBOL(entry_SYSCALL_64);
> >
> > wrmsrl(..., (unsigned long)entry_SYSCALL_64);
> >
> > It would be a bit nifty if we didn't need a forward declaration, but
> > I'm not immediately seeing a way to do this without hacks that we'll
> > probably regret;
> >
> > But this doesn't help the case in which the symbol is an actual
> > C-callable function and we want to be able to call it, too.
>
> The right way to solve this is that the compiler provides a builtin
>
>  function_nocfi() +/- the naming bikeshed
>
> which works for
>
>       foo = function_nocfi(bar);

I agree in general.  But right now, we have, in asm/proto.h:

void entry_SYSCALL_64(void);

and that's pure nonsense.  Depending on your point of view,
entry_SYSCALL_64 is a symbol that resolves to an integer or it's an
array of bytes containing instructions, but it is most definitely not
a function void (void).  So, regardless of any CFI stuff, I propose
that we standardize our handling of prototypes of symbols that are
opaque to the C compiler.  Here are a couple of choices:

Easy one:

extern u8 entry_SYSCALL_64[];

Slightly more complicated:

struct opaque_symbol;
extern struct opaque_symbol entry_SYSCALL_64;

The opaque_symbol variant avoids any possible confusion over the weird
status of arrays in C, and it's hard to misuse, since struct
opaque_symbol is an incomplete type.

--Andy