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

[Andy Lutomirski]

On Fri, Apr 16, 2021 at 3:14 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Fri, Apr 16, 2021 at 03:06:17PM -0700, Andy Lutomirski wrote:
> > On Fri, Apr 16, 2021 at 3:03 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
> > >
> > > On Fri, Apr 16, 2021 at 02:49:23PM -0700, Sami Tolvanen wrote:
> > > > __nocfi only disables CFI checking in a function, the compiler still
> > > > changes function addresses to point to the CFI jump table, which is
> > > > why we need function_nocfi().
> > >
> > > So call it __func_addr() or get_function_addr() or so, so that at least
> > > it is clear what this does.
> > >
> >
> > This seems backwards to me.  If I do:
> >
> > extern void foo(some signature);
> >
> > then I would, perhaps naively, expect foo to be the actual symbol that
>
> I'm just reading the patch:
>
> ... The function_nocfi macro always returns the address of the
> + * actual function instead.
> + */
> +#define function_nocfi(x) ({                                           \
> +       void *addr;                                                     \
> +       asm("leaq " __stringify(x) "(%%rip), %0\n\t" : "=r" (addr));    \
> +       addr;
>
> so it does a rip-relative load into a reg which ends up with the function
> address.

This is horrible.

We made a mistake adapting the kernel to GCC's nonsensical stack
protector ABI, especially on 32-bit, instead of making GCC fix it.
Let's not repeat this with clang please.

Sami, I'm assuming that:

extern void func(void);

results in anything that takes a pointer to func getting a pointer to
some special magic descriptor instead of to func, so that:

void (*ptr)(void);
ptr = func;
ptr();

does the right thing.  Then void (*)(void) is no longer a raw pointer.  Fine.

But obviously there is code that needs real function pointers.  How
about making this a first-class feature, or at least hacking around it
more cleanly.  For example, what does this do:

char entry_whatever[];
wrmsrl(..., (unsigned long)entry_whatever);

or, alternatively,

extern void func() __attribute__((nocfi));

void (*ptr)(void);
ptr = func;  /* probably fails to compile -- invalid conversion */

(unsigned long)func /* returns the actual pointer */

func();  /* works like normal */

And maybe allow this too:

void (*ptr)(void) __attribute__((nocfi);
ptr = func;
ptr();  /* emits an unchecked call.  maybe warns, too.  anyone who
does this needs to be extremely careful. */

--Andy