On Fri, Apr 16, 2021 at 3:14 PM Borislav Petkov <b...@alien8.de> 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 <b...@alien8.de> 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
