Re: RANDSTRUCT structs need linux/compiler_types.h (Was: [nfsd4] potentially hardware breaking regression in 4.14-rc and 4.13.11)

[Kees Cook]

On Wed, Feb 21, 2018 at 2:47 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> And other attribute specifiers we encourage people to put in other
> parts of the type, like __user etc, so they don't have that same
> parsing issue.

Looking at other attributes we use on structs, we may have similar
risks for these:

__packed
____cacheline_aligned
____cacheline_aligned_in_smp
____cacheline_internodealigned_in_smp

But they just haven't been used in places that we could trip over it
as badly, AFAICT.

> I guess one _extreme_ fix for this would be to put
>
>     extern struct nostruct __randomize_layout;
>
> in our include/linux/kconfig.h, which I think we end up always
> including first thanks to having it on the command line.

We could do that for all the above, but I wonder if the real problem
is our convention of using "regular" names for these kinds of
attributes instead of parameterized names. If we always used something
like:

#define __struct(x)   __attribute__(x)

We'd avoid it, but we'd uglify our struct attributes:

struct thing { ... } __struct(randomize_layout);

though trying this now creates other problems. Hmmm.

(Regardless, let me send the nfs fix separately...)

-Kees

>
> Because if you do that, you actually get an error:
>
>     CC [M]  fs/nfsd/nfs4xdr.o
>   In file included from ./include/linux/fs_struct.h:5:0,
>                    from fs/nfsd/nfs4xdr.c:36:
>   ./include/linux/path.h:11:3: error: conflicting types for â__randomize_layoutâ
>    } __randomize_layout;
>      ^~~~~~~~~~~~~~~~~~
>   In file included from <command-line>:0:0:
>   ././include/linux/kconfig.h:8:28: note: previous declaration of
> â__randomize_layoutâ was here
>        extern struct nostruct __randomize_layout;
>                               ^~~~~~~~~~~~~~~~~~
>   make[1]: *** [scripts/Makefile.build:317: fs/nfsd/nfs4xdr.o] Error 1
>
> and we would have figured this out immediately.
>
> Broken example patch appended, in case somebody wants to play with
> something like this or comes up with a better model entirely..
>
>                Linus
>
> ---
>
> diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
> index fec5076eda91..537dacb83380 100644
> --- a/include/linux/kconfig.h
> +++ b/include/linux/kconfig.h
> @@ -4,6 +4,10 @@
>
>  #include <generated/autoconf.h>
>
> +#ifndef __ASSEMBLY__
> + extern struct nostruct __randomize_layout;
> +#endif
> +
>  #define __ARG_PLACEHOLDER_1 0,
>  #define __take_second_arg(__ignored, val, ...) val



-- 
Kees Cook
Pixel Security