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

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Message-ID: <CAGXu5jJo+c4d+=57rFiHaFrOHxDHVSZzMhWSMqNQe+tcw0-4zQ@mail.gmail.com>
Date:   Mon, 5 Mar 2018 11:15:35 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>,
        Patrick McLean <chutzpah@...too.org>,
        Emese Revfy <re.emese@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Bruce Fields <bfields@...hat.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: RANDSTRUCT structs need linux/compiler_types.h (Was: [nfsd4]
 potentially hardware breaking regression in 4.14-rc and 4.13.11)

On Mon, Mar 5, 2018 at 1:27 AM, Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
> Sorry for chiming in late.
>
> I noticed this thread today,
> honestly, the commit made me upset.
>
>
> Can I suggest another way to make it less fragile?
> __attribute((...)) can be placed after 'struct'.
>
>
> So, we can write:
>
>
> struct __randomize_layout path {
>         struct vfsmount *mnt;
>         struct dentry *dentry;
> };
>
>
>   instead of
>
>
> struct path {
>         struct vfsmount *mnt;
>         struct dentry *dentry;
> } __randomize_layout;

Ugh. I had tried this after the struct _name_, not after "struct"
itself. This does fix it, though it remains fragile, as you mention.

> If we force the former notation,
> the undefined __randomize_layout results in a build error
> instead of silent broken code generation.
>
>
> It is true somebody can still place
> __randomize_layout after the closing brace,
> but can we check this by coccicheck or checkpatch.pl?
> (we can describe it in coding style documentation, of course)
>
>
> IMHO, we should not (ab)use include/linux/kconfig.h
> to bring in misc things.

I'm happy to send a patch that reverts the other changes and relocates
all the markings...

Linus, how would you like this to go?

-Kees

-- 
Kees Cook
Pixel Security