linux-kernel - Re: [PATCH v2 06/20] randstruct: Whitelist UNIXCB cast

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Date:   Fri, 26 May 2017 13:20:13 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        "Signed-off-by : David S . Miller" <davem@...emloft.net>,
        Laura Abbott <labbott@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/20] randstruct: Whitelist UNIXCB cast

On Fri, May 26, 2017 at 1:17 PM, Kees Cook <keescook@...omium.org> wrote:
> This is another false positive in bad cast detection:
>
> net/unix/af_unix.c: In function ‘unix_skb_scm_eq’:
> net/unix/af_unix.c:1621:31: note: found mismatched rhs struct pointer types: ‘struct unix_skb_parms’ and ‘char’
>
>   const struct unix_skb_parms *u = &UNIXCB(skb);
>                                ^
>
> UNIXCB is:
>
>         #define UNIXCB(skb)     (*(struct unix_skb_parms *)&((skb)->cb))
>
> And ->cb is:
>
>         char                    cb[48] __aligned(8);
>
> This is a rather crazy cast, but appears to be safe in the face of
> randomization, so whitelist it in the plugin.
>
> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Cc: Signed-off-by: David S. Miller <davem@...emloft.net>

Argh, paste-o. I was adding git history folks here to Cc, pardon the
glitch. This should be Cc: and absolutely not a S-o-B from Dave.

-Kees

> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  scripts/gcc-plugins/randomize_layout_plugin.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c b/scripts/gcc-plugins/randomize_layout_plugin.c
> index e126ac7874af..bf110915a5aa 100644
> --- a/scripts/gcc-plugins/randomize_layout_plugin.c
> +++ b/scripts/gcc-plugins/randomize_layout_plugin.c
> @@ -43,6 +43,8 @@ struct whitelist_entry {
>  };
>
>  static const struct whitelist_entry whitelist[] = {
> +       /* unix_skb_parms via UNIXCB() buffer */
> +       { "net/unix/af_unix.c", "unix_skb_parms", "char" },
>         /* walk struct security_hook_heads as an array of struct list_head */
>         { "security/security.c", "list_head", "security_hook_heads" },
>         { }
> --
> 2.7.4
>



-- 
Kees Cook
Pixel Security