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

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Date:   Fri, 26 May 2017 13:17:10 -0700
From:   Kees Cook <keescook@...omium.org>
To:     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,
        linux-kernel@...r.kernel.org
Subject: [PATCH v2 06/20] randstruct: Whitelist UNIXCB cast

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>
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