[PATCH] LSM: Make security_hook_heads a local variable.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

A sealable memory allocator patch was proposed at
http://lkml.kernel.org/r/20170519103811.2183-1-igor.stoppa@huawei.com ,
and is waiting for a follow-on patch showing how any of the kernel
can be changed to use this new subsystem. So, here it is for LSM hooks.

The LSM hooks ("struct security_hook_heads security_hook_heads" and
"struct security_hook_list ...[]") will benefit from this allocator via
protection using set_memory_ro()/set_memory_rw(), and it will remove
CONFIG_SECURITY_WRITABLE_HOOKS config option.

This means that these structures will be allocated at run time using
smalloc(), and therefore the address of these structures will be
determined at run time rather than compile time.

But currently, LSM_HOOK_INIT() macro depends on the address of
security_hook_heads being known at compile time. But we already
initialize security_hook_heads as an array of "struct list_head".

Therefore, let's use index number (or relative offset from the head
of security_hook_heads) instead of absolute address of
security_hook_heads so that LSM_HOOK_INIT() macro does not need to
know absolute address of security_hook_heads. Then, security_add_hooks()
will be able to allocate and copy "struct security_hook_list ...[]" using
smalloc().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Kees Cook <keescook@chromium.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: James Morris <james.l.morris@oracle.com>
Cc: Igor Stoppa <igor.stoppa@huawei.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
---
 include/linux/lsm_hooks.h |  6 +++---
 security/security.c       | 10 ++++++++--
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 080f34e..865c11d 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1884,8 +1884,8 @@ struct security_hook_heads {
  */
 struct security_hook_list {
 	struct list_head		list;
-	struct list_head		*head;
 	union security_list_options	hook;
+	const unsigned int		idx;
 	char				*lsm;
 };
 
@@ -1896,9 +1896,9 @@ struct security_hook_list {
  * text involved.
  */
 #define LSM_HOOK_INIT(HEAD, HOOK) \
-	{ .head = &security_hook_heads.HEAD, .hook = { .HEAD = HOOK } }
+	{ .idx = offsetof(struct security_hook_heads, HEAD) / \
+		sizeof(struct list_head), .hook = { .HEAD = HOOK } }
 
-extern struct security_hook_heads security_hook_heads;
 extern char *lsm_names;
 
 extern void security_add_hooks(struct security_hook_list *hooks, int count,
diff --git a/security/security.c b/security/security.c
index 54b1e39..d6883ce 100644
--- a/security/security.c
+++ b/security/security.c
@@ -33,7 +33,7 @@
 /* Maximum number of letters for an LSM name string */
 #define SECURITY_NAME_MAX	10
 
-struct security_hook_heads security_hook_heads __lsm_ro_after_init;
+static struct security_hook_heads security_hook_heads __lsm_ro_after_init;
 char *lsm_names;
 /* Boot-time LSM user choice */
 static __initdata char chosen_lsm[SECURITY_NAME_MAX + 1] =
@@ -152,10 +152,16 @@ void __init security_add_hooks(struct security_hook_list *hooks, int count,
 				char *lsm)
 {
 	int i;
+	struct list_head *list = (struct list_head *) &security_hook_heads;
 
 	for (i = 0; i < count; i++) {
+		const unsigned int idx = hooks[i].idx;
+
+		if (WARN_ON(idx >= sizeof(security_hook_heads) /
+			    sizeof(struct list_head)))
+			continue;
 		hooks[i].lsm = lsm;
-		list_add_tail_rcu(&hooks[i].list, hooks[i].head);
+		list_add_tail_rcu(&hooks[i].list, &list[idx]);
 	}
 	if (lsm_append(lsm, &lsm_names) < 0)
 		panic("%s - Cannot get early memory.\n", __func__);
-- 
1.8.3.1