Re: [PATCH] LSM: Make security_hook_heads a local variable.

[Casey Schaufler]

On 5/22/2017 7:03 AM, Christoph Hellwig wrote:
> On Sun, May 21, 2017 at 08:14:05PM +0900, Tetsuo Handa wrote:
>> A sealable memory allocator patch was proposed at
>> http://lkml.kernel.org/r/20170519103811.2183-1-igor.stoppa@xxxxxxxxxx ,
>> 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@xxxxxxxxxxxxxxxxxxx>
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Paul Moore <paul@xxxxxxxxxxxxxx>
>> Cc: Stephen Smalley <sds@xxxxxxxxxxxxx>
>> Cc: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
>> Cc: James Morris <james.l.morris@xxxxxxxxxx>
>> Cc: Igor Stoppa <igor.stoppa@xxxxxxxxxx>
>> Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
>> ---
>>  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;
> Eww, struct casts.  This whole security_hook_heads scheme stink,
> even with the slight improvements from Tetsuo.  It has everything we
> shouldn't do - function pointers in structures that are not hard
> read-only, structure casts, etc.
>
> What's the reason why can't just have good old const function tables?

The set of hooks used by most security modules are sparse.

> Yeah, stackable LSM make that a little harder, but they should not be
> enable by default anyway.

With the number of security modules queued up behind full stacking
I can't say that I agree with your assertion.

> But even with those we can still chain
> them together with a list with external linkage.

I gave up that approach in 2012. Too many unnecessary calls to
null functions, and massive function vectors with a tiny number
of non-null entries. From a data structure standpoint, it was
just wrong. The list scheme is exactly right for the task at
hand.

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